Status Report "focussing of Intense Ion Beams"

In the Winterberg scheme, a charge neutralized intense high-Z ion beam is axially injected into a mirror magnetic field, where it undergoes isothermal compression without significant loss of forward beam velocity. During compression , an azimuthal current is induced in the beam which then leads to ohmic heating of the beam. Simultaneously, the beam density increases to a collision dominated regime. Thus, during this process, energy balance between ohmic heating, radiation loss (Bremsstrahlung, line emission, and recombina-tion), and kinetic energy increase due to the magnetic compression becomes important for determination of the final beam configuration. Winterberg's 18 3 calculation anticipates a final beam configuration of about 10 ions/cm and a radius of 0.1 cm. A critical temperature Tc is identified such that for T > Tc, the radiation cooling time is less than the ohmic heating time. In order to assess the feasibility and determine the operating conditions, we have identified the following critical problem areas as essential for understanding the Winterberg scheme. 1. Development of a self-consistent theory giving the evolution of such beam variables as density, parallel and perpendicular temperatures, beam velocity and current during compression by the magnetic field. Such a theory may be obtainable by extending presently available theories of ion ring compression schemes to include collisonal heating and radiation. In this regard, the relative heating and cooling rates depend critically on the above mentioned beam variables. 2. In Winterberg's work, the radiation rate is calculated on the basis of an approximate radiation formula including Bremsstrahlung, line emission and recombination radiation (cf. Artsimovich, 1964). However, this formula is only an approximate estimate for a plasma in steady-state lasting significantly longer than the confinement and compression time scales given by Winterberg. Correction to this formula or a new formula could affect the radiation rate, thus the overall energy relation critically. 3. The radiation cooling mechanisms are effective only if the beam is collision-dominated. If the beam is not sufficiently collisional except at the end of the compression stage, then the bulk of the cooling occurs after compression. Inthis regard, it is important to understand the dynamic evolution of an intense high-Z ion beam. Another question of interest is the stability of the intense high-Z ion beam during and subsequent to compression. Due to the curvature of the magnetic field lines, stability properties against gross MHD modes may be favorable. We will report on progress in Items #1 …