Ion temperature fluctuation measurements using a retarding field analyzer

Abstract

The retarding field analyzer (RFA) is a widely used diagnostic tool for the ion temperature measurement in the scrape-off-layer (SOL) of the thermonuclear plasma devices. However, the temporal resolution in the standard RFA application is restricted to the ms timescale. In this paper, a dc operation of the RFA is considered, which allows for the measurement of the plasma ion temperature fluctuations. The method is based on the relation for the RFA current-voltage (I-V) characteristic resulted from a common RFA model of shifted Maxwellian distribution of the analyzed ions, and the measurements of two points on the exponentially decaying region of the I-V characteristic with two differently dc biased RFA electrodes. The method has been tested and compared with conventional RFA measurements of the ion temperature in the tokamak ISTTOK SOL plasma. An ion temperature of T(i) = 17 eV is obtained near the limiter position. The agreement between the results of the two methods is within ∼25%. The amplitude of the ion temperature fluctuations is found to be around 5 eV at this location. The method has been validated by taking into account the effect of fluctuations in the plasma potential and the noise contamination, proving the reliability of the results obtained. Finally, constrains to the method application are discussed that include a negligible electron emission from the RFA grids and the restriction to operate in the exponentially decaying region of the I-V characteristic.

Ion temperature fluctuations in the ASDEX Upgrade scrape-off layer

“…The measurements of the ion temperature in the filaments, fil i T would also provide useful background information for turbulence modelling. Unfortunately, with the exception of some preliminary measurements on CASTOR [11] and ISTTOK [12], The i-side (e-side) RFA faces the outer (inner) divertor along the magnetic field lines.…”

Investigation of the Ion Energy Transport in the Scrape-Off Layer on the J-TEXT Tokamak Using a Retarding Field Analyzer

“…Co-and counter-side ion temperature profiles for Configuration (The result indicates that T i drops from 80 eV to 20 eV within the 13 mm of radial distance. Decayed ion temperature profiles across the SOL have also been observed in Refs [3,[17][18][19][20][21][22][23][24]…”

“…To investigate the ion energy transport, a reliable measurement of the T i profile in the SOL is needed. The most widely accepted diagnostic for the SOL and boundary T i measurements is the RFA, which have been applied to measure the ion temperature on many tokamaks, such as JET [17] , Alcator C-Mod [18,19] , MAST [20] , Tore Supra [11,21] , ISTTOK [22] , STOR-M [23] , and HL-2A [24] .…”

Measurements of the electron temperature fluctuations with a retarding field analyzer on the tokamak ISTTOK

“…10, the operational ranges of the RFA, where δT e = T e /T e is below 10% for the measured T e , are: 2 > e V r /T e > 0.5 (V r1 = V shift ) and 0.3 > e(V r1 −V shift )/T e > 0. For an average electron temperature of T eav ∼ 6 eV and shift potential, V shift ∼ 2 V during the discharge measured in the standard swept mode of RFA operation we have 12 V ≥ V r ≥ 3 V and 4 V ≥ V r1 ≥ 2 V. Notice that it is advantageous to operate close to the low limits of V r1 and V r due to the decrease in the signal-to noise ratio with increasing retarding potential.…”