Parametric decay instability during high harmonic fast wave heating experiments on the TST-2 spherical tokamak

Oosako, T.; Takase, Y.; Ejiri, A.; Nagashima, Yoshihiko; Adachi, Yusuke; Kasahara, Hiroshi; Yamada, Teppei; Watanabe, Osamu; Tojo, H.; Kainaga, Soichiro; Sugiyama, Junichi; Yamaguchi, Takashi; An, Byung Il; Hayashi, Hiroyuki; Kobayashi, Hiroaki; Kurashina, Hiroki; Matsuzawa, H.; Yamada, Kotaro; Kumazawa, R.; Shimpo, F.; Ono, Yasushi; Masuda, Takatsugu; Sasaki, M.

doi:10.1088/0029-5515/49/6/065020

Cited by 20 publications

(15 citation statements)

References 9 publications

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“…10 A two-strap antenna was later employed on TST-2 at a higher injected power of $240 kW, where parametric decay instabilities (PDI) were investigated. 334 The HHFW high power heating experiments on NSTX have been conducted utilizing a 12 strap antenna system, powered by six 30 MHz high power rf transmitters ($1 MW each) with real time phase control. 335 The NSTX 12 strap antenna array is shown in Fig.…”

Section: Plasma Waves For Heating and Current Drive A Motivationmentioning

confidence: 99%

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Recent progress on spherical torus research

Ono

Kaita

2015

Physics of Plasmas

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The spherical torus or spherical tokamak (ST) is a member of the tokamak family with its aspect ratio (A = R0/a) reduced to A ∼ 1.5, well below the normal tokamak operating range of A ≥ 2.5. As the aspect ratio is reduced, the ideal tokamak beta β (radio of plasma to magnetic pressure) stability limit increases rapidly, approximately as β ∼ 1/A. The plasma current it can sustain for a given edge safety factor q-95 also increases rapidly. Because of the above, as well as the natural elongation κ, which makes its plasma shape appear spherical, the ST configuration can yield exceptionally high tokamak performance in a compact geometry. Due to its compactness and high performance, the ST configuration has various near term applications, including a compact fusion neutron source with low tritium consumption, in addition to its longer term goal of an attractive fusion energy power source. Since the start of the two mega-ampere class ST facilities in 2000, the National Spherical Torus Experiment in the United States and Mega Ampere Spherical Tokamak in UK, active ST research has been conducted worldwide. More than 16 ST research facilities operating during this period have achieved remarkable advances in all fusion science areas, involving fundamental fusion energy science as well as innovation. These results suggest exciting future prospects for ST research both near term and longer term. The present paper reviews the scientific progress made by the worldwide ST research community during this new mega-ampere-ST era.

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Section: Plasma Waves For Heating and Current Drive A Motivationmentioning

confidence: 99%

“…348 In TST-2, the PDI processes were investigated using probes and reflectometer, where the decay waves were found to be consistent with HHFW or IBW for the lower side band and IQM for the lower frequency mode. 334 On NSTX, such an instability was indeed observed to cause strong ion heating at the plasma edge. 349 …”

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confidence: 99%

Recent progress on spherical torus research

Ono

Kaita

2015

Physics of Plasmas

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“…[6]. In addition, the requirement for high wave heating efficiency has accelerated recent PDI studies in spherical tokamaks [8,9]. PDI is a local nonlinear process with the scale length of the pump wave power profile.…”

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confidence: 99%

“…The outboard plasma boundary is determined by the limiter (R ¼ 0:63 m R lim ) attached to the rf antenna. In the TST-2 rf high-harmonic fast wave experiment, PDI is frequently observed [12,13], and PDI frequency spectra were investigated by microwave reflectometry, magnetic probes (MPs), and Langmuir probes (LPs) [9]. The f LSB decreases with B t while keeping the relationship f pump ¼ f LSB þ f LFO , suggesting that the LFO corresponds to the ion-cyclotron quasimode (resonance with the cyclotron motion of ions) [9].…”

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Observation of Beat Oscillation Generation by Coupled Waves Associated with Parametric Decay during Radio Frequency Wave Heating of a Spherical Tokamak Plasma

et al. 2010

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We present an observation of beat oscillation generation by coupled modes associated with parametric decay instability (PDI) during radio frequency (rf) wave heating experiments on the Tokyo Spherical Tokamak-2. Nearly identical PDI spectra, which are characterized by the coexistence of the rf pump wave, the lower-sideband wave, and the low-frequency oscillation in the ion-cyclotron range of frequency, are observed at various locations in the edge plasma. A bispectral power analysis was used to experimentally discriminate beat oscillation from the resonant mode for the first time. The pump and lower-sideband waves have resonant mode components, while the low-frequency oscillation is exclusively excited by nonlinear coupling of the pump and lower-sideband waves. Newly discovered nonlocal transport channels in spectral space and in real space via PDI are described.

