PreprintPDF Available
Preprints and early-stage research may not have been peer reviewed yet.

Abstract and Figures

We report on the concept of an innovative source to produce polarized proton/deuteron beams of a kinetic energy up to several GeV from a laser-driven plasma accelerator. Spin effects have been implemented into the PIC simulation code VLPL to make theoretical predictions about the behavior of proton spins in laser-induced plasmas. Simulations of spin- polarized targets show that the polarization is conserved during the acceleration process. For the experimental realization, a polarized HCl gas-jet target is under construction using the fundamental wavelength of a Nd:YAG laser system to align the HCl bonds and simultaneously circular polarized light of the fifth harmonic to photo-dissociate, yielding nuclear polarized H atoms. Subsequently, their degree of polarization is measured with a Lamb-shift polarimeter. The final experiments, aiming at the first observation of a polarized particle beam from laser-generated plasmas, will be carried out at the 10 PW laser system SULF at SIOM/Shanghai.
Content may be subject to copyright.
High Power Laser Science and Engineering, (2019), Vol. 7, e16, 6 pages.
© The Author(s) 2019. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (
licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.
Polarized proton beams from laser-induced plasmas
Anna H¨
utzen1,2, Johannes Thomas3, J¨
urgen B¨
oker4, Ralf Engels5, Ralf Gebel4, Andreas Lehrach4,6,
Alexander Pukhov3, T. Peter Rakitzis7,8, Dimitris Sofikitis7,8, and Markus B ¨
1Peter Gr¨
unberg Institut (PGI-6), Forschungszentrum J ¨
ulich, Wilhelm-Johnen-Str. 1, 52425 J ¨
ulich, Germany
2Institut f¨
ur Laser-und Plasmaphysik, Heinrich-Heine-Universit¨
at D¨
usseldorf, Universit¨
atsstr. 1, 40225 D ¨
usseldorf, Germany
3Institut f¨
ur Theoretische Physik I, Heinrich-Heine-Universit¨
at D¨
usseldorf, Universit¨
atsstr. 1, 40225 D ¨
usseldorf, Germany
4Institut f¨
ur Kernphysik (IKP-4), Forschungszentrum J ¨
ulich, Wilhelm-Johnen-Str. 1, 52425 J ¨
ulich, Germany
5Institut f¨
ur Kernphysik (IKP-2), Forschungszentrum J ¨
ulich, Wilhelm-Johnen-Str. 1, 52425 J ¨
ulich, Germany
6JARA-FAME und III. Physikalisches Institut B, RWTH Aachen, Otto-Blumenthal-Str., 52074 Aachen, Germany
7Department of Physics, University of Crete, 71003 Heraklion-Crete, Greece
8Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas, 71110 Heraklion-Crete, Greece
(Received 4 October 2018; revised 14 November 2018; accepted 12 December 2018)
We report on the concept of an innovative source to produce polarized proton/deuteron beams of a kinetic energy up
to several GeV from a laser-driven plasma accelerator. Spin effects have been implemented into the particle-in-cell
(PIC) simulation code VLPL (Virtual Laser Plasma Lab) to make theoretical predictions about the behavior of proton
spins in laser-induced plasmas. Simulations of spin-polarized targets show that the polarization is conserved during the
acceleration process. For the experimental realization, a polarized HCl gas-jet target is under construction using the
fundamental wavelength of a Nd:YAG laser system to align the HCl bonds and simultaneously circularly polarized light
of the fifth harmonic to photo-dissociate, yielding nuclear polarized H atoms. Subsequently, their degree of polarization
is measured with a Lamb-shift polarimeter. The final experiments, aiming at the first observation of a polarized particle
beam from laser-generated plasmas, will be carried out at the 10 PW laser system SULF at SIOM, Shanghai.
Keywords: laser-driven plasma accelerator; particle-in-cell simulations; polarized gas-jet target; polarized proton beams
1. Introduction
Ion acceleration driven by super-intense laser pulses has
undergone impressive advances in recent years. Due to in-
creased laser intensities, much progress in the understanding
of fundamental physical phenomena has been achieved[14].
Nevertheless, until today large-scale ion accelerators are
used worldwide for producing energies up to 100 MeV:
from basic research, through semiconductor doping and
isotope production, right up to medical applications, e.g.,
more efficient cancer treatment[5]. However, appropriate
accelerators such as cyclotrons, tandems, linear accelerators
as well as storage rings are quite large, very energy-intensive
and expensive in purchase and maintenance.
Laser-driven acceleration offers one highly promising al-
ternative thanks to advances in laser technology. Increasing
energies and repetition rates allow even higher ion energies
and intensities, possibly even laser-induced nuclear fusion.
Correspondence to: A. H¨
utzen, Peter Gr¨
unberg Institut (PGI-6),
Forschungszentrum J¨
ulich, Wilhelm-Johnen-Str. 1, 52425 J ¨
ulich, Germany.
In this context, one important feature of modern accelerators
is still missing, namely the production of highly polarized
particle beams. To achieve this, we are pursuing two ap-
proaches. First, polarization build-up by the laser itself and,
second, polarization preservation of polarized targets during
laser acceleration. Given that, one unsolved problem is the
influence of the huge magnetic fields present in the plasmas
acting on the ion spins. The present work aims at the first
production of a polarized proton beam – where the proton
spins are aligned relatively to an arbitrary quantization axis
– from laser-induced plasmas using polarized targets.
Two scenarios are discussed to build up a nuclear po-
larization in the plasma. Either polarization is generated
due to a spin flip according to the Sokolov–Ternov effect
by changing the spin direction of the accelerated particles,
induced by the magnetic fields of the incoming laser pulse.
