Bi-Maxwellian electron energy distribution function in the vicinity of the last closed flux surface in fusion plasma

Abstract

The first-derivative probe technique was applied to derive data for plasma parameters from the IV Langmuir probe characteristics measured in the plasma boundary region in the COMPASS tokamak and in the TJ-II stellarator. It is shown that in the COMPASS tokamak in the vicinity of the Last Closed Flux Surface (LCFS) the Electron Energy Distribution Function (EEDF) is bi-Maxwellian with the low-temperature electron fraction predominating over the higher temperature one, whereas in the far scrape off layer (SOL) the EEDF is Maxwellian. In the TJ-II stellarator during NBI heated plasma the EEDF in the confined plasma and close to the LCFS is bi-Maxwellian while in the far SOL the EEDF is Maxwellian. In contrast, during the ECR heating phase of the discharge both in the confined plasma and in the SOL the EEDF is bi-Maxwellian. The mechanism for the appearance of a bi-Maxwellian EEDF in the vicinity of the LCFS is discussed. The comparison of the results from probe measurements with ASTRA package and EIRENE code calculations suggests that the main reason of the appearance of a bi-Maxwellian EEDF in the vicinity of the LCFS is the ionization of the neutral atoms. Results for the electron temperatures and densities obtained by the first-derivative probe technique in the COMPASS tokamak and in the TJ-II stellarator were used to evaluate the radial distribution of the parallel power flux density. It is shown that in the vicinity of the LCFS where the EEDF is bi-Maxwellian, the radial distribution of the parallel power flux density is double exponential. It is pointed that in calculations of the parallel power flux density at the LCFS the energy losses from ionization mechanisms have to be taken into account.

Interaction of dust ion acoustic solitons with cubic nonlinearity in a magnetized dusty plasma with () distributed electrons

“…[ 16–19 ] Likewise, many laboratory experiments on Tokamaks indicated the non‐Maxwellian behaviour of electrons. [ 20–22 ] It has been found that electrons are distributed in space plasmas with modified shapes both at low and high energies. Qureshi et al, [ 12 ] therefore, devised a new non‐Maxwellian distribution function, namely, generalized () distribution function comprising two spectral indices and to construct the electron distribution function in agreement with the observations in the terrestrial magnetosheath.…”

Measurement of the Local Electron Temperature in Self-Compressed Plasma Stream

“…Electric probes are among the most reliable diagnostic tools among the contact methods used to measure plasma parameters with sufficiently high temporal and spatial resolution in various devices [10 -16], including tokamaks and stellarators [17,18]. Sometimes, when the electron energy distribution function (EEDF) consists of a bi-Maxwellian with well-separated temperatures [17 -19], the two temperatures can be obtained by straightline fits on the semi-logarithmic discharge-voltage characteristic.…”

Formation of solitons and shocks in toroidal ion temperature gradient mode in the presence ofnon‐Maxwellianelectrons

“…[21] Later presence of bi-Maxwellian distribution was observed in COMPASS Tokamak and TJ-2 stellarator by using the first-derivative technique. [22] Experimental evidences of presence of hot electrons in Tokamak plasma has helped in better understanding of various phenomena. Gunn et al modified the theory of Mach probe by including bi-Maxwellian electron distribution which influenced the measured Mach number and ion sound speed.…”