Direct measurement of the transition from edge to core power coupling in a light-ion helicon source

Abstract

We present time-resolved measurements of an edge-to-core power transition in a light-ion (deuterium) helicon discharge in the form of infra-red camera imaging of a thin stainless steel target plate on the Proto-Material Exposure eXperiment device. The time-resolved images measure the twodimensional distribution of power deposition in the helicon discharge. The discharge displays a mode transition characterized by a significant increase in the on-axis electron density and core power coupling, suppression of edge power coupling, and the formation of a fast-wave radial eigenmode. Although the self-consistent mechanism that drives this transition is not yet understood, the edge-to-core power transition displays characteristics that are consistent with the discharge entering a slow-wave anti-resonant regime. RF magnetic field measurements made across the plasma column, together with the power deposition results, provide direct evidence to support the suppression of the slow-wave in favor of core plasma production by the fast-wave in a light-ion helicon source.

Comparison of heating mechanisms of argon helicon plasma in different wave modes with and without blue core

“…TG waves, which satisfy the dispersion relation at low plasma density n p (n p <n LH , where n LH is lower-hybrid (LH) density), are excited and deposit power at the plasma edge [32][33][34]. While helicon waves, which satisfy the dispersion relation at the higher n p , are excited and deposit power inside the plasma core [27][28][29][30]35]. Indeed, regardless of where the power coupling occurs, helicon modes typically have centrally peaked electron density profiles, especially in high-density BC mode [14,17,[22][23][24][35][36][37].…”

“…While helicon waves, which satisfy the dispersion relation at the higher n p , are excited and deposit power inside the plasma core [27][28][29][30]35]. Indeed, regardless of where the power coupling occurs, helicon modes typically have centrally peaked electron density profiles, especially in high-density BC mode [14,17,[22][23][24][35][36][37]. The central heating mechanism was not usually explained by any linear collisional heating theory of the helicon waves due to the weak damping rate of the helicon waves estimated from the dispersion relation.…”

“…Carter et al [37] considered that for high-density plasma (5×10 12 cm -3 ) where the TG waves are nonexistent or damped over a very short distance, the collision damping and focusing of the on-axis helicon waves play an important role. Piotrowicz et al [35] also proposed that the production of centrally peaked density profiles necessitates the deposition of power directly in the core, which is accessible via helicon waves. Recently, Isayama et al [41,42] suggested that the generation of centrally peaked, flat or hollow density profiles is due to the different contributions of TG waves.…”

Power absorption, plasma parameters and wave structure in inductive RF plasma source with low value external magnetic field

“…In order to obtain more physical information from experiments, researchers use theoretical models, numerical calculations and indirect measurements such as discharge mode transition, peculiarities of the RF coupling efficiency, etc. For instance, in [43], time-resolved measurements of an edge-to-core power transition in the high-density light-ion helicon discharge were carried out. The authors obtained direct evidence that the slow wave was suppressed in favor of the fast wave that was responsible for power coupling in the on-axis region.…”

“…The authors obtained direct evidence that the slow wave was suppressed in favor of the fast wave that was responsible for power coupling in the on-axis region. However, the authors of [43] mentioned that the physical mechanism that drove the edge-to-core transition was not fully understood yet. Later, based on the self-consistent helicon discharge model, it was shown in [44] that in the first time phase, the surface source flux by the TG wave power absorption formed the surface localized density profile.…”