Apart from that, the spatial separation of various spin states
due to magnetic-field gradients (Stern–Gerlach effect) may
result in the generation of polarization for different beam
2A. H¨
utzen et al.
Figure 1. Schematic setup for the first proton polarization measurement[2].
Besides these two mechanisms which may lead to a tempo-
ral or spatial polarization build-up, all particle spins precess
around the laser or plasma magnetic fields as character-
ized by the Thomas–Bargmann–Michel–Telegdi (T–BMT)
equation describing the spin motion in arbitrary electric and
magnetic fields in the relativistic regime.
The first and only experiment measuring the polariza-
tion of laser-accelerated protons has been performed at
the ARCturus laser facility at Heinrich-Heine University
usseldorf[2]. Figure 1schematically depicts the setup: for
the measurements a 100 TW Ti:sa laser system with a typical
pulse duration of 25 fs and a repetition rate of 10 Hz was
used for producing an intensity of several 1020 W·cm2
when being focused on a target. Impinging the laser pulse
in a 45angle on an unpolarized gold foil of 3 µm thickness,
protons with an energy of typically a few MeV are produced.
They are accelerated according to the target normal sheath
acceleration (TNSA) mechanism[1]toward a stack of three
radio-chromic-film detectors where the number of protons
is measured. In a silicon target with a thickness of 24 µm
elastic scattering takes place. Thus, the spin-dependent
asymmetries of the differential cross-section for the different
azimuthal angles can be measured by counting the number
of colliding particles per detector area with the help of CR-
39 detectors that are placed a few millimeters downstream.
The result was that no polarization was built up in the laser-
accelerated proton beam.
To estimate the magnitude of possible polarizing mag-
netic fields in this case, particle-in-cell (PIC) simulations
have been carried out with the fully relativistic 2D code
EPOCH[2,7]. A B-field strength of 104T and gradients of
1010 T·m1are expected. Although these values are rather
high, they are yet too small to align the proton spins and do
not yield measurable proton polarization.
One conclusion from this experiment is that for measuring
a proton polarization P6= 0, both a stronger laser pulse
with an intensity of about 1023 W·cm2and an extended
gas instead of a thin foil target are needed. Such a scenario
has been theoretically considered in a paper by Shen et al.[8]
Due to a larger target size, the interaction time between the
laser-accelerated protons and the B-field is increased. The
typical timescale for spin motion is given by the Larmor
frequency. For the numbers in Ref. [8] this is in the order
of 0.1 ps, i.e., sufficiently short compared to the interaction
time of approximately 3.3 ps of the accelerated protons with
the magnetic field and, thus, a spin manipulation is possible.
With respect to gas targets it has been demonstrated that
for nuclear and electron spin-polarized hydrogen at densities
of at least 1019 cm3the polarization lifetime is 10 ns,
which is sufficiently long to generate polarized hydrogen
atoms on the timescale of our experiment[9]. This density
is large enough for laser-driven ion acceleration of spin-
polarized protons.
2. Proton-spin dynamics
We have implemented particle-spin effects into the 3D PIC
simulation code VLPL (Virtual Laser Plasma Lab) in order to
make theoretical predictions about the degree of proton-spin
polarization from a laser-driven plasma accelerator[10,11].
These calculations consider all relevant effects that may lead
to the polarization of proton beams[12].
The Sokolov–Ternov effect is, for example, employed in
classical accelerators to polarize the stored electron beams,
where the typical polarization build-up times are minutes or
longer. This effect can, therefore, be neglected in the case
of laser-induced acceleration. We refer to our forthcoming
publication[12]for a more quantitative estimate.
Our assessment for the Stern–Gerlach force[12]shows
that non-relativistic proton beams with opposite spins are
separated by not more than p9.3×107λLwith the
laser wavelength λL. Moreover, the field strengths are of the
order of EB105T and the field gradients |B| ≈
105T/Rwith the laser radius R, typically λL/R=1/10
and a characteristic separation time would be t=100ω1
where ωLis the laser frequency. Thus, the force on the given
length scale is too weak and the Stern–Gerlach effect does
not have to be taken into account for further simulation work
on proton-spin tracking.
For charged particles the spin precession in arbitrary elec-
tric and magnetic fields is given by the T–BMT equation[13]
in CGS units,
dt= − e
c×E×s= − E
Here sis the proton spin in the rest frame of the proton, e
is the elementary charge, mpthe proton mass, cthe speed of
light, the dimensionless anomalous magnetic moment of the
proton ap=(gp2)/2=1.8 with the g-factor of the free
Polarized proton beams from laser-induced plasmas 3
proton gp,γthe Lorentz factor, vthe particle velocity, Bthe
magnetic field, and Ethe electric field, both in the laboratory
frame. Since E
always has a component perpendicular to s,
the single spins in a polarized particle ensemble precess with
the frequency ωs= | E
|. For protons with an energy in the
range of a few GeV, γ1 and 1 &v/c, so that
Under the assumption |B|≈|E| ≈ Fthis simplifies to
As a consequence, a conservation of the polarization of the
system is expected for time
for ap=1.8. For typical field strengths in our performed
simulations (cf. Figure 2) of F=5.11 ×1012 V/m=17.0×
103T the preservation of the spin directions is estimated for
time t<1 ps. This time is sufficiently long taking into
account that the simulation time is tsim =0.13 ps 1 ps,
so the polarization is maintained during the entire simulation
according to the T–BMT equation.
