ICRF Waves Research Articles

Three-dimensional proton trajectory analyses and simulation of neutral particle transport in an ICRF heated long pulse discharge on the large helical device

A three-dimensional neutral particle transport simulation code is applied to identify a source of the hydrogen outgas in a LHD long pulse discharge by ICRF heating. Radiation collapse induced by uncontrollable plasma density rise terminated the long pulse discharge. Toroidally non-uniform increases in the Hα emission and divertor temperature were observed. A CCD camera detected local heating of a vertically installed divertor plate, which is consistent with the strike points of the trajectories of protons accelerated by ICRF waves. The neutral particle transport simulation with the proton trajectory analyses shows that outgas from the divertor plates heated by the protons is necessary for explaining the observed toroidal non-uniform distribution of the Hα emission in the long pulse discharge.

Improved simulation of the ICRF waves in the VASIMR plasma

This paper describes the recent developments in the modeling of the ICRF heating process in the VASIMR plasma using the EMIR code. The latest upgrade of the EMIR model involves a calculation of the parallel electric field component with a warm plasma conductivity model and considers the Doppler shift effect. The code now gives a much more accurate and a more physical solution than in the previous version, but requires a more advanced solver.

Overview of recent Alcator C-Mod research

Research on the Alcator C-Mod tokamak [1] is focused on high particle- and power-density plasma regimes to understand particle and energy transport in the core, the dynamics of the H-mode pedestal, and scrape-off layer and divertor physics. The auxiliary heating is provided exclusively by RF waves, and both the physics and technology of RF heating and current drive are studied. The momentum which is manifested in strong toroidal rotation, in the absence of direct momentum input, has been shown to be transported in from the edge of the plasma following the L–H transition, with timescale comparable to that for energy transport. In discharges which develop internal transport barriers, the rotation slows first inside the barrier region, and then subsequently outside of the barrier foot. Heat pulse propagation studies using sawteeth indicate a very narrow region of strongly reduced energy transport, located near r/a = 0.5. Addition of on-axis ICRF heating arrests the buildup of density and impurities, leading to quasi-steady conditions. The quasi-coherent mode associated with enhanced D-Alpha (EDA) H-mode appears to be due to a resistive ballooning instability. As the pedestal pressure gradient and temperature are increased in EDA H-mode, small ELMs appear; detailed modelling indicates that these are due to intermediate n peeling–ballooning modes. Phase contrast imaging has been used to directly detect density fluctuations driven by ICRF waves in the core of the plasma, and mode conversion to an intermediate wavelength ion cyclotron wave has been observed for the first time. The bursty turbulent density fluctuations, observed to drive rapid cross-field particle transport in the edge plasma, appear to play a key role in the dynamics of the density limit. Preparations for quasi-steady-state advanced tokamak studies with lower hybrid current drive are well underway, and time dependent modelling indicates that regimes with high bootstrap fraction can be produced.

Recent Experiments on Stability and Heating in the Hiei Tandem Mirror

MHD stability and ion cyclotron heating are investigated in the axisymmetric tandem mirror HIEI, which is a three-cell device with a typical magnetic field strength of 0.05-0.3-0.1-0.3 T, from central cell midplane to outer throat of plug cells. A magnetic divertor configuration introduced in the central cell provides magnetic nulls and stabilizes MHD activities by shorting-out the charge separation in the azimuthal direction. The density fluctuation of m = 1 flute mode is observed to decrease significantly by using the divertor. The stabilization effect of the divertor is stronger as the nulls are located at inner radius. It is observed from the cross correlation measurement that the electrons exhibit E × B motion in unstable plasmas, while they tend to follow the Boltzmann relation in the divertor-stabilized plasmas. The second harmonic ion cyclotron heating is also investigated. In the divertor configuration where the magnetic field strength varies significantly in radial direction, the ICRF waves have a large transverse gradient in amplitudes to enhance the finite Larmor radius effect. The wave propagation and damping are investigated theoretically and experimentally.

