Download Proposal: Concept of a test stand for laser accelerated

Proposal:
Concept of a test stand
for laser accelerated ions for future
synchrotron injection
K. Harres, F. Nürnberg, M. Schollmeier, M. Günther, I. Alber, A. Blažević, D.H.H. Hoffmann,
T. Stöhlker, H. Eickhoff, I. Hofmann, W. Barth, P. Spädtke and M. Roth
e-mail: [email protected], [email protected]
Acceleration of intense ion beams by ultra-intense laser fields
The development of ultrahigh-power laser systems in the last two decades has lead to increasing attention
and enormous scientific activities in the field of laser-plasma interactions. In the focus of such laser beams,
intensities up to 1022 W/cm2 became available for experiments, and thus an entirely new area of research, the
regime of relativistic plasma physics became accessible. The term "relativistic" marks the fact that electrons in
the laser focus are accelerated close to the velocity of light within one half laser period, which happens for light
intensities exceeding 1018 W/cm2 .
A wealth of new phenomena had been explored for the first
time using laser light as a driver. These are for example the
pre plasma
target
generation of high harmonics up to the x-ray regime, the
acceleration of electrons with energies up to 1 GeV and the
laser
ion b
creation of neutrons by fission reactions. One of the most
eam
striking new discoveries was the generation of intense ion
beams from laser solid interaction, see figure 1. Driven and
initiated by energetic short pulse lasers the beams showed
camera
a stronger collimation in the sense of an actual ion beam
in contrast to the evaporation type expansion known previously from nanosecond laser matter interaction. Moreover,
the ion beams were found to be emitted within a few picoseconds only, and they always were directed perpendicularly to the target rear surface of the irradiated target. The
Figure 1: Laser-ion-acceleration from a solid target at the
LLNL NOVA Petawatt laser.
mechanism briefly takes place as follows. Relativistic electrons accelerated by the intense laser light propagate through the target and build up a strong electric field in
the order of TV/m at the rear surface of the solid. Due to the strong field strength the atoms at the target surface
are field-ionized and are accelerated in the direction of the target surface normal. This mechanism is called the
TNSA (target normal sheath acceleration). The measured particle energies so far extends up to tens of MeV
(60 MeV protons, 5 MeV/u palladium) and they showed complete space charge- and current neutralization due
to accompanying electrons. In the experiments particle numbers of more than 1013 ions per pulse and beam
currents in the MA regime were observed. Another outstanding beam parameter is its excellent beam quality
with a transverse emittance of less than 0.004 π mm mrad and a longitudinal emittance of less than 10−4 eVs.
Because of these unmatched beam characteristics a wealth of applications were foreseen immediately. Those
applications range from:
• new diagnostic techniques for short pulse phenomena, since the short pulse duration allows for the imaging of transient phenomena,
• the modification of material parameters (starting from applications in material science up to warm dense
matter research and laboratory astrophysics),
• ion beam radiography and lithography,
• applications in energy research ("Fast Ignitor" in the inertial fusion energy context),
• injector of high power ion beams for large scale basic research facilities and
• medical treatment (proton and carbon therapy, transmutation of short lived radio-isotopes for positron
emission tomography (PET) in hospitals).
Acceleration of intense ion beams by ultra-intense laser fields
1
To prosper in these exciting applications, especially for the latter one, the fusion of laser-ion-acceleration and
conventional ion accelerator technology is of main importance. The establishment of this connection is the main
goal of the laser- and plasma physics group at Technische Universität Darmstadt (TUD) in close cooperation with
Gesellschaft für Schwerionenforschung mbH (GSI). GSI is a unique facility, combining a heavy ion accelerator
with a laser system of the Petawatt-class, PHELIX, which are the ideal conditions for such a project. For this
reason a virtual institute was founded, VIPBUL, Virtual Institute for the generation of intense Particle Beams
with Ultra-intense Laser fields. This cooperative work between the GSI, TUD, Ludwig-Maximilians-Universität
München, Friedrich-Schiller-Universität Jena and Weizmann Institute in Tel Aviv started the exploration of
laser accelerated ion beams as next generation ion sources from 2004-2008. One of the main interests of the
members of the virtual institute was, to make a high power laser beam available at the experimental area of the
GSI plasma physics group (experiment station Z6), to investigate
• the physics of ion beam generation by ultra-intense lasers,
• to explore the applications, especially in combination with secondary laser and ion beams and as a final
goal
• to study the prospect of laser accelerated heavy ion beams as the next generation ion source.
