Neutral Beam Ion Research Articles

Effect of low magnetic shear induced by lower hybrid current drive on high performance internal transport barriers in the Joint European Torus (JET)

A low/negative magnetic shear profile is maintained in the Joint European Torus in a plasma target with plasma current of 2.4 MA using 2.2 MW of lower hybrid (LH) power combined with neutral beam injection and ion cyclotron radiofrequency heating. In this scenario, an internal transport barrier (ITB) with time duration up to about 4 s is produced. The fraction of LH driven current is about 25% the total plasma current. During LH power application, the layer with reversed shear q-profile can be maintained in a suitable radial position to inhibit the onset of turbulence, which might otherwise drive the ITB to collapse. LH power could be used as a tool to drive moderate amounts of noninductive off-axis current and sustain high performance ITBs at high plasma current.

Energetic ion loss diagnostic for the Wendelstein 7-AS stellarator

A diagnostic to measure the loss of energetic ions from the Wendelstein 7-AS (W7-AS) stellarator has been built. It is capable of measuring losses of both neutral beam ions and energetic ions arising from ion cyclotron resonant heating. The probe can measure losses of both clockwise and counterclockwise-going energetic ions simultaneously, and accepts a wide range of pitch angles in both directions. Initial measurements by the diagnostic are reported.

Ion Propulsion Development Projects in U.S.: Space Electric Rocket Test I to Deep Space 1

The historical background and characteristics of the experimental flights of ion propulsion systems and the major ground-based technology demonstrations are reviewed. The results of the first successful ion engine flight in 1964, Space Electric Rocket Test (SERT) I, which demonstrated ion beam neutralization, are discussed along with the extended operation of SERT II starting in 1970. These results together with the technologies employed on the early cesium engine flights, the applications technology satellite series, and the ground-test demonstrations, have provided the evolutionary path for the development of xenon ion thruster component technologies, control systems, and power circuit implementations. In the 1997-1999 period, the communication satellite flights using ion engine systems and the Deep Space 1 flight confirmed that these auxiliary and primary propulsion systems have advanced to a high level of flight readiness.

Fast ion losses in the W7-AS stellarator

A new probe for the detection of fast escaping ions has been installed and operated at the W7-AS stellarator. Fast ions generated from neutral beam injection or ion cyclotron heating were used to study the fast ion confinement properties of the partially drift-optimized magnetic field structure as well as losses induced by magnetohydrodynamic perturbations. The design of the probe is based on the α-particle detectors of the Tokamak Fusion Test Reactor using scintillator plates and allows the detection of co-and countergoing ions simultaneously in a wide range of gyroradii and pitch angles.

Fast ion loss diagnostic plans for the National Spherical Torus Experiment

The prompt loss of neutral beam ions from the National Spherical Torus Experiment is expected to be between 12% and 42% of the total 5 MW of beam power. There may, in addition, be losses of fast ions arising from high harmonic fast wave (HHFW) heating. Most of the lost ions will strike the HHFW antenna or the neutral beam dump. To measure these losses in the 2000 experimental campaign, thermocouples in the antenna, several infrared camera views, and a Faraday cup lost ion probe will be employed. The probe will measure loss of fast ions with E>1 keV at three radial locations, giving the scrape-off length of the fast ions.

Neutron emission study of DT plasmas heated with tritium neutral beams

The neutron emission energy spectrum from deuterium–tritium fusion reactions has been measured in experiments carried out at the Joint European Torus for plasmas heated with high power neutral beam (NB) injection of tritium beams at Eb≈150 keV using a magnetic proton recoil neutron spectrometer. High quality data were obtained in which up to three spectral components due to different ion reactions could be distinguished with the help of dedicated model calculations of the fusion neutron emission based on Monte Carlo simulations. The analysis model involved neutrons from reactions of NB ions on passing and trapped orbits interacting with thermal bulk ions. Moreover, a narrow Gaussian component, due to neutrons from thermal d+t→α+n reactions, could be resolved in some cases. Results are presented and the plasma information obtained is discussed as an illustration of the capabilities of neutron emission spectroscopy diagnostics.

Hard X‐Rays from Neutralized Ion Beams in Solar Flares

Under suitable conditions, hard X-rays (HXRs) may be emitted by a neutralized proton beam due to the "heating" of the electrostatically dragged electrons in collisions with a nearly neutral background atmosphere. A simple estimate is made generalizing this HXR emission mechanism to heavier ions dragging a neutralizing electron current. Recent gamma-ray results on the energy content of flare ions of ≥1 MeV nucleon-1 are used to estimate the total HXR yield above 20 keV or so which would be expected from these processes, and in 19 flares the results are compared with HXR data in the same events. It is found that only in two flares are the neutral beam HXRs clearly important and that in a few others they may be significant. In most events, however, the neutral beam HXR contribution is small, though the ion energy is comparable with that of electrons.

