Lower hybrid current drive at high density in Alcator C-Mod

Wallace, G.; Hubbard, A.; Bonoli, P. T.; Faust, I.; Harvey, R. W.; Hughes, J. W.; LaBombard, B.; Meneghini, O.; Parker, R.; Schmidt, Andréa; Shiraiwa, S.; Смирнов, А. П.; Whyte, D.G.; Wilson, J. R.; Wright, J. C.; Wukitch, S.J.

doi:10.1088/0029-5515/51/8/083032

Cited by 67 publications

(65 citation statements)

References 25 publications

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“…Above the frequency spectrum is eventually dominated by PDI at the outer wall side, similar to what was observed in single null plasmas. However, its strength as measured with the inner-wall probe is somewhat weaker than that observed in Figure 4, and this correlates with the previous observation of a higher hard X-ray count rates in double null plasmas than in single null plasmas [15]. Nonetheless, one needs to be careful when predicting PDI effects with inside launch based only on this result because both the plasma condition and wave propagation would not be same as what they would be with inside launch.…”

Section: Implications For Future Lhcd Experimentssupporting

confidence: 60%

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

et al. 2015

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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.

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Section: Implications For Future Lhcd Experimentssupporting

confidence: 60%

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

et al. 2015

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“…Previously, we reported that the LHCD performance recovered by using a limited magnetic configuration in Ref. [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].…”

Section: Experimental Progresses To Improve Lhcd At High Densitymentioning

confidence: 99%

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

Shiraiwa

Baek

Faust

et al. 2015

AIP Conference Proceedings

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“…A helical internal kink-like mode [7][8][9] with dominant poloidal and toroidal mode numbers m = 1, n = 1 has recently been observed in the Alcator C-Mod tokamak 10 , during the development of advanced fusion scenarios using lower hybrid current drive (LHCD) [11][12][13] . During LHCD operation, soft x-ray (SXR) detectors measure periodic bursts of oscillations as shown in Fig.…”

mentioning

confidence: 99%

“…is the Lorentz factor 13,[20][21][22][23] . Electron cyclotron waves emitted in the core at these frequencies can travel to the outboard plasma edge without being re-absorbed by the thermal electrons.…”

mentioning

confidence: 99%

Non-resonant destabilization of (1/1) internal kink mode by suprathermal electron pressure

Delgado‐Aparicio

Sugiyama²,

Shiraiwa

et al. 2015

Phys. Plasmas

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New experimental observations are reported on the structure and dynamics of short-lived periodic (1, 1) "fishbone"-like oscillations that appear during radio frequency heating and current-drive experiments in tokamak plasmas. For the first time, measurements can directly relate changes in the high energy electrons to the mode onset, saturation, and damping. In the relatively high collisionality of Alcator C-Mod with lower hybrid current drive, the instability appears to be destabilized by the non-resonant suprathermal electron pressure -rather than by wave-particle resonance, rotates toroidally with the plasma and grows independently of the (1, 1) sawtooth crash driven by the thermal plasma pressure. The economic feasibility of a fusion reactor can be measured in terms of β ≡ p/ B 2 /2µ 0 , where p is the plasma pressure and B 2 /2µ 0 is the magnetic field energy density. The presence of magnetohydrodynamic (MHD) instabilities can distort the orbits of fast ions and electrons compared to the equilibrium, causing an off-axis redistribution of the fusion heating and current density to the detriment of the central β, reducing the fusion power and the production of fast neutrons for tritium breeding. Therefore, understanding the stability and behavior of 3D helical modes in the core of an axisymmetric toroidal configuration remains one of the challenges of fusion research 1-6 . It is doubly significant for burning plasmas such as ITER, where the q = 1 radius that bounds these instabilities may reach half the minor radius.A helical internal kink-like mode 7-9 with dominant poloidal and toroidal mode numbers m = 1, n = 1 has recently been observed in the Alcator C-Mod tokamak 10

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Lower hybrid current drive at high density in Alcator C-Mod

Cited by 67 publications

References 25 publications

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

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

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

Non-resonant destabilization of (1/1) internal kink mode by suprathermal electron pressure

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