Tangential Neutral Beam Research Articles

Heterodyne ECE diagnostic in the mode detection and disruption avoidance at TEXTOR

The paper describes the detection of the m/n = 2/1 disruption precursor at TEXTOR. This frequently observed precursor is detected by realtime cross-correlation of two radially separated channels of the heterodyne electron cyclotron emission diagnostic. An electronic module developed to detect the precursor and its functionality is briefly described. A disruption statistic for TEXTOR is presented, from which the probability of the occurrence of the m/n = 2/1 mode is seen. A method to generate reproducible disruption precursors at TEXTOR is also presented. Experiments to avoid disruptions by inducing toroidal velocity shear by tangential neutral beam injection and mitigate disruptions by He-puffing are presented. In the experiments performed, up to 50% of the disruptions could be avoided. Furthermore, calculations of the precursor frequency and its evolution are described and compared with measurements.

Physics basis for a spherical torus power plant

The spherical torus, or low-aspect-ratio tokamak, is considered as the basis for a fusion power plant. A special class of wall-stabilized high- β high-bootstrap fraction low-aspect-ratio tokamak equilibrium are analyzed with respect to MHD stability, bootstrap current and external current drive, poloidal field system requirements, power and particle exhaust and plasma operating regime. Overall systems optimization leads to a choice of aspect ratio A=1.6, plasma elongation κ=3.4, and triangularity δ=0.64. The design value for the plasma toroidal β is 50%, corresponding to β N=7.4, (based on the toroidal β, not the total β) which is 10% below the ideal stability limit. The bootstrap fraction of 99% greatly alleviates the current drive requirements, with tangential neutral beam injection mainly for profile control and rotation drive. The design is such that 45% of the thermal power is radiated in the plasma by Bremstrahlung and trace Krypton, with Neon in the scrapeoff layer radiating the remainder. The scarcity of spherical torus experimental data caused the design to be based largely on theoretical predictions, some of which may turn out to be overly optimistic. This highlights the need for a strong experimental research program in this area.

Effects of finite radial excursion on the slowing-down distribution of toroidally circulating energetic ions produced by tangential neutral beam injection

Slowing-down of the toroidally circulating energetic ions produced by tangential neutral beam injection is described by the reduced drift kinetic equation, in terms of three constants of motion (the energy, the magnetic moment and the canonical toroidal momentum), to take into account the effects of finite radial excursion of guiding-centers of the energetic ions. An approximate analytical solution to the slowing-down equation is obtained to describe the equilibrium slowing-down distribution of the toroidally circulating energetic ions in the core region of a large-aspect-ratio tokamak, with the effects of finite radial excursion included.

Observation of inward turbulent particle transportin edge plasma region of CHS heliotron/torsatron

Reversal of turbulent particle flux near the plasma edge of CHS heliotron/ torsatron was observed on various discharge conditions, i.e. during tangential neutral beam injection, electron cyclotron heating at the far edge, and the sawtooth crash. For all cases, the reversal is realized at a narrow particular layer near the plasma edge, when plasma parameters go into low collisionality. The reversal is caused by the change in the cross phase between density (ne) and poloidal electric field (Eθ) fluctuations. The turbulent flux is always inward when the radial electric field shear (Er') exceeds a certain threshold of ~1×106 V m-2. In the sawtoothing plasmas, the arrival of a heat pulse due to the sawtooth crash enhances both ne and Eθ fluctuations but changes the cross phase to π in low-frequency range. As a result, a large inward flux is generated by the sawtooth crash. The particle flux reversal in these cases did not lead to obvious confinement improvement.

Nonresonant suppression of the sawtooth instability by passing ions in tokamaks

A new interpretation is presented for the well-known observation of sawteeth stabilization, during ion cyclotron resonant heating, in the Joint European Tokamak [Campbell et al., Phys. Rev. Lett. 60, 2148 (1988)] and the Tokamak Fusion Test Reactor [Phillips et al., Phys. Fluids B 4, 2155 (1992)]. It is shown that the radial fluxes of superthermal and thermal ions exchange a finite amount of power with the m=1 mode (associated with the sawtooth instability) which is suppressed. The dominant contribution to this effect is provided by the passing ions, which experience in the presence of the mode a much faster (than the trapped ions) radial diffusion. This is a nonresonant electrostatic interaction between the passing ions and the finite parallel electric field (along the perturbed magnetic field line), which takes place at any mode frequency. For the requirements of the present theory, the concepts of the finite parallel electric field, and the diffusion of passing ions, in the presence of a magnetic mode are also developed. The present work also provides the interpretation for the suppression of the sawtooth instability by passing ions, during tangential neutral beam injection, in the Japan Atomic Energy Research Institute Tokamak Upgrade (JT-60U) [Kramer et al., Nucl. Fusion 40, 1383 (2000)].

