LIGHT – Laser Ion Generation, Handling and Transport

The LIGHT collaboration was founded based on common interests to combine laser-generated ion beams with conventional accelerator technology and explore their future applications. The central goal is to examine the possibilities of beam shaping based on simulations and experiments: collimation, transport, bunching and post-acceleration of the generated proton/ion beam.

Several Helmholtz institutes (GSI Helmholtzzentrum für Schwerionenforschung, Helmholtz-Institut Jena, Helmholtz-Zentrum Dresden-Rossendorf) and German universities (Technische Universität Darmstadt, Goethe-Universität Frankfurt, Friedrich-Schiller-Universität Jena, Technische Universität Dresden) teamed up to form the collaboration. The multidisciplinary team covers the necessary knowledge on target fabrication, laser-driven ion acceleration, high-intensity laser systems, accelerator technology, and pulsed magnetic field design. In 2018, two more partners have joined the collaboration: Technische Universität München and the Lawrence Berkeley Laboratory.

The GSI is an ideal location for this research project, as it combines two high power laser systems as well as the necessary rf infrastructure. With the availability of a petawatt-class laser system and a large complete conventional accelerator, GSI is worldwide unique and offers many possibilities. Moreover, the LIGHT collaboration benefits from the accelerator expertise at the institute. The test beamline was realized at the Z6 experimental area, where experiments to investigate the beam shaping are performed.

The LIGHT experimental beam line

The present test beamline consists of four key elements. The local Petawatt High-Energy Laser for Heavy Ion EXperiments (PHELIX) hits a solid target (E < 25 J, τ = 500 fs, I > 1019 J/cm2) and drives the TNSA (Target Normal Sheath Acceleration) mechanism. A selected part of the ion beam is collimated by a pulsed high-field solenoid and enters a radio-frequency (rf) cavity, in which it is rotated in longitudinal phase space. Then the proton beam travels through a transport line and is finally focused with a second pulsed high-field solenoid.

Ion Generation: the TNSA mechanism
Image courtesy of Dr. Franck Nürnberg
©
Image courtesy of Dr. Franck Nürnberg
Image courtesy of Dr. Franck Nürnberg

The mainly used and most reliable acceleration mechanism for laser generated ions is the TNSA. It is used in the LIGHT experiments as origin of the ion beam. The TNSA mechanism is described in the figure above. An ultra-intense laser pulse coming from the left is focused into the pre-plasma on the target front side generated by amplified spontaneous emission of the laser system (a). The main pulse interacts with the plasma at the critical surface and accelerates hot electrons into the target material (b). The electrons are transported under a divergence angle through the target, leave the rear side and form a dense electron sheath. The strong electric field of the order of TV/m generated by the charge separation is able to ionize atoms at the rear side (c). They are accelerated over a few μm along the target normal direction. After the acceleration process is over and the target is disrupted (~ns), the ions leave the target in a quasi-neutral cloud together with comoving electrons (d).

The PHELIX laser parameters to drive the TNSA acceleration at Z6 are the following:

λ = 1053 nm, E ≈ 15 J, τ = 500 fs,

 

The laser pulse is focused down with a coated glass off-axis parabola (focal length: 300 mm, full deflection angle: 22.5°) on a target with a 3.5 μm (FWHM) focal spot size and an energy of 10-15 J. This results in an intensity higher than 1019 W/cm2.

The TNSA source of the LIGHT beamline showed an exponentially decaying spectrum up to 28 MeV. The beam contains in the forward direction up to 1013 protons with energies above 4 MeV an d a large, energy dependent divergence. This equals a conversion efficiency of laser energy to ion beam energy about ∼ 10 %.

Handling and Transport

As most applications require a collimated beam with a well-defined energy spread, it is necessary to control the TNSA beam divergence and the energy width of the moving ion pulse. For this purpose, a pulsed high-field solenoid followed by a rf cavity is used.

