Electron density measurements from right-hand cutoff of ECE in the TCABR tokamak

Abstract

In tokamak machines with low toroidal magnetic fields and high plasma densities, the accessibility conditions impose restrictions to the detection of the Electron Cyclotron Emission ( ECE ). In these machines, the righthand cutoff condition can be used as an independent method to determine the local electron density from the ECE data in thermal discharges. In this paper is shown the results obatined from the detection of ECE radiation in the TCABR tokamak, in operation at the Institute of Physics of University of São Paulo. The effect of the ECE radiation cutoff was observed for different radial positions of the plasma column. To reach the ECE cutoff condition, the electron density was increased monotonically by the use of an external gas puffing system. For sufficient high densities, the emission at some frequencies is cutoff and the first and the last frequency to be cutoff depends on the shape of the density profile. These measurements do not require the plasma to be optically thick. It was observed that, for a toroidal field BO = 1.14 T , the first cutoff of ECE occurs for a radial position r ∼ = 5 cm . From these measurements the radial electron density was determined. For a symmetric parabolic profile n e = n eo [1 − (r/a) 2 ] α , values of α between 0.86 and 0.97 were experimentally obtained. A good agreement of these values with those obtained from the microwave interferometer measurements ( α ≈ 0.85 ) was found. Therefore, the ECE right-hand cutoff constitutes an independent method to obtain information about the electron density profile.

Comparative electron temperature measurements of Thomson scattering and electron cyclotron emission diagnostics in TCABR plasmas

“…The anticipated exception to this agreement occurs in discharges for which plasma densities are close to the electron cyclotron emission ͑ECE͒ cutoff. 4 …”

ECE radiometry in the TCABR tokamak

“…Time and radial profi les for the electron temperature ( 3D plot ) obtained in the TCABR. In this discharge, the density is higher than the cutoff and we can observe a reduction in the detected ECE radiation [13] between 40 and 50 ms. …”

“…We verify that, for the interval between 30 and 70 ms , there is a decrease of the radiation intensity due to the increase of the plasma electron density that becomes higher than the cutoff density. This behavior of the detected ECE radiation can be used to obtain information on the radial electron density profi le [13].…”

Optical thickness corrections to ECE measurement of electron temperature in IR-T1 tokamak

“…In this figure one can observe the cutoff and resonance regime of cyclotron harmonic frequency. The right-and left-hand cutoff frequencies are given by (Fonseca et al 2004; Figure 3. Topology of characteristic plasma frequencies such as the first (ω ce ) and second (2ω ce ) electron cyclotron frequency harmonic of the ECE radiation, electron plasma frequency (ω p ), right-hand cutoff frequency (ω R ), left-hand cutoff frequency (ω L ), and upper hybrid resonance frequency (ω uh ) for IR-T1 tokamak plasma parameters.…”

“…The optical depth of tokamak plasma is related to the plasma absorption α (ω) (Hartfuss and HÄse 2004). For τ (ω) > 1, plasma is optically thick (Silva et al 2004), and the correction factor is close to one (F (τ(ω), ρ) ≈ 1). The correction factor depends on both optical depth τ (ω) and reflection coefficient of the vessel walls (ρ) (Pandya et al 2007).…”