Physics Design of Poloidal Field, Toroidal Field, and External Magnetic Diagnostics in KSTAR

The Korea Superconducting Tokamak Advanced Research (KSTAR) tokamak will have superconducting magnets for both the poloidal field (PF) coils and the toroidal field (TF) coils. The physical arrangement of the PF configuration has 14 coils external to the TF coils. The analysis of the equilibrium flexibility of the KSTAR PF system determines the coil currents required to maintain prescribed equilibrium configurations over the desired range of operational parameters specified for Ip (q95), βN, and li(3). Constraints on the plasma separatrix and the flux linkage through the geometric center of the plasma are specified for the free-boundary equilibrium calculations. The ripple magnitude due to the finite number of TF coils and the size of the port for the neutral beam (NB) injector determine the number, size, and shape of TF coils. Two ripple criteria for a shaped plasma are used for types of ripple transport. The current design of the TF coil, with 16 coils and a D shape, is big enough to satisfy requirements for the ripple magnitude at the plasma and to provide adequate access for tangential NB injection. The external magnetic diagnostics, magnetic probes and flux loops to detect the plasma boundary are designed by the EFIT code, which uses a realistic distributed current source constrained by equilibrium. The proposed configuration with 52 full toroidal flux loops and 78 magnetic probes results in <0.7 cm deviation at critical points, with the Gaussian-distributed 3% random root-mean-square perturbation in the signal.

Improvement of the highly energetic passing particle confinement in torsatrons and heliotrons

The configurational effects on the confinement of highly energetic passing particles, especially the helical-field ripple effects, are considered in torsatrons and heliotrons from the viewpoint of increasing the efficiency of tangential neutral beam injection (NBI) heating of plasma. The drift resonances of passing particles (ιΩφ=ΩD, where ι is the rotational transform, Ωφ is the particle toroidal transit frequency, and ΩD is the particle drift frequency) are investigated. It is discovered that the resonant passing particles oscillate poloidally through the variation of the field-line curvature. The passing particles with the energies located in the vicinity of the resonance separatrix are revealed to be as dangerous for confinement, as superbananas are. The existence of the highly poloidally localized resonant particles with lower radial deviations than the corresponding values for the nonresonant particles was found. The particles with the energies close to the state of the high poloidal localization are proposed as the most preferable beam particles for the tangential NBI heating of plasma.

An inversion technique to obtain full poloidal velocity profiles in a tokamak plasma

An inversion technique has been developed to calculate local poloidal rotation velocities of impurity ions from line-integrated measurements of an extended emission source in a tokamak plasma. This technique can recover radial spatial resolution that is lost due to the curvature of the field lines. With charge exchange emission from neutral beams, it can be used to obtain poloidal velocity profiles across the entire minor radius of the plasma without requiring restricted neutral beam height to localize the emission. The technique consists of two Abel-like matrix inversions to obtain the emissivity and the velocity-weighted emissivity from which the velocity is obtained. Use of the neutral beams breaks the assumed symmetry, but knowledge of the beam deposition and geometry allow analogous matrices to be generated. The inversion technique is demonstrated with a simulation of a poloidal rotation diagnostic for the TFTR tokamak with tangential neutral beams with a height of 80 cm. Two opposing vertical views are required to handle effects of the charge exchange cross section, which can cause apparent velocities greatly in excess of expected poloidal velocities. The simulated errors due to the inversion process are about 1.5 km/s near the plasma center.

Comparison of toroidal viscosity with neoclassical theory

Toroidal rotation profiles are measured with charge exchange spectroscopy for a plasma heated with tangential neutral beam injection (NBI) in the Compact Helical System (CHS) heliotron/torsatron device [Ida et al., Phys. Rev. Lett. 67, 58 (1991)] to estimate the toroidal viscosity. The toroidal viscosity derived from the toroidal rotation velocity shows good agreement with the neoclassical toroidal viscosity plus the perpendicular viscosity (μ⊥=2 m2/s).

An inversion technique to obtain full poloidal velocity profiles in a tokamak plasma (abstract)

An inversion technique has been developed to calculate local poloidal rotation velocities of impurity ions from line-integrated measurements of an extended emission source in a tokamak plasma. This technique can recover radial spatial resolution that is lost due to the curvature of the field lines. With charge exchange emission from neutral beams, it can be used to obtain poloidal velocity profiles across the entire minor radius of the plasma without requiring restricted neutral beam height to localize the emission. The technique consists of two Abel-like matrix inversions to obtain the emissivity and the velocity-weighted emissivity from which the velocity is obtained. Use of the neutral beams breaks the assumed symmetry, but knowledge of the beam deposition and geometry allow analogous matrices to be generated. The inversion technique is demonstrated with a simulation of a poloidal rotation diagnostic for the TFTR tokamak with tangential neutral beams with a height of 80 cm. Two opposing vertical views are required to handle effects of the charge exchange cross section which can cause apparent velocities greatly in excess of expected poloidal velocities. The simulated errors due to the inversion process are about 1.5 km/s near the plasma center.