This proposal intends to continue and extent the work and results of the virtual institute.
An overview of the laser system PHELIX is given in the next section, followed by the first concept to prepare
and test a laser-accelerated ion beam for synchrotron injection.
PHELIX laser system
PHELIX is a Petawatt High Energy Laser for Ion EXperiments located at GSI in Darmstadt. It offers 1 kJ laser
energy in a nanosecond pulse, that equals 1 TW peak power, or alternatively highest intensity (>1020 W/cm2 )
and power (1 PW) in pulses of less than 500 fs, e.g. for laser-ion-acceleration.
The first laser-ion-acceleration experiments were carried out in 2006 in the laserlab of the Phelix building using
only the pre-amplifier of the laser. A second campaign is planned for September/October 2008 in the laserbay.
Since the main amplifier of the PHELIX is online since 2007 and a Petawatt compressor was installed in the
laserbay, a laser energy on target of more than 100 J in 500 fs is available for this experiment. Figure 2 shows
in the lower part the final stage of the laser beamline in the laserbay. The beam (orange line) leaves the main
amplifier section and is then guided through a last faraday isolator to the experimental area in the laserbay or
is transfered to the external experiment stations Z6 or HHT. The transfer beamline to Z6 is near to completion,
the one to HHT is still under design.
In the framework of VIPBUL a high power laser beam has started to be established at Z6 for laser-ion-acceleration
experiments. To use PHELIX for this application the beam still needs to be compressed after it is transfered from
the laserbay to the experimental area Z6. Therefore, a second compressor tank was installed inside the Z6 area
and integrated into the existing PHELIX beam line. It was designed to use a sub-aperture beam, providing a
maximum laser power of about (100-150) TW. This set-up allows accurate laser-ion-acceleration experiments at
much lower costs per shot in comparison to Petawatt short pulse experiments. The compressor tank is in place,
but the beamline inside the tank and the connection to the target chamber is still under construction.
Additionally to the original VIPBUL design, the authors want to introduce a possible PHELIX upgrade, that
would make all short pulse experiments at Z6 more attractive especially for external users and largely increase
the physics output. At present the main deficit of the laser is the low repetition rate of one shot every two
hours due to the long cooling time of the main amplifier stage. The amplifiers are pumped by flash lamps which
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PHELIX laser system
Figure 2: Overview of the PHELIX laser building.
results in relatively high temperatures inside the laser medium. Laser-ion-acceleration as well as all other laser
plasma experiments would benefit of a high laser repetition rate. Figure 2 shows in the upper right part a
scheme of a concept how the increase in the repetition rate from two hours to a shot every ten minutes with a
high peak power of 150 TW is achievable. To realize this, the existing beam line to the laserlab will be used to
guide the laser beam (red line) leaving the pre-amplifier to two amplifier stages, each consisting of two 150 mm
disk amplifiers. A single stage delivers a gain of 25 at a 20 min. repetition rate. After the pre-amplifier the
beam has an energy of 5 J, so that the output of one double amplifier stage will be 125 J. A moveable mirror
will be used to select one of the amplifier stages, which doubles the repetition rate, so that a laser shot every
ten minutes becomes available. The complete set up can be placed into the laserlab, which is currently used
for pre-amplifier experiments, since the old target chamber was moved to the laserbay for experiments with
the compressed beam. After the amplification the laser can easily be re-injected into the existing beamline and
transfered to Z6 experimental area, where it will be re-compressed. Assuming a compressor efficiency of 60 %,
the final beam parameters are 75 J in 500 fs, that equals 150 TW power on the target.
