Volume and surface rf power absorption in a helicon plasma source

Abstract

We consider the excitation and absorption of waves in a plasma cavity with parameters typical for helicon sources. rf power is shown to be transferred into the plasma via two channels. The first is realized by weakly damping helicon waves which are excited directly by the azimuthal parts of the antenna and penetrate into the bulk plasma. Electrostatic waves arising at a plasma edge owing to the linear mode conversion form the second channel. Strongly damped electrostatic waves can reach a plasma core at low magnetic fields only, while at high fields they deposit energy at the periphery of the plasma column. A principal fraction of the rf power is transferred into the plasma via the electrostatic channel, and so the power input turns out to be volume at low magnetic fields and surface at high fields. An enhanced volume input is possible at high fields in special anti-resonance regimes when the excitation of the electrostatic wave is suppressed. The effective collision frequency is introduced to describe effective damping of helicon waves arising due to their conversion into electrostatic waves. The results obtained permit explanation of both measured field profiles and the high absorption efficiency in helicon sources.

Quasistatic Plasma Sources : Physical Principles, Modelling Experiments, Application Aspects

“…JOURNAL DE PHYSIQUE IV absorbed in helicon sources via electrostatic waves [15,16]. Moreover, devices of this type can operate at densities below the cut-off for helicon waves [17].…”

“…Almost all the energy of helicons is concentrated in a magnetic field, whereas electrostatic waves reserve it mostly in a kinetic energy of electrons oscillations [16]. For this reason, any electron collisions (both with neutrals and ions) are extremely effective for the electrostatic wave as destroying the principal (kinetic) part of its energy.…”

“…The non-linear damping of helicon wave can arise as the beam-like parametric instability with the growth rate few-fold exceeding the driving frequency [18]. In a linear case, these mechanisms arise due to coupling of helicon wave to the electrostatic wave [15,16], that is are displayed as the mode conversion. It may occur both at the surface of a bounded plasma, or in a plasma bulk.…”

“…Such a mechanism of surface mode conversion results in that a principal fraction of the energy of helicon wave is transferred to the electrostatic wave. The efficiency of this mechanism is evaluated in terms of effective collision frequency which makes [16,191 where ro is the plasma radius, and = 2 nlk, a characteristic axial wavelength excited by antenna. Being very efficient, the surface conversion dominates in many situations as the damping mechanism for helicons.…”

“…This is the case if the plasma and wave parameters satisfy the condition of anti-resonance for the electrostatic wave excitation [20] Electrostatic waves excited at the plasma edge due to mode conversion propagate to the plasma centre and deposit energy via collisions. The depth of penetration is An electrostatic wave can penetrate into the bulk plasma at low magnetic fields only, Bo< B, [16].…”