MHD-mode stabilization by plasma rotation in TEXTOR

An experimental investigation into rotating MHD modes has been performed in the TEXTOR tokamak. The effects on the stability of the MHD tearing modes of coupling between m/n = 2/1 and 1/1 modes and of the slowing down of the mode rotation by wall friction have been studied. Tangential neutral beam injection (NBI) has been used to change the toroidal rotation to observe the influence of plasma rotation on the development of MHD modes. Two phases in the rotation frequency behaviour of the modes can be distinguished: a slow reversible decrease and a fast exponential decay, which starts with a short transient increase in frequency. The last phase finally results in mode locking, which mostly leads to a disruption. By injecting low power NBI in the last phase of the slowing-down process, mode locking could be prevented. The mode amplitude and rotation frequency have been stabilized, without major changes to the pressure profile. By decreasing the central plasma velocity, using NBI, a coupling of the two modes has been forced. The coupling of the modes had a destabilizing effect on the MHD activity.

Characterization of fast ion behaviour during tangential neutral beam injection in the Advanced Toroidal Facility

The confinement characteristics and behaviour of energetic ions injected during tangential neutral beam injection (NBI) experiments on the Advanced Toroidal Facility (ATF) are examined using several experimental and computational methods. Measurements by a two dimensional scanning neutral particle analyser of the fast ion, slowing down spectra of the injected ions have been used to examine the characteristics of the fast ion distribution in low density plasmas (ne ⩽ 1.0 × 1019 m-3). The energetic ions are found to behave in a manner consistent with the classical slowing down process embodied in the fast ion Fokker-Planck equation. Neutron measurements have been used to extend these results to higher density plasmas (ne ≈ 7.5 × 1019 m-3). Because of experimental uncertainties, it is difficult to determine whether the measured neutron rates are consistent with classical predictions; however, there are indications in some ATF magnetic configurations that the measured neutron source rates are much lower than expected, indicating an enhanced orbit loss of fast ions in these configurations. In addition, computational studies suggest that the effectiveness of tangential NBI in certain operating regimes in ATF may be limited owing to the presence of loss regions at the trapping boundary

High beta discharges with neutral beam injection in CHS

High beta plasmas with a volume averaged equilibrium beta value of 2.1% were produced in CHS using tangential neutral beam injection. This beta value was achieved with the confinement improvement (reheat mode) observed after turning off strong gas puffing. Wall conditioning with titanium gettering was used to make high density operation (ne ⩽ 8 × 1019 m-3) possible for low magnetic fields (Bt = 0.6 T). The discharges start with the magnetic hill configuration (in vacuum) and finally achieve Mercier stable equilibrium owing to the self-stabilization effect given by the magnetic well which is produced by the plasma pressure. The Shafranov shift was about 40% of a plasma minor radius. Magnetic fluctuations did not increase with increasing plasma pressure when the beta value exceeded 1%. Dynamic poloidal field control was applied to suppress the outward plasma shift with increasing plasma pressure. Such operation gave an additional increase of beta value compared with the constant poloidal field operation

Non-inductive current drive experiments in JT-60U

Abstract Non-inductive current drive experiments by lower hybrid (LH) waves and neutral beams (NBs) in JT-60U are described with emphasis on current profile control. By phasings of LH multijunction launcher, the plasma internal inductance li was either increased by 0.04 or decreased by 0.13 from the ohmic value with 1.5 s, 3 MW LH waves. On the basis of this fundamental experiment, the internal inductance of the target plasma for NB injection was controlled by LH current drive shot by shot. We observed clear improvement in the energy confinement of the neutral beam heated plasma with the increase in li. Current profile control consistent with theory was also demonstrated by on-and off-axis tangential NBs. In addition, the profile of the plasma toroidal rotation was modified as inferred from the expected profile of NB momentum input. Combined full current drive of on-axis LH and off-axis NB current drives was also demonstrated.

Plasma oscillations induced by tangentially injected neutral beams

A stability analysis is carried out for the m=1, n=1 internal kink mode, in the presence of energetic circulating particles. Including the effect of finite radial particle-orbit excursion, the energetic particles that cross the q<1 region can resonantly interact with the mode and destabilize it. For typical tokamak parameters, the growth rate of the instability is comparable to the one of the corresponding trapped-particle induced instability. Such an instability could explain the experimental observations of 'fishbone' oscillations during tangential neutral beam injection.