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“…Efficient electron heating was confirmed experimentally with the National Spherical Torus Experiment (NSTX) [3]. However, parametric decay instability (PDI) in the edge plasma region and degradation of electron heating were observed with NSTX and the Tokyo Spherical Tokamak-2 (TST-2) [4,5]. It is important to confirm the excitation and propagation of HHFW in the plasma core region.…”

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confidence: 99%

Direct Measurements of High Harmonic Fast Wave Profile in the UTST Spherical Tokamak Plasma

Wakatsuki

Nagashima

Oosako

et al. 2010

Plasma and Fusion Research

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The spatial distribution of the radiofrequency (RF) magnetic field associated with a high harmonic fast wave (HHFW) was measured using an array of magnetic probes in the plasma inside the University of Tokyo Spherical Tokamak (UTST). Data obtained from 25 probes (19 locations for toroidal polarization and 6 locations for vertical polarization) distributed along the poloidal cross section were analyzed. The RF magnetic field is polarized in the toroidal direction, indicating that the HHFW is excited in the plasma. The RF field is weak on the inboard side. Analysis of the group delay suggests that the waves travel long distances in the plasma, probably because of poor absorption. No indication of parametric decay was observed up to 80 kW of injected RF power. A high harmonic fast wave (HHFW) is used to heat electrons and sustain the plasma after spherical tokamak (ST) formation by the double null merging technique in the University of Tokyo Spherical Tokamak (UTST) [1]. HHFW is suitable for electron heating of high-β ST plasmas [2]. Efficient electron heating was confirmed experimentally with the National Spherical Torus Experiment (NSTX) [3]. However, parametric decay instability (PDI) in the edge plasma region and degradation of electron heating were observed with NSTX and the Tokyo Spherical Tokamak-2 (TST-2) [4,5]. It is important to confirm the excitation and propagation of HHFW in the plasma core region. In this paper, results of direct measurements of the radiofrequency (RF) magnetic field associated with a HHFW are presented.UTST is an ST device with the following typical parameters: R 0 (major radius) = 0.43 m, a (minor radius) = 0.17 m, B t (toroidal magnetic field) = 0.15 T, and I p (plasma current) = 100 kA. The frequency of the externally applied RF power (21 MHz) is more than ten times the hydrogen ion cyclotron frequency. The HHFW antenna consists of two poloidal current straps separated in the toroidal direction by 22.5• . When they are excited out of phase, the toroidal mode number of the excited HHFW is ±8 (which corresponds to the toroidal wavenumber of k φ = ±13 m −1 ). This is referred to as double-strap excitation. When only one strap is excited (single-strap exci- tation), a broader toroidal wavenumber spectrum centered around k φ = 0 is excited. A 2-dimensional (2-D) 9×9 magnetic probe array is inserted into the plasma at the poloidal section that is 45• away from the antenna toroidally. Magnetic field components in the toroidal direction (B φ ) and vertical direction (B z ) can be measured at each probe location. Data from only 25 probes (8 toroidal fields and 6 vertical fields on the midplane, and 11 toroidal fields off the midplane) were taken simultaneously in this experiment due to limitation of number of digitizers. Probes are separated from each other by 70 mm in the radial direction and 60 mm in the vertical direction. Figure 1 depicts the 2-D (z-φ) wave polarization angle (defined as tan −1 [B z /B φ ]) profile on the midplane. In these

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Parametric decay instability during high harmonic fast wave heating experiments on the TST-2 spherical tokamak

Cited by 20 publications

References 9 publications

Recent progress on spherical torus research

Recent progress on spherical torus research

Observation of Beat Oscillation Generation by Coupled Waves Associated with Parametric Decay during Radio Frequency Wave Heating of a Spherical Tokamak Plasma

Direct Measurements of High Harmonic Fast Wave Profile in the UTST Spherical Tokamak Plasma

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