This upgrade of course does not allow for the same maximum laser energy available at Z6 in comparison to
experiments in the laserbay using the PHELIX main amplifier, but the number of laser shots per day is significantly increased from four to forty eight and the 150 TW option perfectly fits with the task to inject the
laser-accelerated ions into the GSI synchrotron SIS 18. Moreover, by simply moving a mirror (on a translation
stage) at the output of the pre-amplifier, the main amplifier can be used. Instead of a single experiment in a two
hours interval, twelve shots at 150 TW plus one full Petawatt experiment would be feasible.
We want to point out, that this upgrade will not incur large extra-costs. The two amplifier stages will be provided for free by the Lawrence Livermore National Laboratory (LLNL)! The transport and the refurbishment of
PHELIX laser system
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the amplifiers will be paid in full by a BMBF project of the TUD, if a decision is made within this year. The
overall costs of this laser upgrade including optics, electronics and personnel, as well as a calculation of costs
for the injection test stand and the Z6 beam line and compressor is shown on the last page of this proposal. No
new amplifiers need to be purchased, so that the costs are reduced by half a million Euro.
All units can be easily included into the PHELIX control system and connected with the PHELIX capacitor banks,
since they are parts from the decommissioned LLNL NOVA laser, as well as most of the PHELIX components and
they use the same flash lamps, so they can be driven with the existing PHELIX pulse forming network.
All requirements to the laser- and especially to the ion beam, that are necessary to establish an injection test
stand for synchrotron injection are described in the next section, as well as an introduction to the required ion
optics to control and guide the laser-accelerated ions.
Figure 3: Overview of the Z4 and Z6 experimental areas.
Establishment of a test stand for laser-accelerated ions at the Z4 experimental area
Figure 3 shows an overview of a part of the experimental hall at GSI with the stations Z4 and Z6, as well
as the transfer line from the GSI UNILAC (Universal Linear Accelerator) to the SIS 18. The uncompressed
PHELIX beam enters the experimental hall next to Z6 and is guided through the Terawatt compressor into
the target chamber. The chamber is equipped with several plasma diagnostics, ranging from pinhole cameras,
streak cameras for visible light and X-rays, multi channel diode spectrometers (visible and X-ray) to a multi
frame interferometry with up to six separate arms and a Thomson scattering spectrometer. A second laser
system is located at Z6, NHELIX, a Nanosecond High Energy Laser for Ion EXperiments, that consists of a 0.5 ns
low energy laser for the interferometry and a high energy (10-15) ns laser, that for example can be used as a
driver for X-ray backlighting. The analysis of laser-accelerated ions is done by additional, already existing and
successfully tested diagnostics like radiochromic films, image plates and also Thomson parabola spectrometers,
that are all calibrated by our group.
Collimation unit
The most challenging task is the interface between the laser-ion accelerator and conventional ion optics. Within
a laser-solid interaction at laser intensities higher than 1019 W/cm2 an ion bunch with particle numbers up to
1014 is produced with a length of a few picoseconds. This beam shows an exponential particle spectrum with an
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Establishment of a test stand for laser-accelerated ions at the Z4 experimental area
B-field
ctories
raje
proton t
150 TW
laser
75 mm
Proton energy (MeV)
Figure 4: CST-ParticleStudio simulation of the solenoid with calculated proton trajectories. The magnetic field strength is 20 T. The
inset shows the first prototype of the solenoid.
energy spread of 100 % and is strongly divergent with energy dependent half opening angles up to 25 degrees.
Therefore, the first step is to collimate the beam. This will be done by a strong pulsed solenoid with a magnetic
flux density of 20 T. It is made of a brass coil with a length of 75 mm and an inner diameter of 44 mm. To
produce the magnetic field a current of 30 kA at 5 kV is necessary, that is provided by a capacitor discharge.
Figure 4 shows a CST-ParticleStudio simulation of the solenoid. The calculation of electromagnetic fields as
well as a particle tracking is implemented in the code. A 100 µm particle source emits protons with an energy
of 10±2 MeV and a half opening angle of 25 degrees. The source is placed 3 mm in front of the solenoid. The
simulation shows clearly the collimation effect of the solenoids toroidal field. In the chosen energy range of
10±2 MeV more than 97 % of the protons from a divergent laser-ion source can be collimated.
