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A European project was undertaken to improve the available SOL ICRF physics simulation tools and confront them with measurements. This paper first reviews code upgrades within the project. Using the multi-physics finite element solver COMSOL, the SSWICH code couples RF full-wave propagation with DC plasma biasing over “antenna-scale” 2D (toroidal/radial) domains, via non-linear RF and DC sheath boundary conditions (SBCs) applied at shaped plasma-facing boundaries. For the different modules and associated SBCs, more elaborate basic research in RF-sheath physics, SOL turbulent transport and applied mathematics, generally over smaller spatial scales, guides code improvement. The available simulation tools were applied to interpret experimental observations on various tokamaks. We focus on robust qualitative results common to several devices: the spatial distribution of RF-induced DC bias; left-right asymmetries over strap power unbalance; parametric dependence and antenna electrical tuning; DC SOL biasing far from the antennas, and RF-induced density modifications. From these results we try to identify the relevant physical ingredients necessary to reproduce the measurements, e.g. accurate radiated field maps from 3D antenna codes, spatial proximity effects from wave evanescence in the near RF field, or DC current transport. Pending issues towards quantitative predictions are also outlined.

The lower hybrid current drive efficiency on the EAST tokamak is estimated on a large database of low loop voltage discharges (VL of these discharges, can account for the high efficiency according to the expected scaling with Zeff and . Modelling with a ray-tracing code coupled to a Fokker-Planck solver supports this result, assuming that the fast electron transport is reduced in the zero loop voltage discharge with high efficiency.

We study the effect of lower hybrid waves on edge turbulence using a set of two Langmuir probes inserted in the scrape-off layer of the Tore Supra tokamak. We use the cross-correlation coefficient to assess the interplay between turbulence and the LH waves. We found that turbulence is affected at high plasma current even at relatively low LH powers. When the plasma current is below 1 MA, this effect disappears. Finally, we show that the level of fluctuations and the cross-correlation amplitude are correlated whereas the increase of the first leads to the increase of the second. The role of the plasma current could reflect that the exchange of the RF waves with edge turbulence is occurring in the closed field lines rather in the open field lines range.

The lower hybrid current drive (LHCD) system plays a crucial role in the mission of the Experimental Advanced Superconducting Tokamak (EAST) and is a prerequisite for reaching long pulse, high confinement plasmas on EAST [1, 2]. LHCD experiments and modelling [3] have been carried out on EAST in 2015-2016, with the aim to optimising EAST long pulse scenarios, and at the same time gain experience for the exploitation of WEST [4]. Experiments have been carried out to study the LH current drive efficiency in different plasma configurations (Upper Single Null and Lower Single Null). The effect of the gas feed location on the LH wave coupling was investigated by comparing gas fuelling from high field side, low field side and upper divertor. In view of long pulse H-mode scenarios, a series of H-mode experiments were conducted where all the heating power was provided by RF heating methods only, i.e. LHCD, ECRH and ICRH. H-modes were sustained in both Upper Single Null (W divertor) and Lower Single Null (carbon divertor) configurations, with loop voltage maintained as low as 50 mV.

Inverted 3He and D ion cyclotron minority heating scenarios were recently tested in JET-ILW. They confirm the good heating efficiency at low concentrations of ∼3%. The 3He minority heating scheme is only modestly affected by the change from a carbon (JET-C) to a Beryllium (JET-ILW) wall but unlike what was the case in JET-C, the intrinsic Be ions D-like particles in terms of charge-over-mass ratio do not prevent the D (or 4He) minority regime from being exploited. Direct and indirect evidence of the existence of fast particle subpopulations was found in both cases.

During the 2015-2016 JET campaigns many efforts have been devoted to the exploration of high performance plasma scenarios envisaged for ITER operation. In this paper we model the combined ICRF+NBI heating in selected key hybrid discharges using PION. The antenna frequency was tuned to match the cyclotron frequency of minority hydrogen (H) at the center of the tokamak coinciding with the second harmonic cyclotron resonance of deuterium. The modelling takes into account the synergy between ICRF and NBI heating through the second harmonic cyclotron resonance of deuterium beam ions which allows us to assess its impact on the neutron rate RNT. We evaluate the influence of H concentration which was varied in different discharges in order to test their role in the heating performance. According to our modelling, the ICRF enhancement of RNT increases by decreasing the H concentration which increases the ICRF power absorbed by deuterons. We find that in the recent hybrid discharges this ICRF enhancement was in the range of 10-25%. Finally, we extrapolate the results to D-T and find that the best performing hybrid discharges correspond to an equivalent fusion power of ∼7.0 MW in D-T.

Combined ICRF and NBI heating played a key role in achieving the world-record fusion yield in the first deuterium-tritium campaign at the JET tokamak in 1997. The current plans for JET include new experiments with deuterium-tritium (D-T) plasmas with more ITER-like conditions given the recently installed ITER-like wall (ILW). In the 2015-2016 campaigns, significant efforts have been devoted to the development of high-performance plasma scenarios compatible with ILW in preparation of the forthcoming D-T campaign. Good progress was made in both the inductive (baseline) and the hybrid scenario: a new record JET ILW fusion yield with a significantly extended duration of the high-performance phase was achieved. This paper reports on the progress with the hybrid scenario which is a candidate for ITER longpulse operation (∼1000 s) thanks to its improved normalized confinement, reduced plasma current and higher plasma beta with respect to the ITER reference baseline scenario. The combined NBI+ICRF power in the hybrid scenario was increased to 33 MW and the record fusion yield, averaged over 100 ms, to 2.9x1016 neutrons/s from the 2014 ILW fusion record of 2.3x1016 neutrons/s. Impurity control with ICRF waves was one of the key means for extending the duration of the high-performance phase. The main results are reviewed covering both key core and edge plasma issues.