Comparative Investigation of End Plate and Limiter Biasing in the Hiei Tandem Mirror

It is demonstrated that the transition leading to the improved radial confinement is induced by the end plate biasing in the HIEI tandem mirror, in which the plasma is MHD stabilized, heated, and axially confined by the ICRF waves. On application of the bias voltage, the fluctuation level decreases, the density increases to over 1013 cm−3, and the plasma rotation changes significantly indicating a formation of the internal transport barrier. The observed strong nonlinear phenomena; the hysteresis change of the fluctuation level and the neutral line emission intensity, demonstrate bifurcation feature, which is similar to that in the previous limiter biasing experiment. This bifurcation feature may be explained by the model from the azimuthal momentum balance including the flow shear stabilization of the drift wave instabilities.

ICRF Wave Propagation and Absorption in a Microwave Discharge Ion Thruster

Preliminary experiments are made on the EICR plasma thruster which can generate plasma with ECRH (Electron Cyclotron Resonance Heating) and accelerate ions with ICRFPI (Ion Cyclotron Range of Frequency Heating). A set of plasma diagnostics is being developed and installed. The electron temperature Te, electron density ne and ion temperature Ti were measured in our device. To examine ICRF wave propagation in the plasma, the magnetic fields were measured and the results show that the excited waves were attenuated before reaching the ICR layer. Moreover, ICRF wave absorption and propagation were calculated with an ICRF code. The strong energy absorption in the plasma was obtained at the ICR layers.

On the role of different phasings of the ICRF antennas in optimized shear discharges in JET

In discharges with internal transport barriers produced by combined NBI and ICRF heating using the hydrogen minority scheme in JET, confinement and fusion performance are strongly affected by the direction of propagation of the ICRF waves. When the waves propagate along the plasma current, the formation of an internal transport barrier is prompter and the neutron yield is up to a factor of two higher than that for propagation against the current. An ICRF induced pinch of resonating trapped ions is put forward as a candidate for explaining the observations. Simulation results are presented which show that this effect is strong enough to provide a credible explanation for the experimental results.

Energetic particle physics in JET

Results achieved on JET during the 1997-1999 experimental campaigns in the physics of energetic ions and runaway electrons are reviewed. Heating of deuterium-tritium (DT) plasmas by fusion born alpha particles is found to be similar to that achieved by comparable ICRF heating of deuterium plasmas. The stability of alpha particle driven Alfvén eigenmodes (AEs) in the highest fusion power ELM-free H mode discharges is shown to be consistent with the existing theoretical analysis for AEs. Direct measurements of the trapped alpha particle and knock-on deuteron distribution functions by a neutral particle analyser (NPA) are described. New ICRF heating scenarios tested in JET DT plasmas are presented in view of the possible use of the ICRF heating of ITER-like DT plasmas on route to ignition. The energetic ion pinch in the presence of toroidally asymmetric ICRF waves is studied experimentally on JET. Recent experimentally observed effects of ICRF accelerated ions on sawteeth in JET are reviewed. Detailed time and space resolved X ray images of the electron runaway beam in flight spontaneously generated by disruptions on JET are described.

Overview of steady state operation of HT-7 and present status of the HT-7U project

Significant progress has been made on the HT-7 superconducting tokamak recently. Repeatable plasma discharges for up to 10.7 s have been routinely obtained, which nearly reached the present limit of the PF system. LHCD experiments have been successfully carried out. A full LH wave current drive for up to 3 s has been achieved. A quasi-steady-state H mode was observed with a very high plasma density for LHCD. ICRF waves were used for wall conditioning, startup, and fast wave and ion Bernstein wave heating. A new RF conditioning technique is shown to be a very powerful means of conditioning future large superconducting tokamaks. The physical design of HT-7U has been nearly finished. The engineering design and some R&D have started. A 2:3 model of the TF coil and a 1:4 model of the central solenoid with a cable in conduit conductor cable have been finished. Testing of the coils will start soon. The engineering design is under way in accordance with the schedule.

Alpha particle physics experiments in the Tokamak Fusion Test Reactor

Alpha particle physics experiments were done on TFTR during its DT run from 1993 to 1997. These experiments utilized several new alpha particle diagnostics and hundreds of DT discharges to characterize the alpha particle confinement and wave-particle interactions. In general, the results from the alpha particle diagnostics agreed with the classical single particle confinement model in MHD quiescent discharges. The alpha loss due to toroidal field ripple was identified in some cases, and the low radial diffusivity inferred for high energy alphas was consistent with orbit averaging over small scale turbulence. Finally, the observed alpha particle interactions with sawteeth, toroidal Alfvén eigenmodes and ICRF waves were approximately consistent with theoretical modelling. What was learned is reviewed and what remains to be understoodis identified.

