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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.
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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 (http://creativecommons.org/
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doi:10.1017/hpl.2018.73
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 ¨
uscher1,2
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)
Abstract
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.
Email: a.huetzen@fz-juelich.de
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
trajectories[6].
1
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
D¨
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
L,
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,
ds
dt= − e
mpcap+1
γBapγ
γ+1v
c
·Bv
c
ap+1
1+γv
c×E×s= − E
×s.(1)
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
ωs<e
mpcs(ap+1)2B2+ap
22
B2+ap+1
22
E2.
(2)
Under the assumption |B|≈|E| ≈ Fthis simplifies to
ωs<e
mpcFq9
4a2
p+3ap+5
4.(3)
As a consequence, a conservation of the polarization of the
system is expected for time
t2π
ωs
2π
3.7e
mpcF(4)
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
J¨
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.
Acknowledgements
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.
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... While the use of SPH targets seems rather straightforward for proton acceleration [35,36], the acceleration of polarized electrons would first require the removal of unpolarized electrons from the accelerating plasma which are unavoidably present in the (otherwise unused) halide atoms. Two solutions have been put forward here, either removal via ionization [37] or spatial separation of the SPH from the unwanted photoproducts leaving behind only hydrogen atoms of high polarization (P = 90%) [34]. ...
... The acceleration of polarized ions from hypothetical, overdense targets was considered by Hützen et al. in [36]. Two sets of PIC simulations were carried out: at first a fully polarized Hydrogen layer of 1 µm thickness and 128n crit density was considered (see Fig. 9). ...
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Spin-polarized particle beams are of interest for applications like deep-inelastic scattering, e.g. to gain further understanding of the proton's nuclear structure. With the advent of high-intensity laser facilities, laser-plasma-based accelerators offer a promising alternative to common radiofrequency-based accelerators, as they can shorten the required acceleration length significantly. However, in the scope of spin-polarized particles, they bring unique challenges. This paper reviews the developments in the field of spin-polarized particles on the basis of the interaction of laser pulses and high-energy particle beams with plasma. The relevant scaling laws for spin-dependent effects in laser-plasma interaction, as well as acceleration schemes for polarized leptons, ions and gamma quanta are discussed.
... This would have the advantage that the 3 He ions should be dominantly emitted under 0 • and at significantly higher energies (10)(11)(12)(13)(14)(15) [27]. In parallel we developed a polarized HCl gas target [28] for laser-or beamdriven acceleration of polarized proton and electron beams [29][30][31][32][33][34][35]. This target could also be operated with DI gas [36] and be combined with polarized 3 He in order to further study options for polarized fusion with high-power lasers. ...
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The preservation of nuclear spin alignment in plasmas is a prerequisite for important applications, such as energy production through polarized fusion or the acceleration of polarized particle beams. Although this conservation property has been the basis of numerous theoretical papers, it has never been experimentally confirmed. Here, we report on first experimental data from a polarized 3He target heated by a PW laser pulse, showing evidence for persistence of the nuclear polarization after acceleration to MeV energies. The finding also validates the concept of using pre-polarized targets for experiments at high-power laser facilities.
... This would have the advantage that the 3 He ions should be dominantly emitted under 0 • and at significantly higher energies (10)(11)(12)(13)(14)(15) [27]. In parallel we developed a polarized HCl gas target [28] for laser-or beamdriven acceleration of polarized proton and electron beams [29][30][31][32][33][34][35]. This target could also be operated with DI gas [36] and be combined with polarized 3 He in order to further study options for polarized fusion with high-power lasers. ...
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Full-text available
The preservation of nuclear spin alignment in plasmas is a prerequisite for important applications, such as energy production through polarized fusion or the acceleration of polarized particle beams. Although this conservation property has been the basis of numerous theoretical papers, it has never been experimentally confirmed. Here, we report on first experimental data from a polarized 3^3\mkern-2muHe target heated by a PW laser pulse, showing evidence for persistence of the nuclear polarization after acceleration to MeV energies. The finding also validates the concept of using pre-polarized targets for experiments at high-power laser facilities.
... 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. ...
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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. ...
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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.
... Notably, gaseous targets have been used in the polarized proton acceleration schemes, and the acceleration processes primarily depend on the density gradients in these gas targets [22]. However, in actual experiments, the density modulation of gas targets, particularly the spin-prepolarized ones, has elaborate specifications for the pressure of the gas nozzle [23] and the delay of laser pulses [15]. Nevertheless, using a composite target comprising a multilayer slab mixed with uniform pre-polarized HCl gas may overcome this difficulty [24]. ...
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Herein, we propose a scheme based on collision-less shock acceleration (CSA) involving the use of composite targets comprising a micro-structured foil and a 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 to 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 and other target components to enhance proton quality via the CSA process.
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High-brightness particle beams generated by advanced accelerator concepts have the potential to become an essential part of future accelerator technology. In particular, high-gradient accelerators can generate and rapidly accelerate particle beams to relativistic energies. The rapid acceleration and strong confining fields can minimize irreversible detrimental effects to the beam brightness that occur at low beam energies, such as emittance growth or pulse elongation caused by space charge forces. Due to the high accelerating gradients, these novel accelerators are also significantly more compact than conventional technology. Advanced accelerators can be extremely variable and are capable of generating particle beams with vastly different properties using the same driver and setup with only modest changes to the interaction parameters. So far, efforts have mainly been focused on the generation of electron beams, but there are concepts to extend the sources to generate spin-polarized electron beams or positron beams. The beam parameters of these particle sources are largely determined by the injection and subsequent acceleration processes. Although, over the last decade there has been significant progress, the sources are still lacking a sufficiently high 6-dimensional (D) phase-space density that includes small transverse emittance, small energy spread and high charge, and operation at high repetition rate. This is required for future particle colliders with a sufficiently high luminosity or for more near-term applications, such as enabling the operation of free-electron lasers (FELs) in the X-ray regime. Major research and development efforts are required to address these limitations in order to realize these approaches for a front-end injector for a future collider or next-generation light sources. In particular, this includes methods to control and manipulate the phase-space and spin degrees-of-freedom of ultrashort plasma-based electron bunches with high accuracy, and methods that increase efficiency and repetition rate. These efforts also include 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. Emittance cooling via the emission of synchrotron radiation using current technology requires kilometer-scale damping rings. For future colliders, the damping rings might be replaced by a substantially more compact plasma-based approach. Here, plasma wigglers with significantly stronger magnetic fields are used instead of permanent-magnet based wigglers to achieve similar damping performance but over a two orders of magnitude reduced length.
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