3. Particle-in-cell simulations
In order to reproduce the results of the seminal experiment
presented in Section 1[2]and to verify the quantitative esti-
mates of Ref. [2], 3D simulations with the above-mentioned
VLPL code including spin tracking have been carried out on
the supercomputer JURECA[14]. These were performed for
a focused 3D laser pulse of Gaussian shape with wavelength
λL=800 nm, a normalized laser amplitude a0=12
calculated for the ARCturus laser system, a duration of 25 fs
and a focal spot size of 5 µm.
It is important to consider that to simulate the plasma
behavior, a PIC code first defines a three-dimensional Carte-
sian grid which fills the simulated volume where the plasma
evolves over the simulated time. Moreover, not each physical
particle is treated individually but they are substituted by
so-called PIC particles. This is why the continuous spin
vector of a PIC particle represents the mean spin of all
substituted particles. Thus, not the spin of each single
particle is simulated but the polarization Pof every PIC
particle. Therefore, the sum of spin vectors of different
PIC particles within a certain volume (polarization cell)
corresponds to the local polarization of the ensemble[12,15].
Figure 2shows preliminary simulation results for proton-
spin tracking with the PIC code VLPL. Two different
simulation scenarios were investigated regarding the devel-
opment of proton spins in the interaction with a laser pulse.
For this purpose, the simulations were carried out with many
particles per cell and a fully polarized hydrogen layer.
The upper two images depict the magnetic field Bzand
the polarization Pzdistribution for a pure hydrogen target
(thickness 1 µm, density 128ncr ). For the simulation a grid
cell size of hx=hy=hz=0.02 µm was chosen. Within the
target geometry the polarization is preserved after interaction
with the laser pulse, impinging from the left side of the
simulation box. The resulting field strengths are in the range
of 7.5×104T, so one can assume that the polarization is
preserved for up to 0.24 ps.
In the lower two pictures a more complicated scenario
is chosen, which is very close to the setup described in
Section 1. The laser impinges on an aluminum foil target
(2.5µm, 35ncr) covered with a fully polarized proton layer
(0.5µm, 117ncr). A grid cell size of hx=0.025 µm and
hy=hz=0.05 µm was used. An acceleration of the protons
due to the TNSA mechanism is in evident. The fields that
interact in the target here are more static and we estimate a
proton polarization preservation for at least 0.18 ps.
Thus, VLPL simulations on proton polarization demon-
strate the conservation of polarization according to the
T–BMT equation when accelerated by the TNSA
mechanism[13,15]. Our analysis of the spin-rotation angle
in the simulations shows a precession of most PIC particles
by less than 15, which confirms the conservation of
polarization. Considering that, a compact target is needed
in which the nuclear spins are already aligned at the
time of irradiation with the accelerating laser. For an
in-depth analysis of particle acceleration with polarized
targets, we refer to Ref. [12] which will be published
shortly. However, solid foil targets suitable for laser
acceleration with TNSA mechanism are not available so far
and an experimental realization is extremely challenging.
In previous experiments hydrogen nuclear polarization
mostly results from a static polarization, e.g., in frozen
spin targets[16]or with polarized 3He gas[17,18]. For the
acceleration of protons until now only polarized atomic
beam sources based on the Stern–Gerlach principle[19]are
available, which however have the disadvantage of a too
small particle density. In order to provide a dynamically
polarized hydrogen gas target for laser–plasma applications,
a new approach is needed.
4. Experimental realization
For the experimental realization of our new concept for
a dynamically polarized ion source, three components are
required: a suitable laser system, a vacuum interaction cham-
ber including a gas jet and a polarimeter. The schematic view
of the setup is depicted in Figure 3.
As a component of the gas target, hydrogen halides are
a viable option[20,21]. A hydrogen chloride (HCl) target is
4A. H¨
utzen et al.
(a) (b)
(c) (d)
Figure 2. 3D VLPL simulations showing the conservation of proton polarization in two polarized target geometries after interaction with a laser pulse
(λL=800 nm, normalized laser amplitude a0=12, 25 fs duration, 5 µm focal spot size) impinging from the left side of the simulation box.
Figure 3. Schematic view of the setup for the proton polarization
measurement using a polarized hydrogen gas target.
preferred in this case due to the rather high polarizability
and the easy availability. The HCl gas is injected into the
interaction chamber by a standard gas nozzle with a high-
speed short-pulse piezo valve that can be operated at 5 bar
inlet-gas pressure to produce a gas density in the range
of 1019 cm3. Few millimeters below the nozzle, the
interaction between gas and laser beams takes place.
The polarizing laser system is a pulsed Nd:YAG laser
from EKSPLA[22]. Its peculiarity is the quasi-simultaneous
output of the fundamental wavelength at 1064 nm and the
fifth (213 nm) harmonic. The repetition rate of the laser
system is 5 Hz and the pulses are of 170 ps duration which
is sufficiently short with regard to the transfer time of the
electron spin polarization to the nucleus due to hyperfine in-
teraction (1 ns)[20]. The linearly polarized 1064 nm beam
with a pulse energy of 100 mJ is focused with an intensity
of 5×1013 W·cm2into the interaction chamber to align
the HCl bonds (cf. Figure 4). By this, the signal intensity
is increased and the amplification factor xis calculated to be
x2 assuming an interaction parameter of =10 and,
thus, hhcos2θii = 0.7 since the polarizability interaction is
governed by a cos2θpotential with the angle θbetween the
molecular axis and the electric field distribution[23].
