High Energy Neutral Beam Research Articles

A 2D Particle in Cell model for ion extraction and focusing in electrostatic accelerators

Negative ions are fundamental to produce intense and high energy neutral beams used to heat the plasma in fusion devices. The processes regulating the ion extraction involve the formation of a sheath on a scale comparable to the Debye length of the plasma. On the other hand, the ion acceleration as a beam is obtained on distances greater than λD. The paper presents a model for both the phases of ion extraction and acceleration of the ions and its implementation in a numerical code. The space charge of particles is deposited following usual Particle in Cell codes technique, while the field is solved with finite element methods. Some hypotheses on the beam plasma transition are described, allowing to model both regions at the same time. The code was tested with the geometry of the NIO1 negative ions source, and the results are compared with existing ray tracing codes and discussed.

Development of the gas puff charge exchange recombination spectroscopy (GP-CXRS) technique for ion measurements in the plasma edge

A novel charge-exchange recombination spectroscopy (CXRS) diagnostic method is presented, which uses a simple thermal gas puff for its donor neutral source, instead of the typical high-energy neutral beam. This diagnostic, named gas puff CXRS (GP-CXRS), is used to measure ion density, velocity, and temperature in the tokamak edge/pedestal region with excellent signal-background ratios, and has a number of advantages to conventional beam-based CXRS systems. Here we develop the physics basis for GP-CXRS, including the neutral transport, the charge-exchange process at low energies, and effects of energy-dependent rate coefficients on the measurements. The GP-CXRS hardware setup is described on two separate tokamaks, Alcator C-Mod and ASDEX Upgrade. Measured spectra and profiles are also presented. Profile comparisons of GP-CXRS and a beam based CXRS system show good agreement. Emphasis is given throughout to describing guiding principles for users interested in applying the GP-CXRS diagnostic technique.

A high spatial resolution Stokes polarimeter for motional Stark effect imaging

We describe an enhanced temporally switched interfero-polarimeter that has been successfully deployed for high spatial resolution motional Stark effect imaging on the KSTAR superconducting tokamak. The system utilizes dual switching ferroelectric liquid crystal waveplates to image the full Stokes vector of elliptically polarized and Doppler-shifted Stark-Zeeman Balmer-alpha emission from high energy neutral beams injected into the magnetized plasma. We describe the optical system and compare its performance against a Mueller matrix model that takes account of non-ideal performance of the switching ferro-electric liquid crystal waveplates and other polarizing components.

Studies on neutral beam ion confinement and excitation of beta-induced Alfvén acoustic eigenmodes in the HL-2A tokamak

Experiments with high-energy deuterium neutral beam (NB) injection were performed on the HL-2A tokamak. To obtain information on NB deposition and the slowing down of beam ions in HL-2A plasmas, very short-pulse deuterium NB injection, or the so-called ‘blip’ injection, was applied to MHD-quiescent ohmic deuterium plasmas. Analysis of neutron decay following the NB ‘blip’ injection indicates that tangentially injected beam ions are well confined, slowing down classically in the HL-2A tokamak. In contrast to the MHD-quiescent plasma, anomalous losses of beam ions were observed when a core-localized mode with a frequency up-chirping from 15 to 40 kHz appeared in the plasma. The core-localized mode was identified as a beta-induced Alfvén acoustic (BAAE) mode by its frequency sweeping behaviour and numerical calculation. Such a high energetic particle driven mode led to fast-ion loss, showing the strong influence of the core-localized fast-ion-driven BAAE mode on the fast-ion transport. Furthermore, a clear frequency splitting was first observed on the Alfvén-acoustic-type mode, and is found to be strongly linked to the effect of resonant wave–particle interaction, providing further insights into how frequency splitting structures are generated in the plasma.

Simulations of combined neutral beam injection and ion cyclotron heating with the TORIC-SSFPQL package

A source describing the injection of fast ions due to the ionization of high-energy neutral beams has been added to the surface-averaged quasilinear Fokker–Planck code SSFPQL (Brambilla 1994 Nucl. Fusion 34 1121). For this purpose, the multiple-beam NBI code SINBAD (Feng et al 1995 Comput. Phys. Commun. 88 161) has been included as a module in SSFPQL, with the modifications required to handle arbitrary axisymmetric equilibria. Alternatively, the neutral beam injection (NBI) source can be built using the output of a Monte Carlo NBI code. We have also added a term describing losses of fast ions during thermalization, and a subroutine evaluating the neutron production rate by nuclear reactions. With these extensions, iterations between SSFPQL and the full-wave solver TORIC (Brambilla 1999 Plasma Phys. Control. Fusion 41 1) can now be used to investigate the strong interplay between NBI and ion cyclotron (IC) heating.By comparing the predicted and measured neutron production rates from D–D reactions in a discharge with combined NBI and IC heating in ASDEX Upgrade we obtain a plausible estimate of the importance of fast-ion losses (FILs), even if their cause cannot be identified. We find, however, that the plasma composition, in particular the presence of low Z impurities, plays a more critical role than FILs in limiting the efficiency of this heating scheme.

