Wave-induced helicity current drive by helicon waves

Abstract

Helicon waves are launched in a toroidal device of small aspect ratio, to form and sustain high-density plasma with 1018m−3 peak density and 12eV electron temperature. Owing to the strong poloidal asymmetry in the wave magnetic field structures, a nonresonant current is driven in plasma by the dynamo electric field, which arises due to the wave helicity injection by helicon waves. A study of the parametric dependence of plasma current driven in the electron-magnetohydrodynamic regime, at very high frequency, along with numerical estimations of nonresonant components, are presented here. The measured magnitude of plasma current is in good agreement with the estimated values. Numerical estimation, using experimentally measured variations of wave magnetic field components, clearly delineates the plasma current due to wave-induced helicity from other possible resonant or nonresonant sources in the present parameter regime.

Current drive by helicon waves

“…The convective term, ͑m / n 0 q 2 ͓͒͑J e · ٌ͒J e ͔, present in the momentum equation of fluid motion, turns out to be 10 −3 times ͗J ជ e ϫ B ជ ͘ term at n e =10 18 m −3 , which is very small owing to the small Larmor radius conditions in the present experiment. 34 The estimated magnitude of the poloidal magnetic field ͑B ͒, due to toroidal plasma current, in present experiment is Ϸ2 G. Almost exact reversal of plasma current during concomitant reversal of toroidal magnetic field and helical antenna confirmed the insignificant contribution of diffusion driven term due to pressure anisotropy ٌ͑ · P e ͒ in the net plasma current measured. Since, apart from ͗J ជ e ϫ B ជ ͘ term, the contribution from pressure gradient force and convective force terms could not account for the magnitude of plasma current measured during helicon discharge in THPD, so a small but finite contribution from the resonant wavemomentum transfer force could not be ruled out during the present study.…”

“…During these experiments, analogous jumps in plasma density and plasma current could not be observed due to early excitation of helicon waves. Such a parametric study of plasma current 34 indicates linear variation in plasma current with toroidal magnetic field in two different fill pressures ͑2 ϫ 10 −3 and 8 ϫ 10 −3 mbar͒; congruous to the trend of helicon wave dispersion relation, as shown in Fig. 4.…”

“…34 In the present parameter regime, contribution of displacement current is negligible. However, for long wavelength modes such as helicon waves, curvature effects of torus of small aspect ratio can no longer be neglected.…”

“…͑9͒ and ͑10͒, are very close to the magnitude of plasma current measured. Our estimations 34 show that the dynamo term ͗J ជ e ϫ B ជ ͘, generated by helicon wave induced helicity, plays a major role in the present current driven mechanism. Results of the comparative study of plasma current in different density regimes are shown in Table I.…”

“…35 is calculated using the plasma parameters obtained at present operational regime. The value of plasma resistivity obtained in present parameter regime is Ϸ8 ⍀ m. 34 Plasma resistivity plays an important role in deciding the magnitude of contribution of wave helicity in the plasma current driven at present operational regime. For high density plasma, lower plasma resistivity leads to enhanced wave induced helicity current drive.…”