At the same time but under a 90angle, the circularly
polarized fifth harmonic with an energy of 20 mJ is also
focused at an intensity of 1012 W·cm2into the vacuum
chamber to interact with the HCl gas. The aligned HCl
molecules are photo-dissociated by UV excitation via the
A151state, which has a total electronic angular-momentum
projection of = +1 along the bond axis. Hence,
the resulting H and Cl(2P3/2)photofragments conserve
this +1 projection of the laser photons, producing H and
Polarized proton beams from laser-induced plasmas 5
Figure 4. Schematic overview of the production of polarized proton beams.
Cl(2P3/2)atoms each with the projections of approximately
ms= +1/2 (so that they sum to +1), and thus the H-
atom electron spin is approximately ms= +1/2[24]. In
a weak magnetic field (Zeeman region), all H atoms are
in a coherent superposition of the total angular-momentum
states |F,mFiwith the coupling F=S+Iof the electron
spin Sand the nuclear spin I. When the electron spin is
fixed due to the polarization of the incident laser beam,
e.g., ms= +1/2, then only the spin combinations |ms=
+1/2,mI= +1/2iand |+1/2,1/2ican be found in the
free hydrogen atoms. The hyperfine state |+1/2,+1/2i =
|F=1,mF= +1iis an eigenstate and will stay unchanged
in time. Since the states |−1/2,+1/2iand |+1/2,1/2iare
not eigenstates, they will be expressed as linear combinations
of the eigenstates |F=1,mF=0iand |F=0,mF=0i,
which have different energies. Therefore, atoms produced in
the |+1/2,1/2istate will oscillate to the |−1/2,+1/2i
state and back. If now the electron-polarized hydrogen
atoms are produced during a very short time t<1 ns,
they will oscillate in phase. Therefore, after 0.35 ns only
the spin combinations |+1/2,+1/2iand |−1/2,+1/2iare
found. This means that the electron polarization of the
hydrogen atoms, produced by the laser beam, is transferred
into a nuclear polarization. If now the hydrogen atoms are
ionized and accelerated, the out-coming protons will remain
polarized, even if they undergo spin precessing according to
the T–BMT equation[20].
Using a Lamb-shift polarimeter the polarization of an
atomic hydrogen ensemble can be measured in a multi-
step process[25,26]. One important condition is that the
atomic beam can be efficiently converted into metastable
atoms in the 2S1/2state by ionization with an electron-
impact ionizer and a charge reversal in cesium vapor. With
a spin filter, individual hyperfine sub-states are selected by
applying a static magnetic field, an electric quench field
and a high-frequency transition. By varying the resonance
condition when changing the magnetic field, single hyperfine
components can be detected. Finally, the transition into the
ground state within the quenching process is verified by
Lyman-αradiation emitted at 121.5 nm. The intensity of
the individual hyperfine components allows to measure their
occupation number and, therefore, calculate the polarization
of incoming protons and in combination with an ionizer even
for hydrogen atoms. The entire setup, including laser system,
interaction chamber and Lamb-shift polarimeter, is realized
over a length of less than 5 m as a tabletop experiment.
To summarize, our novel gas target will offer nuclear
polarized hydrogen atoms at a density of 1019 cm3or
above with a one-to-one mixture of (unpolarized) chlorine
atoms. The suitability of such type of target, i.e., containing
hydrogen and an admixture of heavier nuclei, for proton
acceleration, has already been demonstrated with the help
of PIC simulations (although without considering spin ef-
fects) in Ref. [27]. It was found that laser intensities of
>1022 W·cm2promise to reach proton energies above
1 GeV. Such a laser system will be available in the near
future at the Shanghai Institute of Optics and Fine Mechanics
(SIOM). The Shanghai Superintense-Ultrafast Lasers Fa-
cility (SULF) will offer pulse energies of 300 J at 30 fs
pulse duration and a repetition rate of 1 shot/min. An-
other important conclusion from Ref. [27] is that the heavy
ions are not accelerated from the gas target. However,
they are vital to provide the proton acceleration in a so-
called electron bubble-channel structure. In this acceleration
scheme protons, which are trapped in the bubble region of
the wake field, can be efficiently accelerated in the front of
the bubble, while electrons are mostly accelerated at its rear.
After the acceleration process the proton polarization will
be determined by a detector similar to that one described in
Section 1.
5. Discussion and conclusion
In conclusion, the T–BMT equation, describing the spin
precession in electromagnetic fields, has been implemented
into the VLPL PIC code to simulate the spin behavior during
laser–plasma interactions. One crucial result of our simula-
tions is that a target containing polarized hydrogen nuclei is
needed for producing polarized relativistic proton beams. A
corresponding gas-jet target, based on dynamic polarization
of HCl molecules, is now being built at Forschungszentrum
ulich. By interacting the fundamental wavelength of a
Nd:YAG laser and its fifth harmonic with HCl gas, nuclear
polarized H atoms are created. Their nuclear polarization
will be measured and tuned with a Lamb-shift polarimeter.
First measurements, aiming at the demonstration of the
feasibility of the target concept, are scheduled for fall 2018.
The ultimate experiment will take place at the 10 PW SULF
facility to observe an up to GeV polarized proton beam from
laser-generated plasma for the first time.
We thank our colleagues B. F. Shen, L. Ji, J. Xu and L. Zhang
from Shanghai Institute of Optics and Fine Mechanics for the
various fruitful discussions and their expertise that greatly
6A. H¨
utzen et al.
assisted our research. This work has been carried out in
the framework of Space: the JuSPARC (J ¨
ulich Short-Pulse
Particle and Radiation Center) project and has been sup-
ported by the ATHENA (Accelerator Technology HElmholtz
iNfrAstructure) consortium. We further acknowledge the
computing resources on grant VSR-JPGI61 on the super-
computer JURECA.
