Ion Bernstein Wave Heating Experiments Research Articles

Particle-in-cell simulations of parametric decay instability of radiofrequency wave in the ion cyclotron range of frequency in an inhomogeneous plasma

Particle-in-cell simulations have been conducted to investigate the parametric decay processes of radiofrequency waves in the ion cyclotron range of frequency in inhomogeneous plasmas. By choosing the parameters close to those in the ion Bernstein wave heating experiments on the HT-7 tokamak (Li et al 2001 Plasma Phys. Control. Fusion 43 1227–38), decay channels similar to experimental observations have been found in simulations. It is shown that due to strong inhomogeneity of tokamak edge plasma, layers may exist where selection rules of the mode–mode coupling are satisfied, and resonant parametric decays can occur if the input wave power is sufficiently high. As a result, daughter waves at half-, second- and third-harmonic frequencies of the pump wave can be excited. The nonresonant parametric decay is also observed, and ions are heated via nonlinear Landau damping.

Progress of LHCD and IBW Heating Experiments on HT-7 Tokamak

Experimental progress of lower hybrid current drive (LHCD) and ion Bernstein wave (IBW) heating was achieved significantly in HT-7. Long pulse plasmas, up to 400 s, were obtained by LHCD. High power lower hybrid wave (LHW), with PLHW of 800 kW at 2.45 GHz, was launched and the current drive efficiency was studied for different wave powers. Electron temperature and Zeff were measured in LHCD discharges. Heating experiment and recent results of high power ion Bernstein wave, with PIBW of 600 kW at 27 MHz, were summarized.

Ion Bernstein Wave Heating Experiments in HT-7 Superconducting Tokamak

Ion bernstein wave (IBW) experiments have been carried out in HT-7 superconducting tokamak in recent years. The electron heating experiments concentrated on deuterium plasma with an injected RF power up to 350 kW. The global heating and the localized heating can be seen clearly by adjusting the ICRF resonance layer positions. The on-axis and the off-axis electron heating were realized by arranging the target plasma parameters properly. Experimental results show that the maximum increment of electron temperature was more than 1 keV, the electron temperature profile was modified by IBW under different plasma conditions and the improvements of both energy and particle confinements have been obtained.

Experimental Results of IBW Heating on the HT-7 Tokmak

Ion Bernstein waves (IBWs) have been proposed to be useful for heating and improving transport in tokamak plasmas. An Ion Bernstein wave heating experiment using different frequency (27 MHz and 30 MHz) was carried out on HT-7 superconducting tokamak in recent experiments. At a frequency of 30 MHz, ne peaked and Hα dropped have been pervasively observed, τp was improved by a factor of 2-4, and τe increased by a factor of 1-1.5. An obvious confinement of particle was observed during the IBW pulse. At 27 MHz, both global and localized electron heating were observed depending on the location of the ion resonant layer by changing Bt. Central electron heating effect was obtained in the global heating mode and electron temperature strongly increased near the 2ΩD resonant layer for localized heating mode.

Observation of parametric decay instability during ion Bernstein wave heating experiments onHT-7

The parametric decay processes with a power threshold of 70 kW have beenobserved on the HT-7 tokamak. Two types of parametric decay instabilities havebeen identified. The pump wave decays into an ion cyclotron quasi-mode and anion Bernstein wave, or decays into a low-frequency electron Landau dampedquasi-mode and an ion Bernstein wave. The experimental observation showsgood agreement with theoretical prediction. Other peaks at a frequency ofnΩD are also observed. The main features of the observed radiofrequency spectra is due tothe parametric decay instability of the launched slow electron plasma pumpwave into an ion cyclotron quasi-mode and an electron plasma wave, dominatedby plasma inhomogeneity convective loss. As the plasma displacement shifts towardsthe antenna, a higher ω/Ω value at the antenna position and a higher wavefrequency are used to reduce the possibility of parametric decay instability (PDI) production. Theexperimental evidence clearly shows that a higher wave frequency, edge densityand value of ω/Ω in the scrap-off layer are favourable to prevent the production of PDIs.The PDI was effectively reduced and successfully avoided witha power level up to 300 kW.