The solenoid is designed with the goal to capture a large part of the divergent TNSA beam. It has been designed and produced by the Helmholtzzentrum Dresden-Rossendorf. Protons, which enter the drift tube inside the solenoid, experience its magnetic force. Because of the dependency of the solenoid’s focal length on the proton energy, different energies are focused in various distances by setting the magnetic field strength: particles with a chosen energy are focused at a certain distance behind the solenoid. At the same time, particles with a higher energy diverge and slower ions are focused at a shorter distance and diverge afterwards, hitting the beam pipe at some place and getting lost. Hence, the solenoid serves as an energy filter, cutting out protons/ions with energies from a dedicated energy interval out of the exponentially decaying spectrum.

While the solenoid addresses the transverse beam dynamic, adding now a radiofrequency structure (rf cavity), that is used for acceleration and phase rotation in conventional accelerators, to the beamline makes the inclusion of the longitudinal dynamics necessary. The cavity used at GSI is a three gap spiral resonator, which was characterized and implemented at the UNILAC before. Rf cavities are typically built as cylindrical resonant cavities for electro-magnetic waves in the radiofrequency region, creating a fast oscillating standing wave between the gaps. According to the phase the ion pulse is injected, the electric field can accelerate or decelerate the ion pulse. As a result, the bunch is rotated in its longitudinal phase space. This rotation depends on the two parameters rf phase and rf amplitude. The later determines the rotation angle. Through this procedure, the proton/ion bunches can be compressed in energy (small energy spread) or in time (short bunch duration).

For experiments mostly a small focus at a target is desired. Therefore, the proton/ion pulse, transversally collimated by the solenoid and longitudinally shaped by the rf cavity, is transported to a second target chamber Z4 and injected into a second solenoid, which focuses the beam. An achieved focal spot and the temporal shape of a temporally compressed proton beam are shown:

The experimental campaigns so far

The LIGHT beamline yielded a multitude of ion beams, the most significant of which are enumerated in the table below. In 2014, the emphasis was on energetic, whereas in 2019, we attempted to temporally compress the proton beam. In 2021 and 2022, the focus shifted to stopping power experiments necessitating the shortest possible bunches, which is why temporal compression was selected. Furthermore, the transport of fluor ions has been demonstrated (F7+) as fluorine ion beams are difficult to access by conventional accelerators, because fluorine ions are highly corrosive and therefore difficult to generate sustainably in an ion source. It has been established that TNSA at the Z6 target chamber enables the generation of proton energies of up to 28.4 MeV, C4+ energies of up to 68.5 MeV and F7+ energies of up to 180 MeV.

parameter

protons (2014)

protons (2019)

C4+

protons (2022)

mean energy

9.7 MeV

7.72 MeV

0.60 MeV

0.63 MeV

energy spread (width at 20%)

2.7 %

4.9 %

not measured

not measured

temporal bunch width (FWHM)

not measured

0.74 ns

1.23 ns

0.76 ns

estimated number of ions

1.7 ⋅ 109

7 ⋅ 108

2 ⋅ 108

6 ⋅ 108

The most recent transported beams were used to conduct stopping power experiments in a solid carbon foil, to demonstrate the feasibility of the planned experiment. The energy loss for protons was measured to be dE = (29 ± 6) keV, while for carbon ions, the energy loss was dE = (61 ± 10) keV. These results align with the energy losses predicted by SRIM. With these measurements, an uncertainty of 7 % for protons and 6 % for carbon ions is predicted for the stopping power experiments with the plasma target. This will lead to meaningful data for benchmarking stopping power theories. These preliminary experiments demonstrate the viability of the proposed stopping power experiment.

Future Experiments

The next step is to conduct the planned stopping power experiment with a plasma target. This experiment will allow for benchmarking of theories in the regime of the stopping maximum with higher accuracy than in previous stopping power experiments performed at GSI. Future stopping power experiments should aim to reduce uncertainties in ion stopping measurements. Besides increasing the time of flight distance, future experiments will also aim to optimize the time of flight diagnostics. With that the main goal is to build a platform for the comprehensive study of stopping power of ions in a plasma. The upcoming stopping power experiment will be the first of many utilizing the infrastructure and platform at the experimental area Z6 at GSI. The LIGHT beamline can cover a wide range of parameters for stopping power experiments by using various configurations of different targets, plasma parameters, and ion beams. These experiments aim to enhance our understanding of ion stopping in plasma and the validity of different stopping power models in various regimes. With an increase in experimental data, new advanced theoretical models, approaches, and corrections are likely to emerge.

