Diagnostics of RF coupling in H− ion sources as a tool for optimizing source design and operational parameters

Abstract

Radio frequency (RF) driven H− ion sources are operated at very high power levels of up 100 kW in order to achieve the desired performance. For the experimental setup, these are demanding conditions possibly limiting the source reliability. Therefore, assessing the optimization potential in terms of RF power losses and the RF power transfer efficiency η to the plasma has moved to the focus of both experimental and numerical modeling investigations at particle accelerator and neutral beam heating sources for fusion plasmas. It has been demonstrated that, e.g., at typical neutral beam injection ion source setups, about half of the RF power provided by the generator is lost in the RF coil and the Faraday shield due to Joule heating or via eddy currents. In a best practice approach, it is exemplarily demonstrated at the ITER RF prototype ion source how experimental evaluation accompanied by numerical modeling of the ion source can be used to improve η. Individual optimization measures regarding the Faraday shield, the RF coil, the discharge geometry, the RF driving frequency, and the application of ferrites are discussed, which could reduce the losses by a factor of two. The provided examples are intended as exemplary guidelines, which can be applied at other setups in order to achieve with low-risk effort an optimized ion source design in terms of reduced losses and hence increased reliability.

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

“…The variance in R network is driven by the ferrite-wound transformer, whose measured losses are discussed later. Measurements [14,15] on the BATMAN source with no isolation transformer confirmed this finding, where negligible variance in R network was found: agreeing with the green line in figure 5. A transformer is necessary for operating the ion source at high voltage in order to extract an ion beam, however it masks the in LTspice.…”

Preliminary experimental research on RF coupling efficiency of CRAFT NNBI prototype ion source

“…These losses can be quantified under different discharge conditions by measuring the real part R network of the entire network complex resistance, including the coil. The efficiency η is estimated using the formula 1 [10]:…”

“…Due to such substantial loss, the power transmitted to the plasma will be considerably low. Its magnitude will heavily depend on the discharge conditions (such as gas filling pressure and discharge power) as well as the geometry (including Faraday shield slits, coil, driver size) [10,11].…”

“…Direct experimental measurement of RF coupling efficiency is a well-established research method, and numerous studies have been conducted on RF ion source test facilities [10,12]. This study primarily considered the impact of the main operation parameters, such as RF power and feeding gas pressure, on the RF coupling efficiency.…”

Simple experimental and analytical methods to estimate the power coupling efficiency in plasma discharges

“…These include inductive plasma discharge systems, [3][4][5] IV probe in RF discharge, [6] radio-frequency glow discharges used as optical sources, [7][8][9] resonant RF network antennas where the driven antenna can be approximated by a parallel resonance equivalent circuit whose input impedance can be measured to interpret the power dissipation in the plasma. [10][11][12][13] These methods are mainly based on direct or indirect measurements of the plasma current. For example, Rauner et al have quantified the power transfer efficiency by using a subtractive method that relies on the measurement of the delivered RF power and of the RF current through the plasma coil both with and without discharge operation.…”