Full-wave calculation of sheared poloidal flow driven by high-harmonic ion Bernstein waves in tokamak plasmas

A full-wave, one-dimensional spectral model is developed to study sheared poloidal flow driven by high-harmonic ion Bernstein waves (IBWs) in tokamak plasmas. The local plasma conductivity is corrected to lowest order in ρ/L where ρ is the ion Larmor radius and L is the equilibrium scale length. This correction takes into account gradients in equilibrium quantities and is necessary for conservation of energy. It is equivalent to the “odd-order derivative” terms in finite difference models. No assumption is made regarding the smallness of the ion Larmor radius relative to wavelength, and results are applicable to all orders in k⊥ρ where k⊥ is the perpendicular wave number. Previous numerical results for flow drive have relied on expansions in k⊥ρ, and are thus limited to cyclotron harmonics of two and below. In this article, we consider higher-harmonic cases corresponding to recent IBW flow drive experiments on the Tokamak Fusion Test Reactor [B. P. LeBlanc, R. E. Bell, S. Bernabei et al., Phys. Rev. Lett. 82, 331 (1999)] and the Frascati Tokamak Upgrade [R. Cesario, C. Castaldo, V. Pericoli-Ridolfini et al., “Recent Results of the Ion Bernstein Wave Heating Experiment on FTU,” in Proceedings of the 13th Topical Conference on Radio Frequency Power in Plasmas, 1999, Annapolis, MD, edited by S. Bernabei and F. Paoletti (American Institute of Physics, New York, 1999), p. 100]. In these cases, a directly launched high-harmonic IBW is used to drive poloidal flow near the fourth- and fifth-harmonic ion cyclotron layers. Other applications include high-harmonic fast wave heating in low aspect ratio tokamaks such as the National Spherical Torus Experiment [S. M. Kaye, M. Ono, Y.-K. M. Peng et al., Fusion Technol. 36, 16 (1999)].

ICRF heating and profile control techniques in TFTR

In fast wave to ion Bernstein wave mode conversion experiments in DT supershot plasmas, localized efficient ion heating rather than electron heating was observed, which was due to Doppler broadened tritium cyclotron resonance overlap into the mode conversion region. The ion temperature heat pulse associated with RF power modulation in this regime could provide a diagnostic tool for measuring the local ion thermal conductivity in various confinement regimes. In direct launch ion Bernstein wave heating experiments, core power coupling was limited by the excitation of parasitic edge modes. However, a sheared poloidal flow was observed that is consistent in both magnitude and direction with theoretical models based on RF driven Reynolds stress. With the modest power coupled to the core (∼360 kW), the magnitude of the shear in the observed flow was estimated to be a factor of 3-4 too low to trigger transport barrier formation through localized shear suppression of turbulence.

Coaxial mode excitation and dissipation in ion Bernstein wave experiments

In recent ion Bernstein wave (IBW) heating experiments on the Tokamak Fusion Test Reactor (TFTR) [J. R. Wilson. R. E. Bell, S. Bernabei, K. Hill et al., Phys. Plasmas 5, 1721 (1998)] a velocity shear layer in the plasma core was obtained. The magnitude of velocity shear was believed to be too small to create an internal transport barrier, because of parasitic edge processes which reduced the power coupled to the core. In this paper we investigate these rf (radio frequency) edge processes by employing a model which includes both coaxial modes and their dissipation in rf plasma sheaths. The coaxial mode (here, an electron plasma wave trapped in the halo plasma between the lower hybrid layer and the vessel wall) can propagate at low poloidal wave numbers. This feature is shown to relate to the observed poloidal phasing dependence of the antenna loading. Results of analytical models and a three-dimensional antenna code are presented. The experimentally observed loading is also nonlinear, being larger at very low powers. This feature is explored using an rf sheath dissipation model. Loading into the coaxial mode is expected to maximize when the density gradient at the lower hybrid layer is steep, preventing efficient mode transformation to the IBW. The role of ponderomotive force in modifying the density profile is also discussed.

Poloidal phasing effects on particle transportduring ion Bernstein heating

Ion Bernstein wave (IBW) heating experiments are performed with a poloidally phased antenna. Between two kinds of poloidal phasing, no difference is found with the heating effect which is expected from the mechanism of heating presented previously. The significant differences due to the difference of poloidal phasing are found in observations of electron density variation, emission intensities of impurity lines and scrape-off layer plasma. All the results show that the poloidal phasing brings about variations of particle transport, and a ponderomotive effect of the antenna near-field might be the most probable mechanism. The phasing effect should be considered on an intense radio frequency (RF) injection by poloidally aligned antenna elements.