Toroidal rotation and ion heating during neutral beam injection in PBX-M

Determination of the profiles of the ion temperature and the plasma toroidal rotation has been accomplished by charge exchange recombination spectroscopy in PBX-M. The angular momentum and the thermal ion energy transport have been studied mainly during the H mode phase of a high βp discharge (Ip ≈ 330 kA, 3.5 × 1019 ⩽ ne ⩽ 6.5 × 1019 m-3) having different heating beam configurations (combination of two perpendicular and two tangential neutral beam injections, abbreviated as 2 perp. NBI and 2 parall. NBI). The toroidal rotation velocity Vϕ rises substantially in the region of r/a ⩾ 0.5 after the L-H transition, and the Vϕ profile (peakedness) is more highly dependent on the beam configuration than the Ti profile. The angular momentum confinement time varies from 147 ms (rigid rotation for 2 perp. NBI) to 39 ms (viscous rotation for 2 parall. NBI). In contrast, the thermal energy confinement time is 44-48 ms and is almost independent of the configuration. The transport analysis shows that the radial angular momentum diffusion is caused mainly by the viscous losses and that the angular momentum diffusivity χϕ is reduced substantially in the outer minor radius region during the 2 perp. NBI H mode. The neoclassical friction effect between the bulk ions and the impurities may influence the χϕ profiles locally, where the ion temperature gradient is steep

Destabilization of the internal kink by energetic-circulating ions.

A stability analysis is carried out for the m=1 internal kink mode, in the presence of energetic circulating particles. It is found that, including the effect of finite radial particle-orbit excursion, the m=1 internal kink mode is strongly destabilized by the resonance interaction with the energetic passing particles. Such an instability could explain the experimental observations of the ``fishbone'' oscillations during tangential neutral beam injection [W. W. Heidbrink et al., Phys. Rev. Lett. 57, 835 (1986)].

The configuration development of the superconducting magnets for the SSAT-S tokamak device

The Steady State Advanced Tokamak (SSAT) device will be developed to demonstrate steady-state tokamak operations and optimize plasma conditions in a steady-state environment. The authors describe the configurational development of the SSAT device to meet the physics objectives and subsystem design requirements. Superconducting toroidal field (TF) and poloidal field (PF) coils are planned, as they are well suited for the long-pulse, high-duty-factor operation of the SSAT mission. A two-coil TF module forms the basis of the tokamak core. PF coils, consisting of a solenoid and six ring-coils, surround the TF coils. The physics requirement of low field ripple and sufficient plasma access for tangential neutral beams influenced the configuration in determining the size and number of TF coils. Further configuration influences included low activation considerations, divertor pumping, service line access, requirements for remote maintenance, and compatibility with the existing TFTR test cell facility. >

Measurements of the radial structure and poloidal spectra of toroidal Alfvén eigenmodes in the Tokamak Fusion Test Reactor

Toroidal Alfvén eigenmodes (TAE) have been excited by tangential neutral beam injection in the Tokamak Fusion Test Reactor (TFTR) [Proceedings of the Thirteenth International Conference on Plasma Physics and Controlled Nuclear Fusion Research, 1990, Washington, D.C. (International Atomic Energy Agency, Vienna, 1990), Vol. I, p. 9]. Beam emission spectroscopy (BES) has been used to study the radial structure and the poloidal power spectra of these modes. Radial profiles show a global, standing wave structure with a node near r/a=0.6 and a maximum displacement of about 5–10 mm. The cross-phase profiles and the power spectra both imply that the mode is composed of a mixture of components with various poloidal and toroidal mode numbers, as expected for the TAE. Measurements of the poloidal mode spectrum via BES show good agreement with theoretical simulations performed by a nonvariational, kinetic magnetohydrodynamics stability code (nova−k [Cheng, Phys. Rep. 211, 1 (1992)]). In particular, the dominant harmonics in the poloidal spectrum obey the expected relation m+1/2≊q(r)n.

Status and Plans for JT-60U

The present status and the experimental results of the initial phase of JT-60U up to the February 1992 are summarized. After the completion of the major modification in March 1991, total of more than three months of shut down work has been done to install four tangential neutral beams. Along with the present main target to conduct an effective conditioning of machine and heating systems in order to get the highest performance of JT-60U, a number of new findings are being studied in areas of plasma control, divertor, LH current drive, etc. Future plans of new 500 keV negative-ion beam system and a proposal of further modification are also described.