Heavy Ion Beam Transport in Laser Initiated High Current Gas Discharge Channels

A critical issue in inertial confinement fusion driven by heavy ions is the transport of the ion beams through a reactor chamber. In all plant scenarios the reactor has radii of about 3-5 m and in some it is envisaged to incorporate a background gas fill to reduce the x-ray damage to the first wall. An ion beam propagating in such environment will not meet the geometrical and temporal parameters required at the target position because of scattering and stripping in the background gas. Therefore gas density reduction and neutralization of the ion beam space charge and electrical current are necessary to successfully achieve ignition of the fuel pellet. A promising technique uses high current discharge channels to transport the heavy ion beam to the target. The aims of the experiments presented in this work are related to the study of initiation and generation of long and stable discharge channels and the examination of their ion optical properties. To fulfill the above issues a cylindrical metallic chamber has been integrated into the Z4 ion beam line of the GSI-UNILAC accelerator. With the help of a tunable CO2 laser a 0.5 m long path in low-pressure ammonia gas is heated by resonant absorption. Because of the subsequent gas expansion a rarefaction channel with preferential breakdown conditions is created along the discharge chamber axis. Dumping of the energy stored in a capacitor bank into the gas produces a straight plasma channel along the laser path. An additional, independent low current discharge before the ignition of the main discharge has proven to be beneficial for the stability of the plasma column, especially at high pressures and currents. Streak, framing and fast shutter cameras were used to study the hydrodynamics of the discharge channel. These experiments proved that stable and reproducible discharge channels could be produced up to times longer than the first half period of the current waveform. A special dB/dt probe containing 8 collinear coils has been designed and constructed for measuring the induced azimuthal magnetic field. Magnetic field values at the channel edge of 1 T and field gradients inside the channels of more then 300 T/m corresponding to a uniform radial current distribution could be measured. During several beamtimes, different ion beam species have been used to probe the ion optical properties of the channel. Within the limits set by the experimental conditions (channel length and diameter, and discharge current) one full betatron oscillation could be observed. This result agrees with simulations based on magnetic probe measurements. For the first time an alternative technique that uses an ion beam to initiate and guide a long discharge was tested. Straight and stable discharge channels could be generated in noble or molecular gases. The advantages of this technique are the reduced channel axial jitter compared to laser initiation and the free choice of the discharge gas