Microwave stability at transition

The question of microwave stability at transition is revisited using a Vlasov approach retaining higher order terms in the particle dynamics near the transition energy. A dispersion relation is derived which can be solved numerically for the complex frequency in terms of the longitudinal impedance and other beam parameters. Stability near transition is examined and compared with simulation results

Evaluation of wave dispersion, mode-conversion, and damping for ECRH with exact relativistic corrections

The complex dispersion functions of Eq. (3) in Ref. 1 have recently been computed accurately and reliably over their entire range of parameters, without recourse to the usual slightly-relativistic approximation, which may have difficulty for oblique incidence. In the future, the local dispersion properties of ECRF waves will be reevaluated for parameters of interest to ECRF conditions in several existing and proposed fusion experiments, with particular emphasis on the damping and mode-conversion of both ordinary and extraordinary waves to electrostatic waves near the upper hybrid and cyclotron frequencies

Theory of mode conversion and wave damping near the ion-cyclotron frequency

Using a variational technique, a set of coupled model equations for the mode-conversion process near the ion-cyclotron frequency is derived. The system is truncated to first order in Larmor radius but includes the effects of explicit gradients and a poloidal field. From the equations a conservation rule is extracted which ensures conservation of total energy and provides an explicit expression for the wave damping in differential form. The equations are integrated numerically for the standard cases of fast waves incident from either the low- or high-field sides of the mode-conversion layer. The scaling of the damping processes is discussed and implications for current rf-heating experiments on the Princeton Large Torus are drawn

An algorithm for the analysis of inductive antennas of arbitrary cross-section for heating in the ion cyclotron range of frequencies

The application of Ion Cyclotron Range of Frequency (ICRF) heating to near ignited plasmas will require launching structures that will be capable of withstanding the harsh plasma environment. The recessed antenna configuration is expected to provide sufficient protection for the structure, but to date no analysis has been done to determine if adequate coupling can be achieved in such a configuration. In this work we present a method for determining the current distribution for the antenna in the direction transverse to current flow and predict antenna loading in the presence of plasma. Antennas of arbitrary cross section are analyzed above ground planes of arbitrary shape. Results from ANDES, the ANtenna DESign code, are presented and compared to experimental results

Trapped Ions and Beam Coherent Instability

In accelerators with negatively charged beams, ions generated from the residual gas molecules may be trapped by the beam. Trapped ions may interact resonantly with the beam and cause a beam-ion coherent instability. This coherent instability bears many similarities to the resistive wall instability and can present important limitations to those machines operation. A description of this effect requires a treatment of the beam coherent instability including both the normal machine wake field and the interaction with ions. They present a linear approach incorporating contributions from the machine impedance as well as ion forces. it also includes spreads in beam and ion frequencies and thus Landau damping. The analysis results in a modified stability diagram which will be used together with physical arguments to explain experimental observations in the Fermilab antiproton accumulator

Influence of globalmagnetic perturbations on plasma behavior in Elmo Bumpy Torus

The sensitivity of plasma confinement to magnetic field error effects has been tested experimentally using externally introduced global field errors on the ELMO Bumpy Torus (EBT). Below a critical error field (deltaB/sub r//B)/sub cr/ of approx. = to 0.6-1 x 10/sup -3/ the plasma was observed to be essentially free from convective cells, toroidal currents, and instabilities. This observed critical value is comparable to a neoclassical critical field error (deltaB/sub r//B)/sub cr/ approx. = rho/R, the ratio of the ion Larmor radius to the major radius of the torus

Measurement of Escaping Ions in the Fermilab Antiproton Accumulator

Positively charged ions trapped in the negatively charged beam of the Fermilab antiproton accumulator pose a limit to beam stability and density. To better understand the dynamics and the consequences of the beam-ion interaction, they have built and installed a low energy ion detector and energy analyzer in the Fermilab accumulator. This analyzer is capable of energy analysis of the escaping ions using a probe with energy retarding grids and may also be scanned in the pitch angle of the escaping ions. Measurements have been made in both longitudinal and transverse planes under a variety of machine operating conditions. The experimental measurement results are presented together with attempts to model the ion dynamics and explain observations

Measurements of ICRF (ion cyclotron range of frequencies) loading with a ridged waveguide coupler on PLT

An ICRF ridged waveguide coupler has been installed on PLT for measurements of plasma loading. The coupler was partially filled with TiO/sub 2/ dielectric in order to sufficiently lower the cutoff frequency and utilized a tapered ridge for improved matching. Vacuum field measurements indicated a single propagating mode in the coupler and emphasized the importance of considering the fringing fields at the mouth of the waveguide. Low power experiments were carried out at 72.6 and 95.0 MHz without any external impedance matching network. Plasma loading increased rapidly as the face of the coupler approached the plasma, and, at fixed position, increased with line-averaged plasma density. At the lower frequency, the reflection coefficient exhibited a minimum (<8%) at a particular coupler position. At both frequencies, measurements indicated efficient power coupling to the plasma. Magnetic probe signals showed evidence of dense eigenmodes suggesting excitation of the fast wave. 24 refs., 13 figs

ICRF (Ion Cyclotron Range of Frequencies) edge modeling studies

Theoretical models have been developed, and are currently being refined, to explain the edge plasma-antenna interaction that occurs during ICRF heating. The periodic structure of a Faraday shielded antenna is found to result in strong ponderomotive force in the vicinity of the antenna. A fluid model, which incorporates the ponderomotive force, shows an increase in transport to the Faraday shield. A kinetic model shows that the strong antenna near fields act to increase the energy of deuterons which strike the shield, thereby increasing the sputtering of shield material. Estimates of edge impurity harmonic heating no significant heating for either in or out-of-phase antenna operation. Additionally, a particle model for electrons near the shield shows that heating results from the parallel electric field associated with the fast wave. A quasilinear model for edge electron heating is presented and compared to the particle calculations. The models' predictions are shown to be consistent with measurements of enhanced transport. 19 refs., 9 figs