High density LHRF experiments in Alcator C-Mod and implications for reactor scale devices

Abstract

Abstract. Parametric Decay Instabilities (PDI) appear to be an ubiquitous feature of Lower Hybrid Current Drive (LHCD) experiments at high density. In density ramp experiments in Alcator C-Mod and other machines the onset of PDI activity has been well correlated with a decrease in current drive efficiency and production of fast electron Bremsstrahlung. However whether PDI is the primary cause of the "density limit", and if so by exactly what mechanism (beyond the obvious one of pump depletion) has not been clearly established. In order to further understand the connection, the frequency spectrum of PDI activity occurring during Alcator C-Mod LHCD experiments has been explored in detail by means of a number of RF probes distributed around the periphery of the C-Mod tokamak including a probe imbedded in the inner wall. The results show that i) the excited spectra consists mainly of a few discrete ion cyclotron quasi-modes, which have higher growth than the ion sound branch; ii) PDI activity can begin either at the inner or outer wall, depending on magnetic configuration; iii) the frequencies of the IC quasi-modes correspond to the magnetic field strength close to the LFS or HFS separatrix; and iv) although PDI activity may initiate near the inner separatrix, the loss in fast electron Bremsstrahlung is best correlated with the appearance of IC quasimodes characteristic of the magnetic field strength near the LFS separatrix. These data, supported by growth rate calculations, point to the importance of the LFS SOL density in determining PDI onset and degradation in current drive efficiency. By minimizing the SOL density it is possible to extend the core density regime over which PDI can be avoided, thus potentially maximizing the effectiveness of LHCD at high density. Increased current drive efficiency at high density has been achieved in FTU and EAST through lithium coating and special fueling methods, and in recent C-Mod experiments by operating at higher plasma current. Another approach would be to locate the launcher in the inner wall with double null operation. This would reduce the SOL density by an order of magnitude or more and greatly mitigate the effects of PDI as well as other parasitic losses.

Impact of SOL plasma profiles on lower hybrid current drive: Experimental evidence, mitigation and modeling approaches

“…[13]. An alternate approach to recover the current drive in a diverted configuration was discovered last year when we measured the dependency of CD efficiency in a wide range of plasma current [14]. Figure 2 compares the SOL …”

“…16 in Ref. 14). To make a quantitative comparison with the observation, we estimated the LH power by the sum of ray power propagating near the probe location.…”

Measurements of the parallel wavenumber of lower hybrid waves in the scrape-off layer of a high-density tokamak

“…As a result, the LH waves launched at the outer mid-plane are weakly absorbed on a single-pass, and the waves may undergo multiple parasitic edge loss mechanisms [9,10,11,12,13] at the plasma edge before being absorbed via electron Landau damping (ELD) in the plasma core. In particular, the wave-power reaching the high field side (HFS) of the C-Mod tokamak was found to decrease as density increases above ̅ = 1.0 × 10 20 m −3 [14]. Further, a recent ray-tracing modeling study [15] indicates that 2 collisional absorption could explain the observed anomalous decrease in hard X-ray count rates (thus fast electron populations) in high density plasmas, consistent with the experimental observation of the enhanced ionization rate in the active divertor region with the applied LH power [2].…”

“…In all cases, 2 remains high above 90%. of the onset of ion cyclotron parametric decay instabilities, which may be linked to the observed LHCD density limit problem [14]. Given the frequency separation, this instability is expected to occur at the LFS edge on the first pass from the launcher to the plasma.…”

Parametric Instabilities During High Power Helicon Wave Injection on DIII-D

“…The importance of introducing such a current drive method was already recognized in the ARIES reactor studies where it was necessary to introduce the fast lower hybrid wave (helicon wave in current use) in the 1 GHz range to provide some of the necessary driven current in the outer region of the plasma core, yet well inside the pedestal region [3]. Previous work postulated [4] that significant levels of PDI would be unlikely in such experiments due to the weaker RF electric fields of these predominantly electromagnetic waves, in contrast to those of the quasilongitudinal (electrostatic) slow lower hybrid wave [5,6,7,8]. In this work we show that this is not necessarily the case under present day experimental conditions in medium size tokamaks, e.g., DIII-D, where 1 MW of RF power at 476 MHz is proposed to be injected using a "comb-line" traveling wave antenna [2].…”