Investigation of beam– and wave–plasma interactions in spherical tokamak Globus-M

The experimental and theoretical results obtained in the last two years on the interaction of neutral particle beams and high-frequency waves with a plasma using the spherical tokamak Globus-M are discussed. The experiments on the injection of low-energy proton beam of ∼300 eV directed particle energy are performed with a plasma gun that produces a hydrogen plasma jet of density up to 3 × 1022 m−3 and a high velocity up to 250 km s−1. A moderate density rise (up to 30%) is achieved in the central plasma region without plasma disruption. Experiments on high-energy (up to 30 keV) neutral beam injection into the D-plasma are analysed. Modelling results on confinement of fast particles inside the plasma column that follows the neutral beam injection are discussed. The influence of the magnetic field on the fast particle losses is argued. A neutral beam injection regime with primary ion heating is obtained and discussed. The new regime with fast current ramp-up and early neutral beam injection shows electron temperature rise and formation of broad Te profiles until the q = 1 flux surface enters the plasma column. An energetic particle mode in the range of frequencies 5–30 kHz and toroidal Alfvén eigenmodes in the range 50–300 kHz are recorded in that regime simultaneously with the Te rise. The energetic particle mode and toroidal Alfvén eigenmodes behaviour are discussed. The toroidal Alfvén eigenmode spectrum appears in Globus-M as a narrow band corresponding to n = 1. The first experimental results on plasma start-up and noninductive current drive generation are presented. The experiments are carried out with antennae providing mostly poloidal slowing down of waves with a frequency of 920 MHz, which is higher than a lower hybrid one existing under the experimental conditions. The high current drive efficiency is shown to be high (of about 0.25 A W−1), and its mechanism is proposed. Some near future plans of the experiments are also discussed.

Achievement and improvement of the JT-60U negative ion source for JT-60 Super Advanced (invited)

Developments of the large negative ion source have been progressed in the high-energy, high-power, and long-pulse neutral beam injector for JT-60 Super Advanced. Countermeasures have been studied and tested for critical issues of grid heat load and voltage holding capability. As for the heat load of the acceleration grids, direct interception of D- ions was reduced by adjusting the beamlet steering. As a result, the heat load was reduced below an allowable level for long-pulse injections. As for the voltage holding capability, local electric field was mitigated by tuning gap lengths between large-area acceleration grids in the accelerator. As a result, the voltage holding capability was improved up to the rated value of 500 kV. To investigate the voltage holding capability during beam acceleration, the beam acceleration test is ongoing with new extended gap.

10.1007/s11452-008-2001-9

Results from experimental studies on the injection of high-energy neutral hydrogen beams into the plasma of the Globus-M spherical tokamak are reviewed. In the Introduction, the importance of these studies for implementing the controlled fusion research program and constructing the ITER tokamak is proved. Some problems related to the use of neutral beam injection in small and low-aspect-ratio tokamaks is analyzed. Results are presented from numerical simulations of the experiment by using the ASTRA transport code. It is shown that the use of neutral beam injection in the Globus-M tokamak ensures efficient ion heating and increases the plasma stored energy. The greater part of the review is devoted to the survey of experiments on the injection of 22-to 30-keV hydrogen and deuterium beams with a power of 0.4–0.8 MW into the plasma of the Globus-M spherical tokamak in a wide range of plasma currents and densities. The experimental results are analyzed and compared with the results of numerical simulations. The achievement of top plasma parameters is highlighted.

Tungsten accumulation in H-mode plasmas of JT-60U

It has been observed that with increasing toroidal rotation velocity inside the q = 1 layer in the direction opposite to the plasma current, sawtooth activity becomes moderate and tungsten accumulation becomes significant. The tungsten accumulation level is significantly reduced from this trend in the case when electron cyclotron wave or high energy neutral beam is injected into the plasma core. In contrast, the tungsten accumulation is kept high by the electron cyclotron wave injection into the peripheral region.