1. A. Macchi, M. Borghesi, and M. Passoni, Rev. Mod. Phys. 85,
751 (2013).
2. N. Raab, M. B ¨
uscher, M. Cerchez, R. Engels, l. Engin, P.
Gibbon, P. Greven, A. Holler, A. Karmakar, A. Lehrach, R.
Maier, M. Swantusch, M. Toncian, T. Toncian, and O. Willi,
Phys. Plasma 21, 023104 (2014).
3. M. Wen, H. Bauke, and C. H. Keitel, Dynamical spin effects
in ultra-relativistic laser pulses (2014), arXiv:1406.3659.
4. J. Vieira, C.-K. Huang, W. B. Mori, and L. O. Silva, Phys. Rev.
S Accel. Beams 14, 071303 (2011).
5. Universit¨
atsklinikum Heidelberg, Heidelberger Ionenstrahl-
Therapiezentrum (HIT) https://www.klinikum.uni-heidelberg
.de/Willkommen.113005.0.html (August 1, 2018).
6. B. M. Garraway and S. Stenholm, Contemp. Phys. 43, 3
7. T. D. Arber, K. Bennett, C. S. Brady, A. Lawrence-Douglas,
M. G. Ramsay, N. J. Sircombe, P. Gillies, R. G. Evans, H.
Schmitz, A. R. Bell, and C. P. Ridgers, Plasma Phys. Control.
Fusion 57, 113001 (2015).
8. B. F. Shen, X. Zhang, Z. Sheng, M. Y. Yu, and J. Cary, Phys.
Rev. ST Accel. Beams 12, 121301 (2009).
9. D. Sofikitis, C. S. Kannis, G. K. Boulogiannis, and T. P.
Rakitzis, Phys. Rev. Lett. 121, 083001 (2018).
10. A. Pukhov, Proceedings, CAS – CERN Accelerator School:
Plasma Wake Acceleration (2016), p. 181.
11. J. Vieira, R. A. Fonseca, and L. O. Silva, Proceedings, CAS –
CERN Accelerator School: Plasma Wake Acceleration (2016),
p. 79.
12. J. Thomas, A. H¨
utzen, A. Pukhov, A. Lehrach, and M.
uscher, Tracking of Particle Spins with PIC Codes,
publication in preparation.
13. V. Bargmann, L. Michel, and V. Telegdi, Phys. Rev. Lett. 2,
435 (1959).
14. J¨
ulich Supercomputing Centre, J. Large-scale Research
Facilities A62, 2 (2016).
15. P. S. Farago, Rep. Prog. Phys. 34, 1055 (1971).
16. C. D. Keith, J. Brock, C. Carlin, S. A. Comer, D. Kashy, J.
McAndrew, D. G. Meekins, E. Pasyuk, J. J. Pierce, and M. L.
Seely, The Jefferson Lab frozen spin target (2012), arXiv:120
17. I. Engin, M. B ¨
uscher, O. Deppert, L. Di Lucchio, R. Engels, S.
Frydrych, P. Gibbon, A. Kleinschmidt, A. Lehrach, M. Roth,
F. Schl¨
uter, K. Strathmann, and F. Wagner, in Proceedings of
Science (PSTP2015), paper 002.
18. H. Soltner, M. B¨
uscher, P. Burgmer, I. Engin, B. Nausch¨
S. Maier, and H. Gl¨
uckler, IEEE Trans. Appl. Supercon. 26, 4
19. A. Nass, C. Baumgarten, B. Braun, G. Ciullo, G. Court,
P. F. Dalpiaz, A. Golendukhin, G. Graw, W. Haerberli, M.
Hennoch, R. Hertenberger, N. Koch, H. Kolster, P. Lenisa, H.
Marukyan, M. Raithel, D. Reggiani, K. Rith, M. C. Simani,
E. Steffens, J. Stewart, P. Tait, and T. Wise, Nucl. Instrum.
Methods Phys. Res. A 505, 633 (2003).
20. D. Sofikitis, P. Glodic, G. Koumarianou, H. Jiang, L. Bougas,
P. C. Samartzis, A. Andreev, and T. P. Rakitzis, Phys. Rev.
Lett. 118, 233401 (2017).
21. D. Sofikitis, L. Rubio-Lago, L. Bougas, A. J. Alexander, and
T. P. Rakitzis, J. Chem. Phys. 129, 144302 (2008).
22. EKSPLA, SL330 series – SBS Compressed Picosecond
Nd:YAG Lasers
nergy-ndyag-lasers-sl330-series/ (July 26, 2018).
23. B. Friedrich and D. Herschbach, J. Phys. Chem. A 103, 10280
24. T. P. Rakitzis, ChemPhysChem 5, 1489 (2004).
25. R. Engels, R. Emmerich, J. Ley, G. Tenckhoff, H. Paetz gen.
Schieck, M. Mikirtytchiants, F. Rathmann, H. Seyfarth, and A.
Vassiliev, Rev. Sci. Instrum. 74, 4607 (2003).
26. R. Engels, E. Emmerichi, K. Grigoryev, J. Ley, M.
Mikirtychyants, H. Paetz gen. Schieck, F. Rathmann, J.
Sarkad, H. Seyfarth, G. Temckhoff, and V. Vasilyev, Rev. Sci.
Instrum. 76, 053305 (2005).