The LIGHT beamline is a noteworthy technological advancement in ion acceleration and beam manipulation. Future developments could focus on improving beam quality, increasing ion energies, and reducing bunch durations. Upgrading the PHELIX laser at the Z6 experimental facility will expand the parameter space of the ion beams generated with the LIGHT beamline. A higher repetition rate would enhance the LIGHT beamline and increase its usefulness in most applications. This will not only benefit studies on ion stopping power but also expand the use of laser-driven ion sources in medical physics, materials science, and radiation therapy. One current investigation concerns the generation of an ion beam with the LIGHT beamline, which will be injected into the heavy ion synchrotron SIS18 of GSI. The injection of a laser-generated ion beam into a synchrotron would represent a significant advancement for the laser-plasma accelerator community. The primary objective is to demonstrate the feasibility of this concept, which could have a significant impact on the design of future accelerator facilities. The concept has the potential to reduce the cost and size of future injectors. Specifically, for GSI there would also be some immediate benefits which include the ability to inject a wider range of ion species and the provision of an additional injector for use in emergencies. Furthermore, the concept could lead to a reduction in injection time, emittance and bunch length, as well as a reduction in the cost and size of injectors in the future. In order to demonstrate the viability of laser-driven ion sources for synchrotrons, it is necessary to show that ions can be injected in a way that is feasible for the operation of the synchrotron. In order to achieve this, we will present a demonstration of the single-shot injection of protons into the SIS18. Furthermore, we will show how a laser proton source with a high repetition rate can be operated in collaboration with the ELI Beamlines facility. This facility already has a laser, namely the L3 beamline, which is capable of generating the necessary proton beams with a repetition rate of 1 Hz. The combination of these two elements will ultimately prove the viability of laser-based injectors for accelerators.

References
  • Busold, S., Almomani, A., Bagnoud, V., Barth, W., Bedacht, S., Blažević, A., Boine-Frankenheim, O., Brabetz, C., Burris-Mog, T., Cowan, T. E., Deppert, O., Droba, M., Eickhoff, H., Eisenbarth, U., Harres, K., Hoffmeister, G., Hofmann, I., Jaeckel, O., Jaeger, R., … Zielbauer, B. (2014). Shaping laser accelerated ions for future applications – The LIGHT collaboration. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 740, 94–98. doi.org/10.1016/j.nima.2013.10.025
  • Ding, J., Schumacher, D., Jahn, D., Blažević, A., & Roth, M. (2018). Simulation studies on generation, handling and transport of laser-accelerated carbon ions. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 909, 168–172. doi.org/10.1016/j.nima.2018.02.103
  • Jahn, D., Schumacher, D., Brabetz, C., Kroll, F., Brack, F. E., Ding, J., Leonhardt, R., Semmler, I., Blažević, A., Schramm, U., & Roth, M. (2019). Focusing of multi-MeV, subnanosecond proton bunches from a laser-driven source. Physical Review Accelerators and Beams, 22(1), 011301. doi.org/10.1103/PhysRevAccelBeams.22.011301
  • Metternich, M., Nazary, H., Schumacher, D., Brabetz, C., Kroll, F., Brack, F.-E., Ehret, M., Blažević, A., Schramm, U., Bagnoud, V., & Roth, M. (2022). Analyzing the filamentation of MeV-range proton bunches in a laser-driven ion beamline and optimizing their peak intensity. Physical Review Accelerators and Beams, 25(11), 111301. doi.org/10.1103/PhysRevAccelBeams.25.111301
  • Nazary, H., Metternich, M., Schumacher, D., Neufeld, F., Grimm, S. J., Brabetz, C., Kroll, F., Brack, F.-E., Blažević, A., Schramm, U., Bagnoud, V., & Roth, M. (2024). Towards ion stopping power experiments with the laser-driven LIGHT beamline. Journal of Plasma Physics, 90(3), 905900302. doi.org/10.1017/S0022377824000576

(Images and text courtesy of Dr. Franck Nürnberg, Dr. Simon Busold and Dr. Diana Jahn)


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