Diagnostics of Laser Initiated Plasma Channels for Ion Beam Transport

Laser initiated, free standing discharge channels offer many attractive advantages for the transport and focusing of intense ion beams [1, 2]. Discharge plasmas can neutralize both current and space charge of such beams, while the azimuthal magnetic field provides strong focusing all the way through the channel. Experiments at GSI have produced 50 cm long stable plasma channels with peak currents in excess of 40 kA in 2 to 20 mbar NH3 gas fill. The discharges are initiated by a CO2 laser pulse, fired into the chamber along the chamber axis. Absorption of the laser causes strong gas heating. Subsequent expansion and rarefaction of the gas prepare the right conditions for a stable, reproducible discharge, suitable for ion beam transport. First experiments to study the ion optical properties of such channels were already reported in [3, 4]. During the last year the channel stability was considerably improved by a new CO2 laser with an option for wavelength tuning. The wavelength can be adjusted to the P(32) transition for peak absorption, matching the ν = 950 cm−1 vibrational mode in NH3 [5]. In this way a large fraction of the laser energy is absorbed as the beam passes through the 50 cm long gas filled chamber, down to pressures of a few mbar (figure 1).