HEAVY-ION BEAM TRANSPORT IN PLASMA CHANNELS

Abstract

Concepts for heavy-ion fusion (HIF) reactors require the transport of kiloampere ion beams over distances of several meters inside the reactor chamber. A possible solution for this task is to use assisted pinched transport (APT) for the final transport of the beam. This scheme uses an adiabatic plasma lens to focus the beam outside the chamber and a plasma channel to transport it inside the reactor chamber towards the fusion target at the center. The plasma channel has three functions: it neutralizes both the space charge and the current of the ion beam and furthermore creates a large azimuthal magnetic field that prevents the beam ions from leaving the channel. The appeal of the APT scheme is that it separates the focusing of the beam from the final transport, thus relaxing the focusing requirements of the beam. The purpose of this study is to demonstrate the creation of long, free-standing channels and to study their magnetohydrodynamical stability and their transport properties for low-current heavy-ion beams inside the channels. It is the result of a collaboration with Lawrence Berkeley National Laboratory and thus contributes to the ARIES fusion reactor study. In combination with the results of detailed transport simulations for high-current heavy-ion beams, which are part of the ARIES study, the results of the experiments make it possible to map out a set of suitable operating parameters for the channel transport of beams with reactor-like parameters, for instance the required discharge current and plasma density. Previous transport experiments at the Gesellschaft fur Schwerionenforschung (GSI) used a discharge chamber that was 50 cm long and 60 cm in diameter. To get closer to reactor scales the chamber was extended by inserting a 50 cm long chamber section, resulting in a total length of 1 m. Since the prolongation changed the geometry of the chamber and thus the electric fields inside of it, it was necessary to optimize the fields by setting each section of the chamber to a separate potential. The electric setup was optimized with the help of detailed electrostatic calculations for the chamber. The channels are created in a three-step process. After the channel initiation, which guides the discharge along the chamber axis, a prepulse, that is a small discharge, heats the gas on the axis, resulting in a rarefaction and thereby stabilizing the subsequent main discharge. Two methods of channel initiation were used successfully. Laser-initiated channels were created in ammonia and ion-beam initiated channels in various other gases, such as krypton and xenon. The evolution of the channels is consistent with results from a one-dimensional MHD simulation. The channels are stable for normal operating conditions. A detailed study of channels in ammonia revealed that the channels become unstable for high gas densities, when the prepulse can no longer be used. The instabilities show the characteristics of kink instabilities, and their growth is consistent with the predictions of a simple analytical model. Proof-of-principle experiments demonstrate the transport of low-current heavy-ion beams. The transport properties of the channel were studied and found to be consistent with the assumption of a homogeneous current density in the channel, leading to betatron oscillations. In combination with the results of simulations for the transport of high-current beams and theoretical estimates about the growth of beam-plasma instabilities, the experimental results yield an estimate for a suitable set of operating parameters for assisted pinched transport in a fusion reactor.