Neutralization principles for the extraction and transport of ion beams

The strict application of conventional extraction techniques of ion beams from a plasma source is characterized by a natural intensity limit determined by space charge. The extracted current may be enhanced far beyond this limit by neutralizing the space charge of the extracted ions in the first extraction gap of the source with electrons injected from the opposite side. The transverse and longitudinal emittances of a neutralized ion beam, hence its brightness, are preserved. Results of beam compensation experiments, which have been carried out with a laser ion source, are resumed for proposing a general scheme of neutralizing ion sources and their adjacent low-energy beam transport channels with electron beams. Many technical applications of high-mass ion beam neutralization technology may be identified: the enhancement of ion source output for injection into high-intensity, low- and high-energy accelerators, for ion thrusters in space technology, for the neutral beams needed for plasma heating of magnetic confinement fusion devices, and for future accelerator-driven systems for fusion, fission and nuclear waste incineration.

High-energy neutral particle measurement system in the large helical device

The development of a high-energy neutral particle measurement system for ion temperature measurements and high-energy particle confinement analysis during neutral beam injection and ion cyclotron resonance frequency heating experiments in the large helical device (LHD) is described here. We have been improving the time-of-flight neutral particle analyzer for the LHD which had been developed in ENEA Frascati. The control and data acquisition systems were designed to be suitable for long discharges in the LHD. The horizontal and vertical movable stage is prepared to investigate pitch-angle distribution and loss cone. We have resolved many difficulties at installation, for example, the strong leakage magnetic field from the LHD and the limitations of magnetized materials near the LHD, the quench of the superconducting magnetic field, the narrow viewing port, and the fully remote control system. The preliminary results in plasma experiments are also described here.

Amorphous carbon films deposited from carbon ions extracted from a discharge in fullerene vapor

Amorphous carbon (a-C) films with physical properties ranging from diamond like to graphite like were deposited at rates of 1 μm/h under 10−6 Torr vacuum by extracting a molecular carbon ion beam from a pure carbon discharge in fullerene vapor. With ion energies fixed near 300 eV, substrate temperature and beam neutralization were the main process parameters controlling film properties. a-C stress and density increased with decreasing deposition temperature, ranging to greater than 3 GPa and 2.9 g/cm3, respectively. Room temperature conductivity ranged from 10 to 10−11(Ω cm)−1, with cool substrates and a well neutralized ion beam producing the most insulating films. a-C conductivity increased irreversibly after imposition of electric fields exceeding 108 V/m through the film planes. The high conductivities of films deposited without a beam neutralizer are attributed to phase changes resulting from dielectric breakdown driven by electric fields induced by surface charging. Conductivity data obtained for films deposited at different temperatures are discussed in the context of activated conduction, variable range hopping, and multiphonon assisted hopping transport processes.

Microwave plasma source for high current ion beam neutralization

A new microwave based plasma source for high current ion beam neutralization is presented. It consists of a tapered WR340 waveguide with a directly coupled 2.45 GHz magnetron and a quartz tube of 40 mm diameter inside the waveguide confined by two graphite electrodes. The upper electrode houses the gas feed, whereas the lower electrode serves as the plasma outlet. Both electrodes can be biased independently. Tuning is done via a magnetically controlled stab inserted into the waveguide. The plasma was characterized by use of the Langmuir probe. The electron concentration of 5×1011 cm−3 at 250 W was measured. Typical electron temperature is below 2.5 eV even for pressure as low as 2×10−3 Pa. The extractable electron current is dependent on microwave power and gas flow. A limitation is caused by the ion saturation current to the negatively biased electrodes. The voltage on the electrodes allows an easy control of electron current. Maximum electron current achieved so far is 400 mA for an argon flow of 5 sccm and microwave power of 300 W.

Curvature Model for an Ion-Machined Free-Standing Thin Film MEMS Device

ABSTRACTThe connection between processing effects and curvature of a free-standing poly-Si thin film MEMS structure is analyzed theoretically and compared to experimental data. Estimates are made for the strain resulting from processing and post-processing effects including Phosphorus implantation and chemically neutral ion-beam machining. The inferred strain distribution is used to predict the curvature of the free-standing structure upon release from its host substrate. Curvature predictions agree closely with experimental measurements for particular choices of the processing strain magnitudes. The analysis procedure is proposed as a means to determine the through-thickness stress distribution in a free-standing structure. The ion-beam machining procedure is found to be a useful method for planarizing free-standing thin film structures with undesirable curvature due to process-induced stress.

