Heat loads on JET plasma facing components from ICRF and LH wave absorption in the SOL

Abstract

In JET, lower hybrid (LH) and ion cyclotron resonance frequency (ICRF) wave absorption in the scrape-off layer can lead to enhanced heat fluxes on some plasma facing components (PFCs). Experiments have been carried out to characterize these heat loads in order to: (i) prepare JET operation with the Be wall which has a reduced power handling capability as compared with the carbon wall and (ii) better understand the physics driving these wave absorption phenomena and propose solutions for next generation systems to reduce them. When using ICRF, hot spots are observed on the antenna structures and on limiters close to the powered antennas and are explained by acceleration of ions in RF-rectified sheath potentials. High temperatures up to 800 °C can be reached on locations where a deposit has built up on tile surfaces. Modelling which takes into account the fast thermal response of surface layers can reproduce well the surface temperature measurements via infrared (IR) imaging, and allow evaluation of the heat fluxes local to active ICRF antennas. The flux scales linearly with the density at the antenna radius and with the antenna voltage. Strap phasing corresponding to wave spectra with lower k ∥ values can lead to a significant increase in hot spot intensity in agreement with antenna modelling that predicts, in that case, an increase in RF sheath rectification. LH absorption in front of the antenna through electron Landau damping of the wave with high N ∥ components generates hot spots precisely located on PFCs magnetically connected to the launcher. Analysis of the LH hot spot surface temperature from IR measurements allows a quantification of the power flux along the field lines: in the worst case scenario it is in the range 15–30 MW m−2. The main driving parameter is the LH power density along the horizontal rows of the launcher, the heat fluxes scaling roughly with the square of the LH power density. The local electron density in front of the grill increases with the LH launched power; this also enhances the intensity of the LH hot spots.

Self consistent radio-frequency wave propagation and peripheral direct current plasma biasing: Simplified three dimensional non-linear treatment in the “wide sheath” asymptotic regime

“…This parametric dependence is valuable for design and experimental studies. In particular, the RFinduced heat loads in the RF antenna vicinity were inferred experimentally to scale as E //ap 1 , 29,30 consistent with the first order theory. The particular simulation showed in Ref.…”

“…From a practical point of view, this is also a critical regime since experimentally the deleterious plasma-wall interaction related to the sheaths gets exacerbated at high RF power. 18,[28][29][30] Sections III and IV detail the asymptotic resolution for several configurations of the excitation and of the target plasma. The way to refine iteratively the solution from its asymptotic value is also outlined.…”

Full wave simulations of fast wave efficiency and power losses in the scrape-off layer of tokamak plasmas in mid/high harmonic and minority heating regimes*

“…However, the understanding of the interaction between ICRF antennas and the scrape-off layer (SOL) plasma is crucial in order to improve the performance of ICRF in a tokamak. In fact, many ICRF heating experiments [2,3,4,5,6,7,8,9] have found regimes in which significant fractions of the coupled RF power does not appear in the core. Experimental studies on erosion and impurity generation in the presence of ICRF have been performed on different machines [4,7,10,8,9].…”

“…In fact, many ICRF heating experiments [2,3,4,5,6,7,8,9] have found regimes in which significant fractions of the coupled RF power does not appear in the core. Experimental studies on erosion and impurity generation in the presence of ICRF have been performed on different machines [4,7,10,8,9]. In the recent years, a large effort has been made to understand the main physical mechanisms behind the interaction between RF waves and the SOL, such as parametric decay [11,3,12,13], sheath effects [14,15], etc..…”

ICRF specific plasma wall interactions in JET with the ITER-like wall

“…It is known that ICRF operation can lead to deposition of additional heat loads due to RF sheath effects, as has been observed with carbon wall in JET [1]. ICRF operation can also lead to release of Be and high-Z impurities such as W [2] and nickel Ni.…”