27. B. F. Shen, Y. Li, M. Y. Yu, and J. Cary, Phys. Rev. E 76,
055402 (2007).
... Before a technical implementation can be envisaged, some principal issues need to be addressed theoretically, for example: (i) is it possible to alter the polarization of an initially unpolarized target through interaction with relativistic laser pulses or (ii) are the spins so inert during the short acceleration period that a pre-polarized target is required (see Ref. [41] for a recent review)? Starting from the work by Hützen et al. [42] (which proposes the use of pre-polarized targets for proton acceleration), Wu et al. [43] and Wen et al. [44] have developed a scheme to generate intense polarized electron beams via the interaction of an accelerating laser pulse with a pre-polarized plasma, which is produced through photo-dissociation of a dense Halide (e.g. ...
... A polarized 3 He gas-jet target [49] that has been used for a first experimental campaign at the Phelix laser facility at GSI Darmstadt. A polarized HCl target for proton acceleration has been prepared at Forschungszentrum Jülich [42]. It is planned to upgrade this target to deliver also polarized electrons. ...
Full-text available
High-brightness beams generated by particle sources based on advanced accelerator concepts have the potential to become an essential part of future accelerator technology. High-gradient accelerators can generate and rapidly accelerate particle beams to relativistic energies while minimizing irreversible detrimental effects to the beam brightness that occur at low beam energies. Due to the high accelerating gradients, these novel accelerators are also significantly more compact than conventional technology. The beam parameters of these particle sources are largely determined by the injection and subsequent acceleration processes. While there has been significant progress crucial parameters that are required for a future collider or more near-term applications, including X-ray free-electron lasers (XFELs), such as a sufficiently small energy spread and small emittance for bunches with a high charge and at high pulse repetition rate. Major research and development efforts are required to realize these approaches for a front-end injector for a future collider in order to address these limitations. In particular, this includes methods to control and manipulate the phase-space and spin degrees-of-freedom of ultrashort LWFA electron bunches with high accuracy, methods that increase the laser-to-electron beam efficiency and increased repetition rate. This also includes the development of high-resolution diagnostics, such as full 6D phase-space measurements, beam polarimetry and high-fidelity simulation tools. A further increase in beam luminosity can be achieve through emittance damping. For future colliders, the damping rings might be replaced by a substantially more compact plasma-based approach. Here, plasma wigglers are used to achieve similar damping performance but over a two orders of magnitude reduced length.
... Remarkably, with the advent of 100 nm lasers, 41 the molecular photodissociation technique might be applied to pure hydrogen, potentially enabling 100% plasma pre-polarization. The method of plasma pre-polarization via laser-induced molecular photodissociation was initially proposed by Hützen et al. and applied to polarized proton beam generation in laser-plasma interaction in Ref. 42. Following these seminal works, other schemes utilizing a pre-polarized plasma were put forward to generate energetic spin-polarized electron (or proton) beams [43][44][45][46][47][48][49][50][51][52][53][54][55] or to investigate polarization effects in inertial confinement fusion. ...
Full-text available
Employing colliding-pulse injection has been shown to enable the generation of high-quality electron beams from laser–plasma accelerators. Here, by using test particle simulations, Hamiltonian analysis, and multidimensional particle-in-cell simulations, we lay the theoretical framework for spin-polarized electron beam generation in the colliding-pulse injection scheme. Furthermore, we show that this scheme enables the production of quasi-monoenergetic electron beams in excess of 80% polarization and tens of pC charge with commercial 10-TW-class laser systems.
... The plasma source could be prepolarized using hydrogen halide molecular dissociation [33][34][35][36][37][38]. A subnanosecond alignment laser perpendicular to the LPA driver (purple) aligns the molecular bonds. ...
Full-text available
Highly polarized, multi-kiloampere-current electron bunches from compact laser-plasma accelerators are desired for numerous applications. Current proposals to produce these beams suffer from intrinsic limitations to the reproducibility, charge, beam shape, and final polarization degree. In this paper, we propose colliding pulse injection (CPI) as a technique for the generation of highly polarized electron bunches from prepolarized plasma sources. Using particle-in-cell simulations, we show that colliding pulse injection enables trapping and precise control over electron spin evolution, resulting in the generation of high-current (multi-kA) electron bunches with high degrees of polarization (up to 95% for >2kA). Bayesian optimization is employed to optimize the multidimensional parameter space associated with CPI to obtain a percent-level energy spread, submicron normalized emittance electron bunches with 90% polarization using 100-TW class laser systems.
... Despite many advances mostly on the theoretical side, several principal issues need to be addressed, for example: (i) is it possible to alter the polarization of an initially unpolarized target through interaction with relativistic laser pulses [31][32][33][34][35]? or (ii) are the spins so inert during the short acceleration period that a pre-polarized target is required [13,14,[36][37][38]? Following the work by Hützen et al [39], Wen et al [13] have proposed to generate high-current polarized electron beams in the interaction of an ultra-intense laser pulse with a pre-polarized gas plasma, which is produced through photo-dissociation by a circularly polarized ultra-violet (UV) laser pulse [40]. The work of Vieira et al showed that spin is depolarized mainly in the injection phase [41]. ...