Energetic particle transport in compact quasi-axisymmetric stellarators

Hamiltonian coordinate, guiding center code calculations of the confinement of suprathermal ions in quasi-axisymmetric stellarator (QAS) designs have been carried out to evaluate the attractiveness of compact configurations which are optimized for ballooning stability. A new stellarator particle following code is used to predict the confinement of thermal and neutral beam ions in a small experiment with R=145 cm, B=1–2 T and for alpha particles in a reactor size device. As for tokamaks, collisional pitch angle scattering drives ions into ripple wells and stochastic field regions, where they are quickly lost. In contrast, however, such losses are enhanced in QAS so that high edge poloidal flux has limited value in improving ion confinement. The necessity for reduced stellarator ripple fields is emphasized.

Energetic particle transport and alpha driven instabilities in advanced confinement DT plasmas on TFTR

The article reviews the physics of fusion alpha particles andenergetic neutral beam ions studied in the final phase of TFTRoperation, with an emphasis on observations in reversed magnetic shear(RS) and enhanced reversed shear (ERS) DT plasmas. Energy resolvedmeasurements of the radial profiles of confined, trapped alphas in RSplasmas exhibit reduced core alpha density with increasing alphaenergy, in contrast to plasmas with normal monotonic shear. Themeasured profiles are consistent with predictions of increased alphaloss due to stochastic ripple diffusion and increased first orbit lossin RS plasmas. In experiments in which a short tritium beam pulse isinjected into a deuterium RS plasma, the measured DT neutron emissionis lower than standard predictions assuming first orbit loss andstochastic ripple diffusion of the beam ions. A microwavereflectometer measured the spatial localization of low toroidal modenumber (n), alpha driven toroidal Alfvén eigenmodes (TAEs) in DTRS discharges. Although the observed ballooning character of the n = 4mode is consistent with predictions of a kinetic MHD stability code,the observed antiballooning nature of the n = 2 mode isnot. Furthermore, the modelling does not show the observed strongdependence of mode frequency on n. These alpha driven TAEs do notcause measurable alpha loss in TFTR. Other Alfvén frequency modeswith n = 2-4 seen in both DT and DD ERS and RS discharges arelocalized to the weak magnetic shear region near qmin. In 10-20% ofDT discharges, normal low n MHD activity causes alpha loss at levelsabove the first orbit loss rate.

Numerical Analysis of Ripple Loss in Reversed Shear Tokamak Operation.

Ripple loss of energetic alpha particles and neutral beam ions was calculated for reversed shear discharges in ITER-FDR(Final Design Report). The result indicates that, compared with normal operation with positive shear, reversed shear operation dramatically enhances the ripple loss. Ripple loss of alpha particles can reach 25 % with the maximum heat load of 3.7 MW/m2 on the outboard wall, while that of a 1 MeV negative ion source beam can be as high as 20 % as well. The heat load due to alpha particle loss is marginal in the light of a wall tolerance. The calculation suggests that the Toroidal Field (TF) ripple in a fusion reactor should be designed to be less than 0.6 % at the plasma surface so that ripple loss of alpha particles or beam ions can be acceptably low. Ferritic steel insert to the vacuum vessel, is a probable solution to reduce the ripple loss to an allowable level in the ITER-FDR design with the TF ripple reduction of by a factor of 0.4. With TF ripple reduction, ripple loss for alpha particles and beam ions is expected to decrease to 10 % and 4 %, respectively.

Electron and ion internal transport barriers in Tore Supra and JET

Formation of core regions in Tore Supra and JET tokamaks with reduced transport coefficients is reported. Characteristics of the enhanced confinement regions and the physics process involved in their formation and maintenance should be considered separately when the electron or ion components are predominantly heated. In Tore Supra and JET, central electron temperature transitions are observed by injecting lower hybrid waves at modest power levels during the current ramp-up phase of the discharges. Transport analyses stress the importance of the low magnetic shear in the core to explain the anomalous electron transport reduction. With high-power dominant ion heating schemes in JET (neutral beam injection and ion cyclotron resonance heating), internal transport barriers have been obtained in plasmas fuelled with a mixture of deuterium-tritium (D-T) ions leading to a successful production of fusion power (8.2 MW) in this regime. Similar additional power levels to those applied in pure deuterium (D-D) plasmas are required to establish internal transport barriers in D-T plasmas. In D-D and D-T plasmas, ion thermal diffusivities are reduced close to their neoclassical levels in the plasma core and electron thermal diffusivities decrease by one order of magnitude at midplasma radius. The combined role of magnetic shear and velocity shear can explain the formation and evolution of plasma core regions with low energy transport coefficients.

