SOL characterization and LH coupling measurements on JET in ITER-relevant conditions

Abstract

Lower hybrid (LH) current drive experiments have been carried out on JET with an antenna-plasma separatrix distance varying between 0.09 and 0.15 m, and LH power in the range 0-3.2 MW. For different plasma configurations, the electron density n e of the scrape-off layer has been studied by the mean of a reciprocating Langmuir probe magnetically connected to the LH antenna. For pulses in the high confinement regime (H mode) characterized by strong particle bursts in the plasma edge, the edge localized modes (ELMs), profiles of the saturation current (J sat ) are obtained with a sufficient time resolution to distinguish 'between ELMs' and during the rise and decay of the ELMs.It is found that gas injection from a valve located near the LH launcher and magnetically connected to it allows one to raise the density and improve the LH coupling. The J sat profiles indicate quite clearly that this density rise affects mainly the plasma layer in front of the antenna with a typical thickness of 5 cm. The resulting profile can be extremely flat in this region. The effect of the near-launcher gas injection but also of the LH power and the total gas injection on the density at the wall is quantitatively documented. It is shown in particular that with increasing LH power, the required gas injection for obtaining good LH coupling is decreasing, with no saturation obtained so far. Effect of the ELMs on the LH coupling is also discussed. Modelling with the EDGE2D code indicates that such flat profiles of J sat /n e can be obtained when LH power dissipation is taken into account. Detailed analysis of the heat flux carried by

Lower hybrid current drive for the steady-state scenario

“…This cut-off density which scales like the square of the frequency is in the 2-8×10 17 m -3 range for a frequency in the 3.7-8GHz range. This condition is generally fulfilled naturally in the Scrape-Off Layer (SOL) but can be critical in particular in Hmode discharges and requires sometimes particle fuelling near the antenna for rising the density by local LH-induced ionization [11,21]. The wave can then propagate into the plasma as long as the density is lower than a critical value which depends on the local value of the electron density and magnetic field.…”

Impurity Radiation for Detecting Arcs during High Lower Hybrid Power Transmission at JET

“…The database shows clearly a regular increase of the radiation with the LH power (with a power exponent between 1 and 2).. This is also measured by the visible spectroscopy camera viewing the top of the antenna and suggests an increase of density in front of the launcher due to enhanced ionization provided by interaction of the LHCD wave with the plasma edge [3]. For strong radiation cases, the maximum occurs on BOLO2, near the mid-plane, in 60% of the pulses.…”

“…In most cases the maximum of radiation is measured on BOLO1 (45%) or BOLO2 (48%). For arc-free discharges, this could indicate a higher density in the upper part of the launcher resulting from the mismatch of the poloidal shape of the antenna with respect of the magnetic surfaces [3]. The database shows clearly a regular increase of the radiation with the LH power (with a power exponent between 1 and 2)..…”

Optimization of a lower hybrid current drive launcher for ITER

“…The launcher should also sustain high thermal and electromechanical loads (plasma radiation, particle fluxes, RF losses and disruptions) and the resulting stresses, besides neutron activation -required for ITER plasma facing components. In this work density is assumed to vary from ∼n ec (between ELMs) to ∼10n ec (during ELMs) [3], specifically 3-30 × 10 17 m −3 , with a profile consisting of a step n e at the grill aperture, followed by a linear gradient n e having a decay length = n e / n e = 2 cm.…”