Dynamical plasma response to additional heating

Abstract

Abstract-The response of a Tokamak plasma to an additional heating pulse is enigmatic. This paper presents results of the dynamical response on the electron temperature, deduced from the soft X-ray emissivity profile, to additional Alfven Wave Heating Power. Most of this study used quasi-steady state sinusoidal modulation of the delivered r.f. power, but data are also included on the step-function response at the end of the r.f. pulse, and the impulse response to short periodic r.f. pulses. These different approaches all show similar behaviour of the radial profile of the response phase, which is dominated by a central flat region within the q = 1 surface, with, at the higher frequencies used, a minimum phase of close to 180'. Moving further outside the q = 1 radius the phase increases extremely sharply. This basic character of the phase profile is not altered by varying the calculated power deposition profile but is still defined by the q = 1 radius, demonstrated by a plasma current scan at constant toroidal field. In addition to the soft X-ray flux baseline modulation, the sawtooth period is modulated, but out of phase with r.f. power. These effects have been studied as a function of modulation frequency, plasma current, r.f. power and Alfven wave mode.

Torsional Alfvén wave excitation by mode conversion in a tokamak plasma

“…(In addition to the transfer of power to the n = -2 surfaces, significant power is expected to be transferred to the |n| = 1 surfaces near the edge of the main plasma.) This should be contrasted to the results of Joye et al [4], who found no evidence of significant coupling of power from an unshielded antenna to Alfve'n resonance surfaces.…”

“…The major effort has been that of the TCA group in Lausanne. Their experiments showed that energy deposition in the plasma is determined predominantly by an uncontrolled density increase [3]; experiments in which the soft X-ray emissivity profile was measured while modulating the Alfve'n heating power exhibited no correspondence between the power deposition profile and the position of the Alfve'n resonance surfaces [4], leading to the conclusion that Alfve'n resonant absorption is not the major power deposition mechanism. Borg et al [5] suggested that this may be due to Langmuir currents flowing from the antennas to the scrape-off plasma.…”

Experimental study of the vertical stability of high decay index plasmas in the DIII-D tokamak

“…The goal is to estimate the modelled transfer function H*(s) which, when stimulated by the given experimental input x(t), in our case the square-wave modulation, outputs a waveform y(t), which is as similar as possible to the measured experimental waveform y(t). A system identification tool based on this diagram [5] had already been developed for dynamical studies on TCA [6]. The transfer function of the model is of the form H*(s) = g no- (10) Such a form represents any physical system described by ordinary differential equations.…”

Experimental and numerical investigations of electron transport enhancement by electron-cyclotron plasma-wave interaction in tokamaks

“…By using a power-modulated heating source in a quasi steady-state plasma, it is possible to study its dynamical response to this perturbation and to deduce some transport-related properties such as diffusion or convection coefficients. Historically, this technique has been widely used to study electron heat transport in tokamaks such as DIII-D (EC waves) [18], TFR (EC waves) [19], TCA (Alfvèn waves) [20], JET (Ion-Cyclotron waves) [21] or DITE (EC waves) [22], but also in stellarators such as W7-A (EC waves) [23]. A multi-machine analysis [24], including ASDEX, TCV, RTP and TCU (EC waves), Tore Supra (fast wave electron heating) and JET (combination of Ion-Cyclotron heating and neutral beam heating) has shown that electron heat transport in the core is governed by turbulent transport, except near the magnetic axis (inside the q = 1 surface) where MHD activity dominates.…”