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
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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).

Summarized, the PHELIX laser accelerates at the target a huge number of protons/ions with a large divergence and an exponentially decaying energy spectrum (blue line in figure below). The solenoid captures a large part of the beam and, due to its achromatic focusing, it collimates and filters particles with energies from a chosen interval (ΔΕ/Ε = 17%, green line). The cavity in the following can shape the bunch by decreasing the energy spread (ΔΕ/Ε = 2,7%, red line, left figure) or compressing the bunch in time and hence creating highly intense bunches (tp < 500 ps, right figure).

The final transverse focus

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 located 6 m behind the first one and injected into a second solenoid, which focuses the beam down to a spot with a diameter of slightly larger than 1 mm.

The experimental campaign so far

Since the start of the project experiments many aspects of the transport of laser generated ions have been investigated. The first experiments improved the collimation of the highly divergent TNSA ion beam and enabled the transport of 30% of the ion beam over 6 m. Therefore, a pulsed solenoid developed at the Helmholtzzentrum Dresden Rossendorf was used as catching device. In a second step a radio frequency cavity was introduced into the beamline. This enabled an energy compression of the broad TNSA spectrum was shown the first time. Also, the bunching of the ion beams down to a 500 ps pulse length was recently conducted. The introduction of a second solenoid enabled not only the longitudinal compression but also a transversal focusing of the ion beam after the transport to generate high intensity ion pulses. In parallel the transport not only of protons but also of heavy ions like carbon was investigated. In the most recent experiments, the carbon ions were focused and analyzed after a 6 m transport. In this heavy ion experiments the TNSA target is heat to evaporate the hydrocarbons so the target bulk material is accelerated primarily.

The LIGHT beam line is now part of the ATHENA project. Within this project the LIGHT project should deliver ion beams for secondary users. First experiments for ion beam users have been conducted.

Future Experiments

One point is and will always be the improvement of the ion beam transport, increase of the intensity and reliability of the ion beam. The main short-term goal is the focusing, bunching and analyzation of the carbon beam. This beam is planned to be used as probe beam for experiments on stopping power in plasma. Therefore, a full refurbishment of the nhelix is intended so it delivers the energy for the plasma heating of the target. Also, the LIGHT beamline itself is planned to be replaced to have an enhanced experiment site for future ATHENA experiments.

References
  • The transport and energy selection of laser accelerated ions [S. Busold et al., PR-STAB 16, 101302 (2013); S. Busold et al., Nuclear Instr. and Methods in Phys. Res. A 740, 94-98 (2014)] and
  • The use of a radiofrequency cavity for phase rotation [S. Busold et al., PRST-AB 17, 031302 (2014)].
  • The generation of ultra-short MeV-range proton bunches [S. Busold et al., Nature Scientific Reports 5, 12459 (2015)]
  • S. Busold et al., Towards highest peak intensities for ultrashort MeV range ion bunches, SREP 5, 12459 (2015)
  • D. Jahn et al., First application studies at the laser-driven LIGHT beamline: Improving proton beam homogeneity and imaging of a solid target, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 10.1016/j.nima.2018.02.026 (2018)
  • J. Ding et al., 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, 10.1016/j.nima.2018.02.103 (2018)

 

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


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