Transport experiments of a 2.2 GeV gold ion beam in a plasma channel at the GSI-UNILAC facility

Charged particle beam transport in plasma channels is a well established technique for electron and proton beams in the 1 MeV particle energy range. To explore the applicability of this transport mode to heavy ion beam driven inertial fusion energy production, experiments with heavy ion beams were started. These experiments explore the ion optics of a laser initiated discharge channel for heavy ion beam transport. A metallic, cylindrically shaped, chamber with a length of 0.5 m was set up at the end of the Z4 beam line at GSI-UNILAC. We have chosen laser initiation with a CO₂ laser, which resonantly heats the ammonia discharge gas (2-25 mbar) in the chamber to produce a stable discharge. A capacitor bank (2.6-7.8 μF), charged up to 20 kV, is then triggered and a straight current channel is produced along the laser path. The current can be increased up to 40 kA. A pepper pot mask prepares from the 1 cm diameter beam, nine (1 mm diameter) beamlets, eight of them forming a symmetrical cross (2 mm pitch) and the ninth is asymmetrically placed. The mask is serving at the same time as bending mirror for the laser. Hollow electrodes allow the ion beamlets to enter the channel. The beamlets are visualized with the help of a fast plastic scintillator coupled with a fast camera behind the discharge. A 2.2 GeV gold beam was used to probe the ion optical properties of the channel. Under the influence of the azimuthal magnetic field the ions are performing small oscillations around the discharge axis, being trapped inside the channel. The recorded picture of the scintillator reveals that more than one full betatron oscillation can be achieved under certain discharge conditions. The results are in good agreement with the numerical simulations of the ion optics.