Investigation of the power transfer efficiency in a radio-frequency driven negative hydrogen ion source

Abstract

The radio frequency power transfer efficiency is experimentally and numerically investigated in an inductively coupled negative hydrogen ion source. The discharge is operated in a low pressure range of 0.1–3 Pa at a driving frequency of 2 MHz and an applied power of up to 6 kW. In the experiment, the power transfer efficiency value is determined by measuring the applied power and current through the antenna coil both with and without discharge operation. Fundamental properties, such as electron density and effective electron temperature, are obtained by means of a Langmuir probe. The effect of the antenna coil turns, N, is also studied in a range of 5–9 turns. It is found that more coil turns can significantly enhance the power transfer efficiency due to the remarkably increasing quality factor of the system. Moreover, the experimental results show that the power transfer efficiency first increases and then reaches the maximum with increasing applied power, while it first increases quickly and then rises at a slower rate with increasing gas pressure. In order to give a comprehensive knowledge of the power absorption mechanism, a self-consistent hybrid model is developed. It is found that the numerical results are in reasonable agreement with that measured in the experiment. The numerical results and the analytic solutions in the limit cases of low and high pressures can well explain the various trends of the power transfer efficiency obtained in the experiment. These trends mainly depend on the quality factor Q, the electron density, and the effective electron collision frequency.

Measurement of electronegativity during the E to H mode transition in a radio frequency inductively coupled Ar/O2 plasma*

“…More details about the chamber can be found elsewhere. [19][20][21][22][23] A commercial Langmuir probe [24][25][26][27][28][29] (Impedans ALP System TM ) is used to measure the electron density at the axial center and 3 cm below the quartz window. The probe tip is made of tungsten wire with 10 mm in the length and 0.3 mm in the diameter.…”

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

“…The measured value is indeed high at 80%, but plasma self-shielding and the small contribution of capacitive coupling limit the perfect inductive coupling expected for an ideal transformer. It is to be noted that there are many more parameters, for example the gas pressure and RF-coil quality factor, that affect the spatial distributions of the RF fields and, hence, the coupling and resulting plasma parameters [17,18]. In summary, independent methods have been used to determine or estimate that the coupling efficiency from an external RF-coil into the plasma is around 50%-60%.…”

“…This explains why the ISIS ion source is able to operate down to lower powers than the SNS source: the higher frequency (3.2 vs. 2.0 MHz) creates a higher electric field, which presumably eases plasma ignition. A larger number of coil turns also has a beneficial effect [18].…”

“…Altogether, the relevance of the RF frequency on the ohmic power absorption depends on the heating regime; in stochastic (collisionless) heating regime at low gas pressure the anomalous skin depth, δ a , scales with the plasma density (n e ), electron mean velocity (v e ) and the RF frequency ω RF as , whereas at high gas pressure the heating is collisional, and the corresponding inertial skin depth δ i ∝ 1 √ ne is independent of the applied RF frequency (see e.g. [18,19] and the references therein for thorough discussion). The qualitative interpretation is that at low pressure the beneficial effect of increasing the electric field strength by the increase of the RF excitation frequency may be offset by the reduced field penetration implying an optimum frequency.…”

“…In general, the effect of gas flow on coupling efficiency is complicated by the spatial distributions of the RF fields and plasma parameters [14,18]. It is possible to have the efficiency either increasing or decreasing with the gas pressure, depending on the exact setup.…”