Figure 5 demonstrates the effect of the magnetic field on the proton beam. On the left side the transverse phase
space of the protons directly behind the particle source is shown. The beam size is quite small, only one hundred
micrometer, but the half divergence angles reach 25 degrees. For comparison the right side of the figure shows
the phase space after a drift length of 200 mm behind the solenoid. The ellipse has rotated. Most of the particles
now have smaller half divergence angles than 2 degrees!
The first collimation unit prototype is completed and runs the first tests at present. It is designed for the same
energy as used in the simulation, since this energy fits with the injection energy of the ions accelerated by the
UNILAC. During the next laser-ion-acceleration campaign at the PHELIX laserbay this autumn, the solenoid will
be included into the experiment. More than 1011 collimated ions by the solenoid are expected from results of
the simulations.
Debunching unit
The second step after the collimation is the debunching of the beam. That means, the bunch needs to be
compressed in energy to a monoenergtic one and to be stretched in time from about 1 ns after the solenoid to
the µs region, so that an injection into a synchrotron is possible at all. The configuration of the debunching
device will be similar to a drift tube linear accelerator with an alternating electric field between at least two
drift tubes to accelerate the lower ions at the end of the bunch and to decelerate the fastest particles in the
front. Figure 6 shows schematically the principle of a debuncher unit with two accelerating cavities. Between
every pair of drift tubes, the ion bunch is compressed in energy, so that the energy spread can be reduced to
Establishment of a test stand for laser-accelerated ions at the Z4 experimental area
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Figure 5: Left: Phase space of the protons directly behind the target. The divergence angles reach 50 degrees. Right: Phase space
after a drift length of 200 mm behind the solenoid. The divergence angle of the beam is reduced to less than 4 degrees.
a minimum. The plan is to develop a new debunching unit with several cavities to compress the beam to a
monoenergetic one.
Simultaneously with the compression in energy, the bunch expands in time, but to stretch it to a µs length the
beam needs to drift. This can be done between the collimation and the debunching unit as well as ahead of the
injection stage.
After the debunching, the injection into the beamline of the transfer channel can be done by standard ion optics
like dipoles and quadrupoles.
A first debunching device was developed and successfully tested at the J-KAREN laser (JAEA Kansai Advanced
Relativistic ENgineering) of the Kansai Photon Center by our colleagues from University of Kyoto. It operated
at an electric field frequency of 80 MHz and produced quasi-monoenergetic proton bunches with an energy
spread of 1 %. They used an un-collimated beam and a drift distance between the target and the debuncher of
more than a meter, so that the particle numbers in the compressed proton bunch are very low. Nevertheless,
this project has a very high priority within the Japan Atomic Energy Agency and is additionally funded by the
industry.
Beam parameters
The proposed test stand will deliver all interesting information about the beam parameters of the laser-accelerated
ions. As a comparison the parameters of the injection beam into the GSI synchrotron SIS 18, accelerated by the
UNILAC, are summarized in the following:
• energy, e.g. protons or carbon ions: 10 MeV/u ,
• transverse emittance: ∼ 5 π mm mrad ,
• energy spread: < 1 % ,
• number of particles: ∼ 108 / pulse ,
• pulse length: ∼ 100 µs ,
• repetition rate: ∼ 1 Hz .
The beam parameters of the laser-accelerated ions need to be compared with these values, that will largely
exceeded by next generation laser-ion sources. The next step then will be to develop new injection concepts,
which are optimized for a laser-ion source.
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Establishment of a test stand for laser-accelerated ions at the Z4 experimental area
Figure 6: Scheme of a debuncher unit with two acceleration cavities to compress the proton beam in energy. The protons enter
the unit at t = 0. After the first compression, t = t 1 , a strong peak is visible in the spectrum, that will be enhanced and further
compressed by the second cavity, t = t 1 + t L . All particles with energies that do not fit with the acceptance of the debuncher around
the nominal energy E0 can not be used for the injection.
