Profile measurements of the electron temperature on the ASDEX Upgrade, COMPASS, and ISTTOK tokamak using Thomson scattering, triple, and ball-pen probes

Abstract

The ball-pen probe (BPP) technique is used successfully to make profile measurements of the electron temperature on the ASDEX Upgrade (Axially Symmetric Divertor Experiment), COMPASS (COMPact ASSembly), and ISTTOK (Instituto Superior Tecnico TOKamak) tokamak. The electron temperature is provided by a combination of the BPP potential (ΦBPP) and the floating potential (Vfl) of the Langmuir probe (LP), which is compared with the Thomson scattering diagnostic on ASDEX Upgrade and COMPASS. Excellent agreement between the two diagnostics is obtained for circular and diverted plasmas and different heating mechanisms (Ohmic, NBI, ECRH) in deuterium discharges with the same formula Te = (ΦBPP - Vfl)/2.2. The comparative measurements of the electron temperature using BPP/LP and triple probe (TP) techniques on the ISTTOK tokamak show good agreement of averaged values only inside the separatrix. It was also found that the TP provides the electron temperature with significantly higher standard deviation than BPP/LP. However, the resulting values of both techniques are well in the phase with the maximum of cross-correlation function being 0.8.

Improved understanding of the ball-pen probe through particle-in-cell simulations

“…A combination of a BPP and a Langmuir probe can provide fast measurements of the electron temperature using a floating potential from each probe. By rearranging equation 1 for a Langmuir probe and a BPP we obtain This has yielded excellent agreement with Thomson scattering data on multiple tokamak experiments [27], [18]. For a planar Langmuir probe…”

Fast measurements of the electron temperature and parallel heat flux in ELMy H-mode on the COMPASS tokamak

“…Current-voltage characteristics of BPPs mounted on probe manipulators have been systematically studied in the SOL on COMPASS [9] and ASDEX Upgrade [8]. The idea of using the BPP to directly measure the plasma potential is based on the simple LP theory for Maxwellian plasma.…”

“…An alternative option to determine the electron temperature with a high temporal resolution is to use direct measurements of the plasma (Φ) and floating (V fl ) potentials. The electron temperature T e is consequently determined from the difference of both potentials normalized by the proper coefficient for a specific gas and magnetic field [8][9][10][11]. The floating potential is routinely measured by a Langmuir probe (LP) and the plasma potential can be directly measured by a ball-pen probe (BPP).…”

“…The floating potential is routinely measured by a Langmuir probe (LP) and the plasma potential can be directly measured by a ball-pen probe (BPP). The BPP mounted on the midplane reciprocating manipulator has been used to measure the plasma potential in the edge plasma of different fusion devices such as COMPASS [9,12], ASDEX Upgrade [8,9,13,14], MAST [15] and ISTTOK [11]. Recently, a new array of BPPs and dome-shaped LPs was installed in a divertor target of the COMPASS tokamak in order to investigate the plasma potential, electron temperature and parallel heat flux in the divertor region.…”

Progress in diagnostics of the COMPASS tokamak

“…This configuration is suitable for a statistical analysis of plasma structure parameters. A qualitative step forward has been reached by development of so called ball-pen probe (BPP) at IPP Prague, allowing both a direct measurement of the plasma potential and the electron temperature with a submicrosecond temporal resolution, and thus, calculation of the highly time resolved parallel heat flux density [19][20][21][22]. The BPP has a conically shaped collector retracted in an insulating shielding tube, which screens off an adjustable part of the electron current from the probe collector due to a much smaller gyro-radius of electrons.…”