Central impurity toroidal rotation in ICRF heated Alcator C-Mod plasmas

Central impurity toroidal rotation has been observed in Alcator C-Mod ICRF heated plasmas, from the Doppler shifts of argon X ray lines. Rotation velocities of up to 1.3 × 105 m/s in the co-current direction have been observed in H mode discharges with no direct momentum input. There is a strong correlation betweenthe increase in the central impurity rotation velocity and the increase in the plasma stored energy, induced by ICRF heating, although other factors may be involved. This implies a close associationbetween energy and momentum confinement. Co-current rotation is also observedduring purely ohmic H modes. In otherwise similar discharges with the same stored energy increase, plasmaswith lower current rotate faster. For hydrogen minority (D(H)) heating, plasmas with the highestrotation have an H/D ratio between 5 and 10% and have the resonance locationin the inner half of the plasma, i.e. in the same conditions that are conducive to the best ICRFabsorption and heating. Comparisons with neoclassical theoryindicate that the ion pressure gradient is an unimportant contributor to thecentral impurity rotation and the presence of a substantial core radial electric field is inferred during the ICRF pulse. An inward shift of ions induced by ICRF waves could give rise to a non-ambipolar electric field in theplasma core.

Study of acceleration, confinement and sawtooth induced redistribution of fast resonant protons during ICRH in TFTR

A natural diamond detector (NDD), developed in the Russian Federation, has been used for the first time for fast charge exchange H0 atom spectrometry and flux evolution measurements during H+ minority ICRH experiments in deuterium plasmas in TFTR. The behaviour of deeply trapped RF driven H+ ions was studied in quiescent plasmas and under the influence of sawtooth oscillations during central and off-axis ICRH. H+ ion energy spectra measured by the NDD yielded data on the efficiency of H+ minority ion acceleration by ICRF waves and the H+ ion confinement at different ICRH powers and resonance layer positions. The evolution of the hydrogen charge exchange atom flux measured by the NDD was compared with fast proton losses measured by escaping charged particle detectors to demonstrate that fast H+ ions are redistributed from the plasma core to well beyond the q = 1 surface. The redistribution is sufficiently far from the plasma centre for fast ions to leave the plasma due to stochastic ripple diffusion. In contrast with non-deeply-trapped fast ions, Alfvén eigenmode (AE) activity does not significantly affect the spatial and energy distributions of deeply trapped fast ions.

Theoretical analysis of ICRF heating in JET DT plasmas

A number of experiments with heating of DT plasmas using ICRF waves have been carried out at JET. The results of these experiments have been analysed by comparing experimentally measured quantities with the results of numerical simulations. In particular, four scenarios have been examined: (a) heating of minority (∼5-20%) deuterons at the fundamental ion cyclotron frequency, ω = ωcD; (b) second harmonic heating of tritium, ω = 2ωcT; (c) fundamental minority heating of 3He with a few per cent of 3He; (d) second harmonic heating of deuterium, ω = 2ωcD. An important aim of the analysis was to assess whether the present understanding of the ICRF physics is adequate for predicting the performance of ICRF in DT plasmas. In general, good agreement between experimental results and simulations was found which increases the confidence in predictions of the impact of ICRF heating in future reactors. However, when a relatively high deuterium concentration was used in the ω = ωcD scenario, discrepancies were observed. In order to increase confidence in the simulations, the sensitivity of the simulation results to various plasma parameters has been studied.

Ion Transport in Velocity Space Due to Alfvén-Ion-Cyclotron Fluctuations Excited in the RF Driven Tandem Mirror Plasma

Gentle hump structure on energy spectra of end-loss ions was observed in the RF driven tandem mirror plasma by use of an end-loss energy component analyzer (ELECA), which has been developed in order to directly measure velocity distribution functions of the end-loss ions.1 Ions are heated anisotropically in the central cell of the tandem mirror by the ICRF wave injection, and Alfvén ion-cyclotron (AIC) fluctuations are excited due to the anisotropic ion temperature.2 The correlation between the hump structure and the excitation of the AIC fluctuations was observed experimentally. From the viewpoints of the heating characteristics and the resonance condition, the hump energy was estimated and successfully compared with the experimetal results. Influence of the ICRF waves on the hump structure was investigated using a bounce averaged Fokker Planck code.3 The AIC fluctuations enhance the end-loss ions scattered from the trapped region to the loss region, and have direct effects upon the ion confinement. The energy flux of the enhanced end-loss ions due to the AIC fluctuations was estimated and the enhancement factor was obtained. The strong anisotropic ion heating is dangerous for the ion confinement in the open systems which contain necessarily the loss regions.

