Alfvén-wave heating of a high-beta plasma column

Abstract

Alfvén-wave heating experiments are performed on the High Beta Q Machine (HBQM) [Phys. Fluids 25, 262 (1982)] straight plasma column. An investigation is made of Alfvén-wave energy absorption by the Alfvén-resonant layer at which the local shear Alfvén velocity is equal to the phase velocity of the driven plasma standing wave. The radial position of the resonance in the plasma is defined by the equilibrium magnetic field diamagnetic profile and nearly constant electron number density profile. The experiment uses a 250 MW ringing LC circuit in the 615–720 kHz frequency range with the capacitors driving the HBQM compression coil segments that are connected to form a 2.0 m long m=±1 antenna configuration with axial wave number kz=15.7 m−1. Experiments were conducted on both hydrogen and deuterium plasmas. In the hydrogen case the Alfvén resonance is predicted to lie inside the plasma. Internal magnetic field probes were used to measure both the rf and equilibrium magnetic fields. The rf heating produces a larger change in the hydrogen plasma axial magnetic field profile that is centered around r=6 cm, near the Alfvén-resonance location. The rf B̃θ signal shows a phase reversal at the same radius. Other measurements show a 25% increase in plasma pressure for hydrogen. The antenna resistance increases from 0.06 Ω vacuum value to approximately 0.2 Ω in the presence of hydrogen plasma with a 31% absorption efficiency. The rf-induced effects in the deuterium plasma are 20%–50% smaller, agreeing with estimates based on collisional viscous damping, or collisonless model with finite ion cyclotron and Larmor radius effects. In the hydrogen case both these models predict insufficient energy absorption to explain observed heating. However, a magnetofluid calculation taking account of the singularity at the Alfvén-resonant layer reproduces the value of the antenna resistance for the hydrogen case.

Magnetohydrodynamic ponderomotive forces generated about Alfvén resonance layers

“…In contrast, rf momentum M x directed away from the resonance layer would lead to a flat plasma density profile, or even to one that is hollow. These theoretical conclusions are supported by the experiments of Cekic, Nelson, and Ribe, 33 who found steepening of the plasma density profile in the Alfvén resonance region in heating experiments with very high rf power. Petržílka 34,35 and Elfimov, Petržílka, and Tataronis 3 arrived at similar conclusions in their studies of plasma transport produced by ponderomotive forces of helicon waves in a cylindrical configuration.…”

Laboratory observation of Alfvén resonance

“…FLR has enjoyed attention from both fusion and space plasma communities. Like the results presented here, studies have been done in linear machines [Tsushima et al, 1982;Cekic et al, 1991], but the focus of these experiments was on energy absorption. To our knowledge, a clear spectrum of Alfv•n resonances has not been obtained in the laboratory.…”

Heating experiment of field-reversed configuration plasma by low-frequency magnetic pulse

“…6 The plasma heating using the Alfvén resonance has been demonstrated in various plasma configurations. [7][8][9][10] In this letter, we report an effective wave heating method of the FRC plasma by applying a low-frequency magnetic field. The experiment is performed on the confinement section of the FRC Injection Experiment ͑FIX͒ 11 device.…”