Plasma Production by Neutral Beam Injection in Mirror Systems

Injection of energetic neutral atoms is the most promising method so far proposed for maintaining a fusion plasma, especially in mirror systems. Neutral injection studies at LRL are focused on two questions: (1) Whether a fusion-density plasma can be produced by build-up from low levels using neutral beams; and (2) whether a high-density plasma, however created, can be maintained by neutral injection. The Baseball-II experiment attacks the first question by means of a programmed build-up. The plasma temperature and charge-exchange loss rate can be specified in relation to the density for each point in the buildup so that ion-ion Coulomb collisions will always be effective in broadening the trapped-ion distribution in both real and velocity space, thus minimizing the probability of microinstability. Scattering can be realized at a density low enough to insure initial stability by starting the build-up sequence at low energy (∼1 keV). A series of numerical Fokker-Planck calculations shows the degree of collisional broadening which can be expected in the presence of charge-exchange losses. A theoretical estimate of stability against electrostatic modes is made for these calculated distributions. Detailed estimates of plasma accumulation have been made for achievement of the 1-keV starting point in the build-up sequence. These include the effects of cascading and Loientz ionization of excited atoms and collisional ionization of atoms in all levels. At operating vacua (∼10-9 tore), an injected deuterium atom beam intensity of ∼35 equivalent mA is necessary to achieve the desired density. A steady-state 1-A - 1-kV source will be described using extraction from multiple apertures to deliver the required neutral beam current within the required (± 1.4°) solid angle. The second question, which is perhaps even more basic, can be answered most readily in the 2 X-II-experiment, where a high-density plasma is created by pulsed methods. The key requirement for this test is an intense pulsed neutral beam. To this end, the design of an ion source to produce a 10-A, 30-msec pulsed beam of 20-keV deuterium atoms is described. The source utilizes extraction electrodes based on computed ion trajectories and a special hot-cathode discharge. (author)