Beam Ion Transport from Neutron Emissivity Measurement in Deuterium Neutral Beam-Heated Deuterium Plasmas of the HL-2A Tokamak

An experiment at the HL-2A tokamak with a high-energy deuterium neutral beam (NB) injection (30keV, about 0.6 MW) was performed. The emission of d-d fusion neutrons dominated by beam-plasmas reactions when the deuterium NB was injected into the deuterium target plasma was observed by means of a 235U fission chamber. To obtain information on NB deposition and the slowing down of beam ions in HL-2A plasmas, a very short-pulse deuterium NB injection, or the so-called ‘blip’ injection, was performed into MHD-quiescent Ohmic deuterium plasmas. Analysis of neutron decay following the NB ‘blip’ injection indicates that tangentially injected beam ions are well confined, slowing down classically in the HL-2A. In contrast to the MHD-quiescent plasma, anomalous losses of beam ions were observed when tearing mode instabilities were present.

Recent Progress of Neutral Beam Injector and Beam Emission Diagnosis in LHD

Large size hydrogen neutral beam injectors (NBI) used a negative ion source (NNBI) as well as a proton source (PNBI) were developed for the large helical device (LHD). The injected power from NNBI and PNBI have reached 16 MW and 6.8 MW, respectively. These injected powers have outstripped the nominal beam powers. A diagnostic system of beam-emitted hydrogen visible spectrum has been installed along the beam injection axis to estimate the energy fraction on PNBI. The full energy beam component is about half which is equivalent to 70% of injected beam power. The attenuation of high energy neutral beam is also observed on NNBI. The peak density distribution is effective to increase beam deposition power.

Study of plasma heating in discharges with neutral beam injection in the Globus-M spherical tokamak

Results from experimental studies on the injection of high-energy neutral hydrogen beams into the plasma of the Globus-M spherical tokamak are reviewed. In the Introduction, the importance of these studies for implementing the controlled fusion research program and constructing the ITER tokamak is proved. Some problems related to the use of neutral beam injection in small and low-aspect-ratio tokamaks is analyzed. Results are presented from numerical simulations of the experiment by using the ASTRA transport code. It is shown that the use of neutral beam injection in the Globus-M tokamak ensures efficient ion heating and increases the plasma stored energy. The greater part of the review is devoted to the survey of experiments on the injection of 22-to 30-keV hydrogen and deuterium beams with a power of 0.4–0.8 MW into the plasma of the Globus-M spherical tokamak in a wide range of plasma currents and densities. The experimental results are analyzed and compared with the results of numerical simulations. The achievement of top plasma parameters is highlighted.

Improvement of Ion Confinement in Core Electron-Root Confinement (CERC) Plasmas in Large Helical Device

An increase in ion temperature has been observed with superposition of centrally focused electron cyclotron resonance heating (ECRH) to plasmas heated by high-energy neutral beam injection (NBI) in Large Helical Device. The ion-temperature (Ti) rise is accompanied by the formation of electron internal transport barrier (ITB). A transport analysis shows that ion transport as well as electron transport is improved with the reduction of anomalous transport. A neoclassical ambipolar flux calculation shows a positive radial-electric field (Er) in the region of the Ti rise, and Er should suppress the enhancement of ripple transport due to the Ti-rise. These analyses indicate the ion transport improvement in the core electron-root confinement plasmas. Toroidal rotation is driven in the co-direction by applying ECRH, and the toroidal rotation velocity is increased with the Ti rise. A correlation between the Ti rise and toroidal rotation is suggested.

Positive and negative ion sources for magnetic fusion

The positive or negative ion sources which form the primary components of neutral beam injection systems used in controlled nuclear fusion using magnetic confinement have to meet simultaneously several demanding requirements. This paper describes the underlying physics of modern positive ion sources, which provide the required high proton fraction (>90%) and high current density (/spl ap/2 kA/m/sup 2/) at a low source pressure (0.4 Pa) with a high electrical efficiency and uniformity across the accelerator grids. The development of negative ion sources, which are required if high energy neutral beams are to be produced, is described, and the present understanding of the physics of negative ion production in sources is explained. The paper reports that negative ion sources have achieved many of the parameters required of sources for the neutral beam injectors of future fusion devices and reactors, >200 A/m/sup 2/ of D/sup -/ at low source pressure, <0.3 Pa, with a low co-extracted electron content. The development needed to meet all the requirements of future systems is briefly discussed.