Full-text available
Electron beam polarization in the bubble regime of the interaction between a high-intensity laser and a longitudinally pre-polarized plasma is investigated by means of the Thomas-Bargmann-Michel-Telegdi equation. Using a test-particle model, the dependence of the accelerated electron polarization on the bubble geometry is analyzed in detail. Tracking the polarization dynamics of individual electrons reveals that although the spin direction changes during both the self-injection process and acceleration phase, the former has the biggest impact. For nearly spherical bubbles, the polarization of electron beam persists after capture and acceleration in the bubble. By contrast, for aspherical bubble shapes, the electron beam becomes rapidly depolarized, and the net polarization direction can even reverse in the case of a oblate spheroidal bubble. These findings are confirmed via particle-in-cell simulations.
We propose a scheme to mapping electromagnetic field structure in plasma by using a spin polarized relativistic electron beam. Especially by using Particle-in-Cell (PIC) and spin tracing simulations, we have successfully reconstructed a plasma wakefield from the spin evolution of a transmitted electron beam. Electron trajectories of the probe beam are obtained from PIC simulations, and the spin evolutions during the beam propagating through the fields are calculated by a spin tracing code. The reconstructed fields illustrate the main characters of the original fields, which demonstrates the feasibility of field detection by use of spin polarized relativistic electron beams.
Herein, we propose a scheme based on collision-less shock acceleration (CSA) involving the use of composite targets comprising a micro-structured foil and pre-polarized gas for obtaining high-energy polarized proton beams. Femtosecond laser pulses irradiate a microwire-array (MWA) target and efficiently heat the dense plasma, which moves toward the dilute plasma. Shocks are then introduced in the pre-polarized gas and accelerate upstream spin-polarized protons to relativistic velocities. Based on particle-in-cell simulations with added spin dynamics, protons with energies of 30–300 MeV are produced, and the polarization rate of protons in the high-energy region exceeds 90%. The simulations demonstrate an evident increase in the temperature and number of hot electrons owing to the presence of MWA structures, which increase both the longitudinal electric field strength associated with the shock and the energy of the reflected protons. During CSA, the bipolar magnetic field driven by hot-electron currents demonstrates a weak effect on the polarization level of the accelerated protons, resulting in a high polarization rate. The relationship between the energy of the polarized proton beam and the hot-electron temperature enables an optimization of the micro-structured target or other target components to enhance proton quality via the CSA process.
Full-text available
High-energy spin-polarized electron, positron, and \(\gamma\)-photon beams have many significant applications in the study of material properties, nuclear structure, particle physics, and high-energy astrophysics. Thus, efficient production of such polarized beams attracts a broad spectrum of research interests. This is driven mainly by the rapid advancements in ultrashort and ultraintense laser technology. Currently, available laser pulses can achieve peak intensities in the range of \(10^{22}\)–\(10^{23}\) \(\hbox {Wcm}^{-2}\), with pulse durations of tens of femtoseconds. The dynamics of particles in laser fields of the available intensities is dominated by quantum electrodynamics (QED) and the interaction mechanisms have reached regimes spanned by nonlinear multiphoton absorption (strong-field QED processes). In strong-field QED processes, the scattering cross-sections obviously depend on the spin and polarization of the particles, and the spin-dependent photon emission and the radiation-reaction effects can be utilized to produce the polarized particles. An ultraintense laser-driven polarized particle source possesses the advantages of high brilliance and compactness, which could open the way for the unexplored aspects in a range of researches. In this work, we briefly review the seminal conclusions from the study of the polarization effects in strong-field QED processes, as well as the progress made by recent proposals for production of the polarized particles by laser–beam or laser–plasma interactions.
A novel scheme for obtaining high-energy polarized proton beams by the interaction of a femtosecond laser pulse with a foil-gas composite target has been proposed. The carbon plasmas heated by the laser pulse expand toward the prepolarized HCl gas and excite shock waves in the gas target, reflecting and accelerating spin-polarized protons. According to the results from particle-in-cell simulations with the addition of spin dynamics, protons of several MeV are produced with the polarization rate remaining above 90% in the high energy region. The simulation results show that a large number of the reflected protons are subjected to a weak azimuthal magnetic field and with less depolarization. The intensity of laser pulses and the thickness of foils also affect the strength of the azimuthal magnetic field, which affects the depolarization of the proton beams.
We propose obtaining polarized proton beams based on CO 2 -laser-driven collisionless shock acceleration (CSA) of the pre-polarized HCl gas. By tailoring the density profile of the pre-polarized HCl gas, the intense CO 2 laser pulse heats the plasma target and forms a strong shock that reflects the polarized protons to high energy. According to particle-in-cell simulations implemented with the spin dynamics, directional proton beams of several MeV were generated at a total beam polarization of over 80%. Simulations showed that proton spin precession occurred in the azimuthal magnetic fields generated by the Biermann effect and plasma currents. The latter was the main depolarization mechanism in the early stage of shock wave formation. For CSA at CO 2 laser intensities around 10 ¹⁷ –10 ¹⁸ W/cm ² , the proton depolarization was insignificant and the beam polarization purity was preserved. As pre-polarized hydrogen targets were available at gaseous densities in-state-of-art facilities, CSA driven by relatively long wavelength lasers provided a feasible solution for obtaining ultra-fast polarized proton sources.
Full-text available
Plasma accelerators can sustain very high acceleration gradients. They are promising candidates for future generations of particle accelerators for sev- eral scientific, medical and technological applications. Current plasma based acceleration experiments operate in the relativistic regime, where the plasma response is strongly non-linear. We outline some of the key properties of wake- field excitation in these regimes. We outline a multidimensional theory for the excitation of plasma wakefields in connection with current experiments. We then use these results and provide design guidelines for the choice of laser and plasma parameters ensuring a stable laser wakefield accelerator that maximizes the quality of the accelerated electrons. We also mention some of the future challenges associated with this technology.