Deuterium-tritium operation in magnetic confinement experiments: results and underlying physics

A review of experimental results obtained in JET D-T plasmas is presented. In discussing the underlying physics, results previously obtained on tokamak fusion test reactor (TFTR) are also taken into account. In JET, the maximum fusion power output of 16.1 MW has been obtained in an edge localized mode (ELM)-free hot-ion H-mode featuring an edge confinement barrier in a single-null divertor plasma with a where is the total input power to torus. A steady-state H-mode discharge, with plasma shape and safety factor q similar to that of ITER, produced 4 MW for 5 s (22 MJ). The steady-state results extrapolate well to ignition with ITER parameters using the normalized plasma pressure achieved on JET. Also, the advanced tokamak regime using optimized magnetic shear configuration featuring an internal transport barrier produced 8.2 MW of fusion power. With regards to reactor physics issues, a clear identification of electron heating by fusion born -particles has been made both in JET and TFTR. The JET experiments show that the H-mode threshold power has approximately an inverse isotopic mass dependence and that it does not depend on the method of auxiliary heating. The global energy confinement time in the TFTR D-T supershot regime scales as but in the JET H-modes, it is found to be practically independent of isotopic mass where A is the atomic mass of the hydrogenic species. In JET, the plasma core and the edge appear to have different underlying confinement physics, the former follows the gyro-Bohm transport model whereas the edge pedestal energy scales as . The maximum edge pressure in H-modes is analysed in relation to the ion poloidal Larmor radius at the edge. The fast ions driven by neutral beam injections (NBI) or ion cyclotron resonance heating (ICRH) could play an important role in setting the width of the edge pedestal. The thermal ELMy H-mode confinement both in D or T gas fuelled plasmas decreases significantly when the plasma density exceeds 0.75 of the Greenwald limit and the maximum density achieved is . The ICRH scenarios for a reactor have been evaluated. For example, minority in 50:50 D:T and tritium-dominated plasmas showed strong bulk ion heating leading to ion temperatures up to 13 keV with ICRH alone. Deuterium-minority ion cyclotron heating in tritium plasmas at a power level of 6 MW produced steady-state record values of for more than 2.5 s. Finally, the on-site closed-cycle tritium reprocessing plant and remote handling tools at JET have been used routinely and provided an integrated demonstration of safe and reliable operations of a tokamak device in reactor-relevant conditions.

Enhancements to the Compact Helical System fast ion loss probe

A scintillator-based fast ion loss probe has been used to measure 40 keV neutral beam ion loss from Compact Helical System plasmas. Modifications have recently been made to the probe to expand the range of gyroradius covered and to increase the probe acceptance at low pitch angles. In addition, a lamp has been installed inside the probe to facilitate calibration of the scintillator position within the field of view of the video camera. Finally, a Faraday cup structure, integral with the scintillator, has been added to allow direct measurement of the ion current to the probe. This last feature allows much easier absolute calibration of the diagnostic.

Charge state of alkali atoms scattered by Mo and W surfaces

From an experimental study it has been established that the charge states of particles scattered by the surface of refractory metals (W, Ta, Mo) during bombardment by neutral atom and ion beams of alkali metals (Li, Na, K), do not depend on charge states of the primary particles. It has been shown that angular and energy distributions of atoms and ions scattered from the bombarded surface of both ion and atom beams are similar to each other. A method for the direct detection of the neutral fraction yield of particles scattered from metal surface has been developed. In this method a second (auxiliary) target surface was used which had a higher work function than the target surface under investigation. In the yield of scattered particles, the ratio of the ionized component to the neutral component is the same for both target surfaces. This fact has made it possible to measure the yields of neutral particles. Thus, the scattering coefficients for ion and neutral fractions of scattered alkali atoms were determined. The measured and calculated dependencies of the ionization degree of η + for alkali particles are compared.

Experiments on ion beam space-charge neutralization with pulsed electron beams

Space-charge neutralization of heavy ion beams with electron beam pulses generated by electron guns incorporating ferroelectric cathodes has been experimentally investigated. Several experiments are described, the results prove that the intensity of selected ion beam parts with defined charge states generated in a laser ion source can be increased by an order of magnitude. For high charge states the intensity amplification is more significant than for low charge states. A charge enhancement factor of four has been achieved by neutralization with pulsed electron beams for Al 7+ ions generated from an aluminium target.