RF power transfer efficiency of H ion sources: fluid modeling of accelerator source geometries

Abstract

The plasma in RF driven negative hydrogen ion sources is sustained via inductive coupling with large power levels of up to 100 kW and low frequencies around 1 MHz. This leads to currents of around 100 A flowing over the RF coil and corresponding voltages in the kV range. The associated risk of arcing limits the reliability of the whole ion source. The required power level can be reduced via optimizing the RF power transfer efficiency η, which is typically only in the range of 50 to 60% for H- sources used for neutral beam injection systems. In order to study the optimization of η systematically, a self-consistent fluid model has been set up and successfully validated with experimental measurements at the BATMAN Upgrade test bed. For H- sources applied at particle accelerators, no experimental measurements of η are available so far. In order to gain a first insight into the RF power transfer efficiency of these sources, exemplary simulations were carried out with the fluid code. The simulated plasma parameters are in good agreement with results from OES measurements. η shows an increasing trend with larger source radius and a virtually constant value with increasing RF power. For benchmarking these first results, dedicated measurements at an accelerator source setup are inevitable.

Modeling inductive radio frequency coupling in powerful negative hydrogen ion sources: validating a self-consistent fluid model

“…The CAD drawing as shown in figure 1(a) serves as a basis for the simulation domain of the 3D electromagnetic model, where only the driver region is included. The details of this model are discussed elsewhere [30].…”

“…Note that for the validation the experimentally determined network resistance R net is used, since the geometry of the experimental setup does not change during the validation studies. For a detailed discussion on the modeling of the network losses in the ITER RF prototype ion source, see [30].…”

“…This means that the macroscopic transport quantities such as neutral and plasma densities, velocities and temperatures as well as the electromagnetic fields are calculated in space and time using appropriate formulations for the RF coupling in the regime where negative ion sources typically operate. To (a) CAD drawing of the ITER prototype RF ion source at the BUG test bed [1], which serves as basis for the 3D electromagnetic model simulation domain to calculate the network losses [30]. (b) 2D cylindrically symmetric simulation domain of the self-consistent RF power coupling model.…”

“…Figure 1. (a) CAD drawing of the ITER prototype RF ion source at the BUG test bed[1], which serves as basis for the 3D electromagnetic model simulation domain to calculate the network losses[30]. (b) 2D cylindrically symmetric simulation domain of the self-consistent RF power coupling model.…”

RF efficiency measurements of inductively-coupled plasma H− ion sources at accelerator facilities

“…This RF-coil should be placed as close to the plasma as possible to maximise the coupling efficiency. Previous measurements found that the coupling efficiency of fusion ICPs is around 40%-50%, whereas the smaller RF-coils of accelerator-type ICPs were predicted to yield efficiencies closer to 80% [8]. The difference was attributed to the increased network resistance in fusion ICPs owing to the use of a Faraday shield to suppress capacitive coupling, which protects the dielectric from plasma sputtering.…”