Lower hybrid current drive and ion Bernstein wave heating experiments on PBX-M

Abstract This paper presents an overview of the experiments conducted on PBX-M to evaluate the feasibility and effect of current profile and pressure profile control on the plasma stability. Utilizing the inaccessibility of the lower hybrid waves, it has been possible to obtain a certain degree of power deposition localization and off-axis current drive. The effect of fast electron diffusion has been studied and found not to be a limiting factor; consequently, the current profile has been modified in a non-transient manner. More serious is the destabilization of global MHD modes, due to the change of the current profile, which can lead to disruption or to a rapid radial redistribution of the fast electron population. Experiments with ion Bernstein wave heating have shown that power can be deposited off-axis and that the ion temperature can be modified locally. Application of IBW into a strongly neutral beam (NBI) heated H-mode plasma causes a substantial increase of thermal and particle confinement in the core of the plasma: this produces a localized bootstrap current sufficient to significantly raise the value of q(0). We propose to refer to this condition as the CH-mode (or core high-confinement mode).

Study of parametric instabilities during the ion Bernstein wave heating experiment on PBX-M

Parametric instabilities during the ion Bernstein wave (IBW) plasma heating experiment on PBX-M are investigated both theoretically and experimentally. The theoretical work shows that the RF power threshold of the instabilities is very low when the plasma density near the antenna satisfies the condition 2ωpi < ω0 << ωpe. The threshold becomes very high, >>1 MW, and is determined by convective losses due to plasma inhomogeneity when the plasma density near the antenna is sufficiently high that 0.5 < ωpi/ω0 < 1. According to the theory, the parametric instability activity should increase as the plasma is moved away from the antenna, creating a low density electron plasma wave 'gap' region. To test this hypothesis on PBX-M, the plasma position was deliberately varied while monitoring this activity. Under normal IBW operating conditions, very little parametric instability activity was observed ⩽50 dB below the pump wave (ω = ωRF). However, when the plasma edge was moved away from the antenna by about 2 cm, the parametric instability activity increased greatly, exceeding 20 dB of the pump wave. This result shows that the observed parametric instability activity can be explained in terms of the plasma inhomogeneity convective model. It was also demonstrated that, by controlling the plasma position with respect to the antenna, parametric instability activity can be controlled at a low level

Ion Bernstein wave antenna loading measurements on the DIII-D tokamak

Antenna loading measurements carried out during high power ion Bernstein wave (IBW) heating experiments on the DIII-D tokamak indicate that efficient, direct coupling to the IBW at ω ≲ 2ωci as predicted by linear coupling theory did not occur. Whereas strong peaking in the loading resistance near cyclotron harmonics is predicted for high edge densities (ω < ωLH|edge), the observed loading resistance is nearly independent of the toroidal magnetic field. The loading anomaly can be explained by invoking the ponderomotive force which decreases the edge density immediately in front of the antenna, thus allowing coupling to the cold plasma lower hybrid wave (LHW). A linear LHW coupling code including the modified density profile due to the ponderomotive force reproduces the measured dependence of antenna loading on toroidal magnetic field, edge density, antenna frequency and antenna phasing. Further evidence for the ponderomotive force is obtained from reactive loading measurements which indicate that the plasma is pushed away from the antenna as the radiofrequency power level is increased. The results indicate that the lack of central ion heating observed during DIII-D IBW experiments may be due to a lack of efficient mode transformation from the coupled LHW to a centrally propagating IBW, possibly as a result of nonlinear mechanism(s)

Nonlinear ion Bernstein wave heating experiment in the WT-3 tokamak

Ion Bernstein wave (IBW) heating at ion cyclotron half-harmonic frequencies was performed in hydrogen (H) discharges in the WT-3 tokamak. The bulk ion temperature increase was measured by a charge exchange analyser and a spatially scanning polychrometer. The latter diagnostics can provide radial ion-temperature profiles. The incremental ion temperature has shown a peak at the exact location where the IBW frequency omega equals 3/2 of the ion cyclotron frequency, indicating the nonlinear direct acceleration of H ions. The wave electric field was measured by a Langmuir probe in the scrape-off layer. The electric field at the second harmonic of the excited IBW frequency was observed to vary depending on the power level and the toroidal magnetic field. These experimental results are compared with the theoretical prediction of nonlinear IBW damping. The measured dependences of the ion heating and the wave amplitude at 2 omega on the toroidal magnetic field are in reasonable agreement with the prediction.

