Developing high performance RF heating scenarios on the WEST tokamak

Abstract

High power experiments, up to 9.2 MW with LHCD and ICRH, have been carried out in the full tungsten tokamak WEST. Quasi non inductive discharges have been achieved allowing to extend the plasma duration to 53s with stationary conditions in particular with respect to tungsten contamination. Transitions in H mode are obtained lasting up to 4s with weak energy increment at the power crossing the separatrix is close to the threshold. Hot L mode plasmas (Te(0)>3keV) with a confinement time following the ITER L96 scaling are routinely obtained. The weak aspect ratio dependence of this scaling law is confirmed. Tungsten accumulation is generally not an operational issue on WEST. Difficulty of burning through tungsten can prevent from accessing to a hot core plasma in the ramp-up phase or can lead to rapid collapse of the central temperature when radiation is enhanced by a slight decrease of the temperature. Apart few pulses post-boronization, the plasma radiation is rather high (Prad/Ptot~50%) and is dominated by tungsten. This fraction does not vary as the RF power is ramped up and is quite similar in ICRH and/or LHCD heated plasmas. An estimate of the contribution of the RF antennas to the plasma contamination in tungsten is given

Operating a full tungsten actively cooled tokamak: overview of WEST first phase of operation

“…Most of WEST experiments, in which the electron heating is dominant, were performed in deuterium, LSN and USN configurations at magnetic field 3.6-3.7 T, R ∼ 2.5 m, a ∼ 0.45 m, κ ∼ 1.3, δ ∼ 0.5, plasma current in the range of 0.3-0.7 MA (q 95 ∼ 3-6) and up to 1 MA, central electron density from 2.5-8.5 × 10 19 m −3 (Greenwald fraction = 0.3-0.8) [27]. RF power is provided by two LHCD launchers and three ICRH load resilient antennas [4].…”

“…The total injected power has reached a peak of 8.8 MW/0.5 s. Figure 7 illustrates a pulse with the five antennas active and a total RF power of ∼8 MW. The fraction of radiated power is generally 50%-55% in the range of parameters explored [27]. It has to be noted that the five antennas are equipped with W coated side limiters.…”

WEST actively cooled load resilient ion cyclotron resonance heating system results

“…When the isotopic ratio n H /(n H + n D ) is in the expected range for H minority heating scenarios (i.e. from 5 to 10%) and the toroidal phase properly tuned to dipole, the radiated power measured in 2019 and 2020 scales with the total power (ohmic + total RF coupled power) and weak specific effect of LH or ICRH RF power has been found [1,38]. This is illustrated in figure 14 for a large set of plasma scenarios, where each point represents a stable 200 ms time window during which the considered quantity has a relative change below 10%.…”

“…This is illustrated in figure 14 for a large set of plasma scenarios, where each point represents a stable 200 ms time window during which the considered quantity has a relative change below 10%. High-performance plasma experiments conducted on WEST in 2019 have proven that the ICRH system is a key player in achieving edge density pedestal and H-mode transitions [38,39]. Several multiple transitions from L to H modes have been obtained and an H-mode phase of up to 4 s has been sustained with the combination of ICRH and LHCD [1].…”

“…It results in oscillatory edge dynamics also impacting ICRH antennas, decreasing the coupling and eventually the coupled power. Addi- tionally, WEST scenarios are also currently hampered by the issue of burning through the tungsten radiation peak at 1.5-3 keV, due to a limited level of central electron heating including when applying ICRH [38]. Future work will concentrate on optimizing the RF heating scheme to ICRH + LHCD plasma scenarios, especially to increase the central temperature as soon as possible in the discharge.…”