Power threshold and saturation of parametric decay instabilities near the upper hybrid resonance in plasmas

Abstract

Parametric decay instabilities (PDIs) occur for large-amplitude waves in quadratically nonlinear media, where they provide a limit of validity of linear theories and allow efficient coupling between different, well-defined wave modes. We investigate PDIs near the upper hybrid resonance in plasmas by injection of high-power electron cyclotron (EC) waves at the ASDEX Upgrade tokamak. Our measurements of PDIs have an unprecedented frequency resolution, far below the ion cyclotron frequency, allowing the first observations of secondary and tertiary PDIs during the saturation phase in a controlled laboratory setting. Furthermore, we are for the first time able to systematically compare theoretical predictions of the EC wave power thresholds, which must be exceeded to excite such PDIs, to experimental observations, validating the theory. Our findings are relevant for EC wave heating and current drive in tokamaks and stellarators, including future fusion power plants, as well as in low-temperature laboratory and industrial plasmas, inertial confinement fusion, and ionospheric modification experiments.

Numerical investigations of parametric decay into trapped waves in magnetized plasmas with a non-monotonic density background

“…In magnetically confined fusion plasmas, signatures of PDIs near the UH layer were first observed in the Versator II tokamak during electron cyclotron resonance heating (ECRH) 17 . Later, observations have been made at a number devices including during ECRH and EBW heating at the Wendelstein 7-AS 18,19 stellarator, the FT-1 20 tokamak, the TCA 21 tokamak, the MAST 22 tokamak, the TJ-K 23 stellarator and the LHD 24 stellarator; and during collective Thomson scattering (CTS) [25][26][27] at the ASDEX Upgrade tokamak. PDI has also been studied experimentally in other magnetically confined laboratory plasmas such as in tandem mirror machines 28 and linear devices 29,30 .…”

Parametric Decay Instabilities during Electron Cyclotron Resonance Heating of Fusion Plasmas, Problems and Possibilities

“…We investigate n UH e for O-mode ECRH radiation at ITER, as such radiation may drive a PDI involving a trapped UH wave and a low-frequency lower hybrid wave in the presence of an inhomogeneous n e profile with a local maximum slightly above n UH e [53][54][55][56]. Additionally, in connection with O-mode ECRH at ITER, we investigate whether scenarios are likely to occur in which a significant amount of X-mode radiation reaches the UHR upon reflection off the high-field side wall, as PDIs have been observed in such scenarios at ASDEX Upgrade, even without UH wave trapping [23][24][25][26]. We note that the trapping-n e of the UH wave is slightly different from the value in Eq.…”

“…This is the root cause of the microwave diagnostics damage associated with such PDIs [1]. Traditionally, magnetic confinement fusion ECRH PDIs were only expected in connection with extremely high power ECRH provided by pulsed free-electron maser sources [13] or in scenarios with lower ECRH power where a significant fraction of the ECRH power would reach the upper hybrid resonance (UHR) with X-mode polarization [4,5,[9][10][11][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]. PDIs are expected in the above cases due to the occurrence of electric fields with large amplitudes.…”

“…In the free-electron maser case, the electric field amplitude is large enough to overcome the losses in the bulk plasma [13], while the UHR case relies on enhancement of the electric field amplitude associated with X-mode waves near the UHR, where their group velocity becomes very small [34]. Although PDIs involving X-mode waves reaching the UHR can occur at relatively low ECRH power levels without strict requirements on the plasma conditions [25,26], they have generally only been considered to play a role in certain ECRH scenarios. This is due to the fact that X-mode waves cannot reach the UHR directly from the usual lowfield side ECRH launcher position in tokamaks, owing to the R-cutoff always occurring on the low-field side of the UHR.…”

“…This is due to the fact that X-mode waves cannot reach the UHR directly from the usual lowfield side ECRH launcher position in tokamaks, owing to the R-cutoff always occurring on the low-field side of the UHR. The analysis of PDIs at the UHR is distinct for strongly overdense plasmas [18,19,22,29], commonly found in spherical tokamaks, and underdense/weakly overdense plasmas [17,[24][25][26][27][28]30], commonly found in conventional tokamaks and during plasma start-up, owing to the different dispersion properties of the electron Bernstein waves (EBWs) involved in the PDI in these cases. Specific scenarios in which PDIs at the UHR are of interest in connection with magnetic confinement fusion include fundamental X-mode [9][10][11]17] and O-mode [9,11,21,[24][25][26][27][28] ECRH with optically thin/gray resonances, O-X-B heating [4,5,[18][19][20]29], and EBW start-up [35,36].…”