Estimation of heat loads on the wall structures in parasitic absorption of lower hybrid power

Abstract

Parasitic absorption of the short wavelength modes of the LH spectrum is a probable reason for the hot spots seen in the grill region of several tokamaks. Experiments suggest that the heat loads on the wall structures depend on the coupled power. In this work, the parasitic absorption of LH power was studied with self-consistent particle-in-cell simulations. The launched spectra were obtained from the SWAN coupling code. The power and temperature dependences of the absorption in the near field of the LH grill were investigated with a series of simulations. The parasitic absorption was found to grow from 0.6 to 1.1% when the coupled power increased from 26 to 67 MW/m2. When the edge temperature rose from 12.5 to 100 eV, the absorption increased from 0.4 to 1.7%. The maximum kinetic energies were between 0.6 and 1.8 keV. Estimates for the heat loads and surface temperature of the grill limiter are also obtained. The absorption leads to heat loads between 1.5 and 13 MW/m2 and surface temperatures of 510-2390° C.

Radially scanned probe measurements in front of the CASTOR lower hybrid antenna

“…The main differences can be summarized as follows: (i) the cross-correlation coefficient of the V fl signals reaches for all radial positions a value nearly one during the OH (Fig.4), i.e. practically no measurable toroidal electric field is present in this discharge phase; (ii) the cross-correlation of the V fl signals is practically zero during LHW phase deeper in the plasma, while it is recovering just at the radius of the V fl "well" layer; (iii) together with recovering of the correlation, a certain time shift of the cross-correlation function maximum appears in this radially narrow layer; this fact indicates establishment of a toroidal rotation (with a velocity several km/s) of the plasma fluctuations; (iv) simultaneously, an expressive long-living frequency component about 50kHz (visible in the cross-correlation function) appears at the radius of the "well" layer; (v) this 50kHz component existence is confirmed also directly by FFT analysis of V fl signal, see Fig.5; moreover it may be seen from the figure that 50kHz component is slightly enhanced at all radial positions during the LHW (in contradiction to the OH phase), but it is markedly increased just only inside the "well" layer (evidently result of a regular poloidal plasma rotation, observed also in W7-AS under ECRH, see [7]); (vi) amplitude of the toroidal electric field E tor is strongly increased during LHW phase (Fig.6) at all probe radii as expected (see also the point (i) above); (vii) this amplitude increase during the LHW is caused first of all by a massive enhancement of the low frequency component of the E tor spectrum (spectrum has a character of "white noise" during OH), see Fig.7; (viii) however, this distinct difference in spectrum is not in any case the most profound in the "well" layer, where the process of acceleration is supposed to take place; this fact could indicate that the fluctuating toroidal electric field, detected in the experiment, participates in the particle acceleration in the very narrow layer in front of the grill mouth and, in this way, it is absorbed there (this would be similar to the absorption of the high LH field harmonics observed in numerical simulations [8] just in a similar very narrow layer close to the antenna). Fig.…”

“…(v) this 50kHz component existence is confirmed also directly by FFT analysis of V fl signal, see Fig.5; moreover it may be seen from the figure that 50kHz component is slightly enhanced at all radial positions during the LHW (in contradiction to the OH phase), but it is markedly increased just only inside the "well" layer (evidently result of a regular poloidal plasma rotation, observed also in W7-AS under ECRH, see [7]); Fig. 6 Comparison of time course of the toroidal electric field for OH and LHW discharge phases, evaluated for the same three probe radii as given in Fig.4. (vi) amplitude of the toroidal electric field E tor is strongly increased during LHW phase ( Fig.6) at all probe radii as expected (see also the point (i) above); (vii) this amplitude increase during the LHW is caused first of all by a massive enhancement of the low frequency component of the E tor spectrum (spectrum has a character of "white noise" during OH), see Fig.7; (viii) however, this distinct difference in spectrum is not in any case the most profound in the "well" layer, where the process of acceleration is supposed to take place; this fact could indicate that the fluctuating toroidal electric field, detected in the experiment, participates in the particle acceleration in the very narrow layer in front of the grill mouth and, in this way, it is absorbed there (this would be similar to the absorption of the high LH field harmonics observed in numerical simulations [8] just in a similar very narrow layer close to the antenna). Fig.…”

Measurement of lower hybrid hot spots using a retarding field analyzer in Tore Supra

“…It is not known how deep into the plasma they might be observed because the reciprocation depth of the RFA is limited to the LCFS. These observations cannot be explained by the standard theory of parasitic absorption [3] that predicts that waves with high refractive index should be totally absorbed within at most 5 mm from the grill [4], and that the electron current should be stationary in time. points where the time-averaged electron current was more negative than À50 lA or where 10% of the electron bursts were more negative than À20 lA.…”

“…According to theory [3], Landau damping transfers the power carried by the high refractive indices n // of the wave to thermal SOL electrons with energies of a few tens of eV and accelerates them up to a few keV. The high n // spectral components are expected to be absorbed immediately in front of the LH grill within a few mm [4]. Combined Langmuir and emissive probe measurements in front of a low power LH grill in the CASTOR tokamak demonstrated large sheath potentials in a thin layer, which, although indirect, could be consistent with the existence of suprathermal electrons [5].…”

Relativistic Fermi-Ulam map: Application to WEGA stellarator lower hybrid power operation

“…, determined from the equilibrium code in Otte et al 23 . b) Safety factor corresponding to B0.5 T of the respective experiment is q5.…”