An international R&D program for lower-hybrid current drive (LHCD) in ITER is being conducted to deliver 20 MW (CW) using 500 kW klystrons at 5 GHz, with N ||peak = 2.0 ± 0.2 for different plasma scenarios. The launcher is based on the passive-active mulitjunction (PAM), a concept more resilient to conditions expected at the plasma edge, notably densities close to cut-off ( n ec ) and ELM activity, which lead to significant and abrupt reflection of RF power from the plasma, but even under which it may still attain extremely low power reflection coefficients at the input ( R ∼ 1%). It has also a robust and shielded structure; is suitable for long-pulse operation; and has been validated experimentally on FTU and Tore Supra. Here the focus is on the PAM section of the launcher, and the objective is to explore, under broad plasma loading – from n ec to 10 n ec – the impact that design parameters such as the junction lengths, phase-shifter heights, and output waveguide widths have on its performance, particularly on R and on the E-fields inside its waveguides; and to explore also a configuration with a different phase-shifter arrangement, the so-called alternative design . [ABSTRACT FROM AUTHOR]

During the 2011 experimental campaign, one of the three ion cyclotron resonance heating (ICRH) antennas in the Tore Supra tokamak was equipped with a new type of Faraday screen (FS). The new design aimed at minimizing the integrated parallel electric field over long field lines as well as increasing the heat exhaust capability of the actively cooled screen. It proved to be inefficient for attenuating the radio-frequency (RF)-sheaths on the screen itself on the contrary to the heat exhaust concept that allowed operation despite higher heat fluxes on the antenna. In parallel, a new approach has been proposed to model self-consistently RF sheaths: the SSWICH (Self-consistent Sheaths and Waves for IC Heating) code. Simulations results from SSWICH coupled with the TOPICA antenna code were able to reproduce the difference between the two FS designs and part of the spatial pattern of heat loads and Langmuir probe floating potential. The poloidal pattern is a reliable result that mainly depends on the electrical design of the antenna while the radial pattern is on the contrary highly sensitive to loosely constrained parameters such as perpendicular conductivity that generates a DC current circulation from the private region inside the antenna limiters to the free scrape off layer outside these limiters. Moreover, the cantilevered bars seem to be the element in the screen design that enhanced the plasma potential. [ABSTRACT FROM AUTHOR]

Development of a plasma facing module using Cold Isostatic Pressing Graphite (CIPG) was successfully achieved for use in driving a heat-resistant Lower Hybrid Current Drive (LHCD) antenna. A thin stainless film (10 μm), a molybdenum film (10 μm), and a copper film (50 μm) were overlaid on the CIPG materials, the CIPG surfaces were successfully coated with the Cu-layer by diffusion bonding method. This module whose length is 206 mm, has four waveguides, and a water cooling channel. High rf-power long pulse operations with water cooling were successfully tested up to 250 kW (125 MW/m2)/700 s, a stationary temperature and vacuum pressure was performed. But the maximum rf transmission power was limited to 47 MW/m2 for a plasma facing module using Carbon Fiber Composite (CFC) due to poor Cu-plating, a power density large by more than a factor 2.5 was achieved with the CIPG module. The rf power density which meets the requirement of the rf electric field of 5 kV/cm for a design of the LHCD antenna in International Thermonuclear Experimental Reactor (ITER) was successfully performed. [ABSTRACT FROM AUTHOR]

The WEST-project ( W -tungsten E nvironment in S teady-state T okamak) involves equipping Tore Supra with a full tungsten divertor, capable of withstanding heat load of 10 MW/m 2 in steady-state conditions, in discharges sustained by Lower Hybrid Current Drive (LHCD). The LHCD generator, recently upgraded to deliver 9.2 MW/1000 s, is equipped with sixteen TH2103C klystrons powering two launchers. The WEST transformation involves reducing the plasma volume, thus moving the launchers ∼10 cm closer to the tokamak centre. The toroidal curvature of the launchers no longer fits the plasma curvature due to the strong magnetic field ripple effect, leading to a degradation of the LH wave coupling, especially with the Full Active Multijunction Launcher (FAM). The toroidal curvature radius of the FAM launcher mouth will therefore be reshaped from 1700 mm to 2300 mm. The machining process is described in this article. In order to improve the coupling of the LH wave, the local gas injection has been modified to help to meet the requirement of 7 MW/1000 s of LH power coupled to the plasma in the WEST scenarios. Finally, the curvature radius of the waveguide septa are rounded to minimize the excitation of suprathermal electrons near the plasma edge, which can induce high power loads on the plasma facing components. [ABSTRACT FROM AUTHOR]

Three new ion cyclotron resonance heating (ICRH) launchers have been designed for the WEST project (W-Tungsten Environment in Steady-state Tokamak) in order to operate at 3 MW/launcher for 30 s and 1 MW/launcher for 1000 s on H-mode plasmas. These new launchers will be to date the first ICRH launchers to offer the unique combination of continuous-wave (CW) operation at high power and load tolerance capabilities for coupling on H-mode edge. The radio-frequency (RF) design optimization process has been carried out using full-wave electromagnetic solvers combined with electric circuit calculations. Cavity modes occurring between the launchers structures and the vacuum vessel ports have been evaluated and cleared out. [ABSTRACT FROM AUTHOR]