Anisotropic fast neutral particle spectra in the MAST spherical tokamak

The behaviour of fast ions, generated by high energy neutral beams, is investigated in the MAST spherical tokamak using a spatially scanning neutral particle analyser (NPA), which has views that detect charge exchange neutrals from both passing and trapped ions. Almost featureless neutral particle spectra are produced, characteristic of thermalization of the neutral beam energy components, when viewing the charge exchange neutrals from passing fast ions in both L- and H-modes. Using NPA views with low tangency radius, RNPA, both passing and trapped fast ions can be monitored. In L-mode, similar featureless spectra are observed in contrast to H-mode, where they are highly structured. All these aspects have been faithfully reproduced using particle transport codes showing, in particular, that the H-mode spectra found at low RNPA arise from a small fraction of poorly confined trapped fast ions at the edge, an effect enhanced by the relatively high edge ion and neutral densities.

NBI:核融合炉に向けた物理&amp;bull;工学の進展 2.LHD における高エネルギー中性粒子ビーム入射のインパクト

NBI:核融合炉に向けた物理&amp;bull;工学の進展 2.LHD における高エネルギー中性粒子ビーム入射のインパクト

Charge-exchange spectroscopy at the W7-AS stellarator employing a high-energy Li beam

Combined measurements of ion density, temperature and poloidal rotation of impurity ions have been carried out in the gradient region of the Wendelstein 7-AS plasma, employing charge-exchange spectroscopy with the high-energy neutral Li beam and C6+/C5+ ions. The present state of the experimental technique is described. Results are obtained in discharges with high central ion temperatures of 1.3 keV, central electron densities of 7×1019 m−3. Under these conditions the diagnostic covers a region of about 7 cm radial extent, corresponding to ion temperatures between 50 and 800 eV. Poloidal rotation velocities up to (35±2.5) km s−1 were measured in a narrow shear layer at the plasma edge. The contribution of ion diamagnetic and E×B drift is discussed. The radial electric field is derived from a simplified radial force balance equation.

Alfvén eigenmodes driven by Alfvénic beam ions in JT-60U

Instabilities with frequency chirping in the frequency range ofAlfvén eigenmodes have been found in the domain 0.1% < ⟨βh⟩ < 1% and vb||/vA ∼1 with high energy neutral beam injection in JT-60U. One instability with a frequency inside the Alfvén continuum spectrum appears and its frequency increases slowly to the toroidicityinduced Alfvén eigenmode (TAE) gap on the timescale of an equilibrium change ( ≈ 200 ms).Other instabilities appear with a frequency inside the TAE gap and theirfrequencies change very quickly by 10-20 kHz in 1-5 ms.During the period when these fast frequency sweeping (fast FS) modes occur, abrupt large amplitude events (ALEs) often appear with a drop of neutron emission rateand an increase in fast neutral particle fluxes.The loss of energetic ions increases with a peakfluctuation amplitude of B̃θ/Bθ. An energy dependence of the loss ions is observed and suggests a resonant interaction between energetic ions and the mode.

Beam divergence and power loading on the beamline components of the negative-ion based NBI system for JT-60U

In JT-60U, a high-energy neutral beam injection (NBI) program using negative-ion based NBI (N-NBI) for non-inductive current drive and core plasma heating studies in high-density plasma has progressed. The target performance of the N-NBI is a neutral beam injection power of 10 MW for 10 s at 500 keV. A neutral beam power of 5.2 MW at 350 keV with deuterium has already been injected into JT-60U plasma. The beam divergence and power loading onto the beamline components are two important items to evaluate beam performance. The beam divergence, estimated roughly from heat loads at the beam drift duct in beam injection experiments into JT-60U plasma, was around 4 mrad for the horizontal direction and 6 mrad for the vertical direction, which were close to the design values of 5 mrad. The power loading on beamline components were reasonable as compared with the design value.

Spectral Analysis of Highly Ionised Fusion Plasmas Using Beams

Spectral analysis of tokamak plasmas penetrated by medium to high energy (5 keV/amu – 120 keV/amu) neutral deuterium and helium beams is explained. Attention is limited to the diagnostic exploitation of absolute measured spectral intensities. Such measurements reflect the population distributions of the excited emitting species in the beams and in the plasma and the influences upon them. For the neutral beam atom emitters, impact of plasma impurity ions is the primary influence in the medium/high energy regime, but the effective emission at tokamak densities represents the collective effect of many re-distributive processes. Also, the method of analysis is governed by the relaxation lengths of populations in the beam compared with density and temperature scale lengths in the plasma and the spatial resolution expected of spectroscopy measurements. For impurity ion emitters in the plasma, emission follows the primary charge transfer from the neutral beam donor. The distribution of capture is influenced by the various donor states and the consequential effective emission again influenced by re-distributive processes in the plasma. The current state of such charge exchange and beam emission spectroscopic diagnostics is examined from the point of view of the quality of analysis achieved, the factors limiting precision and how enhanced precision might be obtained in the future.