Wall conditioning technique development in Tore Supra with permanent magnetic field by ICRF wave injection

Wall conditioning technique development in Tore Supra with permanent magnetic field by ICRF wave injection

Observations of edge ICRF wave modes in the TORTUS tokamak

In this paper, observations are presented of ICRF wave modes that propagate in the form of narrow beams along magnetic field lines in the plasma edge. A beam composed of evanescent shear waves was observed above the ion cyclotron frequency using edge magnetic probes. Observations of the beam were also obtained around the transition defined by , where the shear wave is propagating for and evanescent for . A previously unobserved localized edge feature above the ion cyclotron frequency is also described and is attributed to high m > 0 propagating fast wave modes.

ICRF-induced DD fusion product losses in TFTR

When ICRF power is applied to TFTR plasmas in which there is no externally supplied minority species, an enhanced loss of DD fusion products results. The loss occurs at the passing trapped boundary, and consists of particles at or near their birth energy. The process appears to be the addition of perpendicular velocity to the fusion products as they interact with the ICRF waves, so that marginally passing particles are moved across the passing trapped boundary into the first orbit loss cone. A simplified model of this process predicts losses of a magnitude similar to those seen. Extrapolations based upon these data for hypothetical ICRF ash removal from reactor plasmas suggest that the technique will not be energy efficient for removing fast alphas from the core

Possibility of simulation experiments for fast particle physics in the Large Helical Device (LHD)

The confinement of fusion produced or high energy particles is one of the most important issues to be studied in the helical confinement system. A preliminary study has been carried out on the possibility of developing techniques for simulation experiments for the study of high energy particle physics in the Large Helical Device (LHD) project. Candidate methods have been considered as follows: (a) a high energy (~3.5 MeV) He0 beam injection method; (b) a medium energy (~200 keV) H0 beam injection method; (c) a method involving high energy rail production by an ICRF wave and/or a method of reaction rate enhancement by an ICRF wave; and (d) a method involving the combination of neutral beam injection and ICRF wave. Various features of each method have been considered. Although the high energy He0 beam injection method has some advantages, the technique for production of this beam is extremely difficult because of the difficulties of the production of both negative helium and ground state neutral helium by neutralization. It is pointed out, on the other hand, that a wide range of simulation experiments for fast particle physics may be carried out even by the medium energy beam method, because the typical orbit deviation (e.g. equivalent super-banana size in a classical sense) can be largely controlled by controlling the magnetic field configuration in the case of a helical system, for example by shifting the magnetic axis. This is one of the unique features of a helical system in contrast to an axisymmetric system

Recent D-T results on TFTR

Routine tritium operation in TFTR has permitted investigations of alpha particle physics in parameter ranges resembling those of a reactor core. ICRF wave physics in a DT plasma and the influence of isotopic mass on supershot confinement have also been studied. Continued progress has been made in optimizing fusion power production in TFTR, using extended machine capability and Li wall conditioning. Performance is currently limited by MHD stability. A new reversed magnetic shear regime is being investigated with reduced core transport and a higher predicted stability limit.

Interaction of ICRF waves with fast particles on TEXTOR

The experiments involving the interaction of radio-frequency heating with fast particles injected by neutral beams on TEXTOR are reviewed. The focus point of the present study is the comparison between experiments and theory. Theory involves the modelling of RF diffusion in velocity space, absorption of radio-frequency waves by non-maxwellian populations, computation of drift trajectories to take trapping and first-orbit losses into account. A more specific theoretical development used in this context is the reconstruction of charge exchange spectroscopy spectra. Three types of experiments are examined. The synergetic increase of beam current drive by radio-frequency power, the large neutron production in third harmonic heating of D beam ions and the control of central fast helium concentration by radio-frequency induced diffusion.