Spectroscopic optimization of a plasma lens for collimating TNSA beams at GSI, Darmstadt*

This Project is closely related to the LIGHT collaboration at the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt and is intended to help permanently upgrade the beamline. The LIGHT collaboration has set itself the goal of installing its TNSA beamline as an injector for the conventional SIS18 ring accelerator.

The TNSA mechanism (Target Normal Sheath Acceleration) describes the laser acceleration through the interaction of ultra-intense lasers with pulse lengths in the femtosecond range with foils of μm widths. This generates a characteristic proton spectrum with divergence angles up to 40 degrees and energy spreads over 25MeV, but ultra-low emittance. Since most applications require a collimated beam with a well-defined energy spreads, a capture element is needed. Solenoids and quadrupoles are currently mainly used for the collimation of laser-accelerated particles. However, due to the lack of magnetic field gradients, large parts of the spectrum cannot be transported. On account of the favorable arrangement of the magnetic field of plasma lenses, they can achieve magnetic field gradients that significantly exceed those of solenoids and quadrupoles.

Plasma lenses are described as gas discharge along the beam-axis of charged particle beams. These are typically ignited from ring electrode to ring electrode in order to let the particle beam propagate through. The resulting high currents generate a magnetic field according to Ampere's law, which focuses or defocuses a charged particle beam symmetrically on both planes perpendicular to the propagation direction. The symmetry and strength increase of the magnetic field with radius are given by the current distribution of the discharge. We have undertaken a spectroscopic approach to analyze the conductivity of the plasma, which is directly proportional to its current density. Spectra were measured simultaneously at different angles and same distances from the beam axis. The electron density and electron temperature were determined to confirm the symmetry and reproducibility of the magnetic field of the plasma lens.