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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Polarized proton beams from laser-induced plasmas
utzen1,2, Johannes Thomas3, J¨
oker4, Ralf Engels5, Ralf Gebel4, Andreas Lehrach4,6,
Alexander Pukhov3, T. Peter Rakitzis7,8, Dimitris Soﬁkitis7,8, and Markus B ¨
unberg Institut (PGI-6), Forschungszentrum J ¨
ulich, Wilhelm-Johnen-Str. 1, 52425 J ¨
ur Laser-und Plasmaphysik, Heinrich-Heine-Universit¨
atsstr. 1, 40225 D ¨
ur Theoretische Physik I, Heinrich-Heine-Universit¨
atsstr. 1, 40225 D ¨
ur Kernphysik (IKP-4), Forschungszentrum J ¨
ulich, Wilhelm-Johnen-Str. 1, 52425 J ¨
ur Kernphysik (IKP-2), Forschungszentrum J ¨
ulich, Wilhelm-Johnen-Str. 1, 52425 J ¨
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
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[1–4].
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. 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),
ulich, Wilhelm-Johnen-Str. 1, 52425 J ¨
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
utzen et al.
Figure 1. Schematic setup for the ﬁrst proton polarization measurement.
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. 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·cm−2
when being focused on a target. Impinging the laser pulse
in a 45◦angle 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) mechanismtoward 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·m−1are 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·cm−2and 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.
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.  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 cm−3the polarization lifetime is ∼10 ns,
which is sufﬁciently long to generate polarized hydrogen
atoms on the timescale of our experiment. This density
is large enough for laser-driven ion acceleration of spin-
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.
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
publicationfor a more quantitative estimate.
Our assessment for the Stern–Gerlach forceshows
that non-relativistic proton beams with opposite spins are
separated by not more than ∆p≈9.3×10−7λLwith the
laser wavelength λL. Moreover, the ﬁeld strengths are of the
order of E≈B≈105T and the ﬁeld gradients ∇ |B| ≈
105T/Rwith the laser radius R, typically λL/R=1/10
and a characteristic separation time would be t=100ω−1
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
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=(gp−2)/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
Ωalways has a component perpendicular to s,
the single spins in a polarized particle ensemble precess with
the frequency ωs= | E
Ω|. For protons with an energy in the
range of a few GeV, γ≈1 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 1and to verify the quantitative esti-
mates of Ref. , 3D simulations with the above-mentioned
VLPL code including spin tracking have been carried out on
the supercomputer JURECA. 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.  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 targetsor with polarized 3He gas[17,18]. For the
acceleration of protons until now only polarized atomic
beam sources based on the Stern–Gerlach principleare
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
utzen et al.
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 cm−3. 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. 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). The linearly polarized 1064 nm beam
with a pulse energy of 100 mJ is focused with an intensity
of ∼5×1013 W·cm−2into 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
x≈2 assuming an interaction parameter of 1ω =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.
At the same time but under a 90◦angle, the circularly
polarized ﬁfth harmonic with an energy of 20 mJ is also
focused at an intensity of ∼1012 W·cm−2into 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 Ω= +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. 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.
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 cm−3or
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. . It was found that laser intensities of
>1022 W·cm−2promise 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.  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
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
utzen et al.
assisted our research. This work has been carried out in
the framework of Space: the JuSPARC (J ¨
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-
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