Experimental and theoretical results
A first campaign to focus and transport laser-accelerated protons was carried out by our group in December
2007 at the Sandia National Laboratories 100 TW laser facility, in cooperation with colleagues from GSI, Los
Alamos National Laboratory (LANL), Ludwig-Maximilians-Universität in Munich, University of Nevada in Reno
and from Forschungszentrum Dresden. We succeeded in focusing protons over a wide energy range to a beam
size smaller than 200 µm FWHM, by using miniature magnetic quadrupoles with a field strength of 500 T/m. A
proton flux increase of nearly two orders of magnitude could be achieved in comparison to the unfocused beam.
This experiment has suffered from the small pinhole size of the magnets of 5 mm, so that only a few percent of
the beam could be transported to the detector. The maximum proton flux was 106 particles.
In addition to the focusing, the de-neutralization of the beam was investigated. A dipole magnet was placed
directly behind the target to extract the co-moving electrons. Despite the high space charge of the proton beam,
the beam quality did not deteriorate after the electron extraction.
The next experiments will be carried out at the GSI PHELIX laser as mentioned above and our group has an
invitation for joint experiments at the J-KAREN laser as well as at the Sandia 100 TW facility for further investigations on the control of the transport and shaping of the laser-accelerated ions. We also collaborate with
the Forschungszentrum Dresden on that topic. A recently established MoU (Memorandum of Understanding)
between the TUD, GSI and LANL focuses on a close collaboration. Moreover, combining the capabilities of the
PHELIX and TRIDENT laser, the GSI and LANL accelerator groups as well as the TUD target fabrication laboratory will support this project.
Besides the experiments we have theoretical support from the GSI accelerator theory group as well as from
Lawrence Berkley National Laboratory. They offered us their particle-in-cell WARP code to simulate a complete
Establishment of a test stand for laser-accelerated ions at the Z4 experimental area
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beamline, going from the collimation of the ions to the injection into a synchrotron. Furthermore, we are in
close collaboration with the institute of theory of electromagnetic fields at the TUD, that covers all aspects of
the CST-StudioSuite.
The diagram below shows the time line to establish a test stand for laser-accelerated ions for future synchrotron
injection at the experimental stations Z4 and Z6 as well as the PHELIX laser upgrade.
2007
Experiments
Theory
2008
Sandia 100 TW
2010
PHELIX laserbay
150 TW at Z6
2011
Test stand running
CST, TRACE3D, WARP
PHELIX
150 TW upgrade
Z6/Z4
Compressor + beam line
Injection test stand
8
2009
Solenoid
Debunching unit + diagnostics
Establishment of a test stand for laser-accelerated ions at the Z4 experimental area
Calculation of costs
parts
sum / !
PHELIX 150 TW upgrade
beamline components
telescopes
injection unit
auxiliary equipment
personnel
Optics + motorization, support, diagnostics
70.000
Optics, vaccum parts, support
105.000
Optics + motorization, support
20.000
PFN Cabling, nitrogen flow
10.000
1 year laser physicist
75.000
1 year construction engineer
amount
70.000
350.000 !
Z6 compressor and beam line
beam line and compressor
components
diagnostics
personnel
beam line, vaccum parts, optics, support
50.000
Optics + support
10.000
1 year construction engineer
70.000
amount
130.000 !
Injection test stand
debunching unit
investment costs
50.000
RF-supply
70.000
diagnostics
investment costs
70.000
building modifications
technical infrastructure
investment costs
70.000
1.5 years debunching unit
100.000
1 year diagnostics
60.000
infrastructure
40.000
debunching unit supply
personnel
theoretical follow-up
amount
72.000
522.000 !
Additional personnel costs
Postdoc
three years
180.000
PhD student
three years
72.000
amount
252.000 !
Overhead
5 % (63.000 !)
Contingency
20% (263.000 !)
Total amount
1.517.000 !
Establishment of a test stand for laser-accelerated ions at the Z4 experimental area
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