Identification and measurement of core plasma turbulence by observation of fast electrons in TEXT-U

Abstract

A method to investigate the nature and level of the turbulence in the core of hot tokamak plasmas is discussed. The method is based on the energy-resolved observation of superthermal electrons by means of vertical electron cyclotron emission. The sensitivity of the electron distribution function and of the related emission spectra to both electrostatic and magnetic turbulence is first investigated theoretically. Then, the method is applied to the study of electron cyclotron resonance heated plasmas in the TEXT-U tokamak. All the measured spectra appear to be dominated by effects due to magnetic rather than to electrostatic turbulence. The experimental spectra are compared with the results of 3D Fokker-Planck simulations including radial diffusion, and the corresponding levels of magnetic turbulence are deduced.

Internal magnetic fluctuations and electron heat transport in the Tore Supra tokamak: Observation by cross-polarization scattering

“…Indirect investigations of internal small scale magnetic activities have been made through suprathermal particle or runaway electron transport [8,[17][18][19][20][21][22][23][24]. Frequency spectra arising from suprathermal ECE and soft X ray emission have been found to be compatible with magnetic transport [23,24].…”

“…Indirect investigations of internal small scale magnetic activities have been made through suprathermal particle or runaway electron transport [8,[17][18][19][20][21][22][23][24]. Frequency spectra arising from suprathermal ECE and soft X ray emission have been found to be compatible with magnetic transport [23,24]. Suprathermal confinement experiments at the edge of TEXT-U have shown that transport could be magnetic for runaway electrons but not for thermal particles [19,21].…”

Diagnostic techniques for measuring suprathermal electron dynamics in plasmas (invited)

“…79 The question of cross-field transport of ECH-generated suprathermal electrons was addressed by one such study, which found that a satisfactory match was only obtained if transport was included, and that magnetic-turbulence-induced diffusion was more successful than its electrostatic counterpart in explaining the observations. 80 In a horizontally viewing arrangement on the HFS, energy and emission locations are inextricably tied: the chosen frequency defines the product ␥R but not the two quantities separately. Radiation will therefore be collected from a region approximately one optical thickness deep as seen from the receiver.…”

The effect of ECRH on the electron velocity distribution function

“…: the cross-field transport of the current carrying suprathermal electrons [24]. Experimental estimates of the suprathermal transport level, generally expressed as a characteristic diffusivity, had been previously obtained primarily in lower hybrid heating experiments [50][51][52], although some cases with ECRH were also reported in the literature [29]. While the reported diffusivity values varied somewhat, they were generally smaller than or comparable to the bulk energy diffusivity.…”