Full-text available
JURECA is a petaflop-scale, general-purpose supercomputer operated by Jülich Supercomputing Centre at Forschungszentrum Jülich. Utilizing a flexible cluster architecture based on T-Platforms V-Class blades and a balanced selection of best of its kind components the system supports a wide variety of high-performance computing and data analytics workloads and offers a low entrance barrier for new users.
Full-text available
Basic principles of particle-in-cell (PIC ) codes with the main application for plasma-based acceleration are discussed. The ab initio full electromagnetic relativistic PIC codes provide the most reliable description of plasmas. Their properties are considered in detail. Representing the most fundamental model, the full PIC codes are computationally expensive. The plasma-based acceler- ation is a multi-scale problem with very disparate scales. The smallest scale is the laser or plasma wavelength (from one to hundred microns) and the largest scale is the acceleration distance (from a few centimeters to meters or even kilometers). The Lorentz-boost technique allows to reduce the scale disparity at the costs of complicating the simulations and causing unphysical numerical instabilities in the code. Another possibility is to use the quasi-static approxi- mation where the disparate scales are separated analytically.
Full-text available
Particle-in-cell (PIC) methods have a long history in the study of laser-plasma interactions. Early electromagnetic codes used the Yee staggered grid for field variables combined with a leapfrog EM-field update and the Boris algorithm for particle pushing. The general properties of such schemes are well documented. Modern PIC codes tend to add to these high-order shape functions for particles, Poisson preserving field updates, collisions, ionisation, a hybrid scheme for solid density and high-field QED effects. In addition to these physics packages, the increase in computing power now allows simulations with real mass ratios, full 3D dynamics and multi-speckle interaction. This paper presents a review of the core algorithms used in current laser-plasma specific PIC codes. Also reported are estimates of self-heating rates, convergence of collisional routines and test of ionisation models which are not readily available elsewhere. Having reviewed the status of PIC algorithms we present a summary of recent applications of such codes in laser-plasma physics, concentrating on SRS, short-pulse laser-solid interactions, fast-electron transport, and QED effects.
Full-text available
The dynamics of single laser-driven electrons and many particle systems with spin are investigated on the basis of a classical theory. We demonstrate that the spin forces can alter the electron dynamics in an ultra-relativistic laser field due to the coupling of the electron's spin degree of freedom to its kinematic momentum. High-energy electrons can acquire significant spin-dependent transverse momenta while passing through a counterpropagating ultra-relativistic infrared laser pulse. Numerical calculations show that the deflection of the electrons by the laser pulse is determined by the laser intensity, the pulse duration, and the initial spin orientation of the electron. We complement our investigation of these dynamical spin effects by performing particle-in-cell simulations and point out possibilities of an experimental realization of the predicted effect with available laser parameters.
Full-text available
We report on the successful use of a laser-driven few-MeV proton source to measure the differential cross section of a hadronic scattering reaction as well as on the measurement and simulation study of polarization observables of the laser-accelerated charged particle beams. These investigations were carried out with thin foil targets, illuminated by 100 TW laser pulses at the Arcturus laser facility; the polarization measurement is based on the spin dependence of hadronic proton scattering off nuclei in a Silicon target. We find proton beam polarizations consistent with zero magnitude which indicates that for these particular laser-target parameters the particle spins are not aligned by the strong magnetic fields inside the laser-generated plasmas.
We measure nuclear and electron spin-polarized H and D densities of at least 1019 cm−3 with ∼10 ns lifetimes, from the photodissociation of HBr and DI with circularly polarized UV light pulses. This density is ∼6 orders of magnitude higher than that produced by conventional continuous-production methods and, surprisingly, at least 100 times higher than expected densities for this photodissociation method. We observe the hyperfine quantum beating of the H and D magnetization with a pickup coil, i.e., the respective 0.7 and 3 ns periodic transfer of polarization from the electrons to the nuclei and back. The 1019 cm−3 spin-polarized H and D density is sufficient for laser-driven ion acceleration of spin-polarized electrons, protons, or deuterons, the preparation of nuclear-spin-polarized molecules, and the demonstration of spin-polarized D-T or D−He3 laser fusion, for which a reactivity enhancement of ∼50% is expected.
Interaction of the strong electric field of an intense laser beam with the anisotropic polarizability of a linear molecule creates pendular states, superpositions of the field-free rotational states, in which the molecular axis librates about the field direction. Angular motion in the low-lying pendular states is thereby restricted by a double-well potential, governed by the laser intensity. The pendular energy levels occur as pairs of opposite parity, with separations corresponding to the frequency for tunneling between the wells. If the molecule is polar or paramagnetic, introducing a static electric or magnetic field connects the nearly degenerate pendular levels and thus induces strong pseudo-first-order Stark or Zeeman effects. This can be exploited in many schemes to control and manipulate molecular trajectories.
In the late 1920s Niels Bohr propagated the idea that the magnetic moment of a free electron could not be observed. This derived from the idea that the spin degree of freedom characterized the electron only when it is bound in an atom. This view initiated a lively discussion, which involved many of the most prominent theoreticians of the time. The independent existence of the electron spin became an issue of principle. In particular it was deemed that quantum effects would destroy the separated classical trajectories in a Stern-- Gerlach type of experiment. We review these discussions and some later developments. Quantum effects do prove to be essential, but they do not overwhelm the magnetic effects of spin. In addition to these arguments, it has been possible experimentally to determine the electron g-factor with high accuracy in electromagnetic traps. In fact no principle seems to prevent the observation of the magnetic moment of the free electron.