High frequency ion Bernstein wave heating experiment in the JIPP T-IIU tokamak

An experiment in a new regime of ion Bernstein wave (IBW) heating was carried out using 130 MHz high power transmitters in the JIPP T-IIU tokamak. The heating regime utilized the IBW branch between the 3rd and 4th harmonics of the hydrogen ion cyclotron frequencies. This harmonic number is the highest one used in IBW experiments conducted previously. The net radiofrequency (RF) power injected into the plasma is around 400 kW and is limited by the transmitter output power. Core heating of ions and electrons was confirmed in the experiment and density profile peaking was found to be a special feature of IBW heating. Peaking of the density profile was also found when IBWs were injected into neutral beam heated discharges. An analysis, using a transport code with these experimental data, indicates that particle and energy confinement should be improved in the plasma core region upon application of IBW heating. It is also found that the ion energy distribution function observed during IBW heating has a smaller high energy tail than those observed in conventional fast magnetosonic wave ICRF heating regimes. The ion energy distribution function obtained during IBW heating is in reasonable agreement with that calculated using the quasi-linear RF diffusion/Fokker-Planck model

Linear propagation and absorption of ion Bernstein waves in toroidal geometry

The ion Bernstein wave (IBW) propagation is investigated in toroidal geometry with particular regard for the absorption mechanism of the power along the trajectory which shows a strong dependence on the value of the poloidal angle at the antenna. A large power fraction launched by poloidally wide antennas turns out to be absorbed in the very edge of the plasma by electron Landau damping and no penetration of the wave up to the plasma core is allowed except for rays starting at poloidal angles very close to the horizontal plane. This result could provide an explanation for the absence of efficient core heating in some of the IBW heating experiments.

Power balance analysis of ion Bernstein wave heating experiments in the Alcator C tokamak

Energy and particle transport processes in Alcator C [Phys. Rev. Lett. 60, 298, (1988)] discharges heated by directly launched ion Bernstein waves (IBW) are investigated using the onetwo transport code [Nucl. Fusion 26, 329 (1986)]. The dependence of the observed ion heating rate (ΔTH/Prf) on plasma density is shown to result mainly from the inherent ion energy confinement that is characteristic of these discharges in the Ohmic phase and not on IBW propagation and absorption characteristics. The inferred value of the Ohmic ion thermal conduction exhibits an increasing anomaly with increasing plasma density. At a plasma density of n̄e ≲1×1020 m−3, the inferred ion thermal conduction is nearly equal to the Chang–Hinton neoclassical prediction. However, at a plasma density of n̄e ≳2×1020 m−3, the inferred ion thermal conduction increases to about 5–8 times the Chang–Hinton neoclassical value. This increasing anomaly, which may result from the ion temperature gradient driven instability, can essentially account for the observed ion heating rate behavior during IBW heating. The effect of edge turbulence on IBW propagation is modeled using a Monte Carlo direct sampling simulation. The result is mainly a radial broadening of the calculated power deposition profile with increasing plasma density.

Modelling of ion Bernstein wave heating in JIPPT-II-U

By using a transport code combined with an ion Bernstein wave tokamak ray tracing code, a modelling code for the ion Bernstein wave heting has been developed. With this code, the ion Bernstein wave heating experiment on the JIPPT-II-U tokamak has been analysed. It is assumed that a resonance layer is formed by the third harmonic of deuterium-like ions, such as fully ionized carbon, and oxygen ions near the plasma centre. As wave absorption mechanisms, electron Landau damping, ion cyclotron harmonic damping, and collisional damping are considered. The characteristics of the ion Bernstein wave heating experiment, such as an increase in ion temperature, a strong dependence of the quality factor on the magnetic field strength and a dependence of the ion temperature increment on the input power are well reproduced.