Neutral Beam Ion Research Articles

The role of current profile broadening in L-mode and H-mode plasmas

Auxiliary heating by neutral beam injection (NBI) or ion cyclotron resonance heating (ICRH) can broaden or steepen the plasma current density profile. A measure of changes to the current density profile Jϕ is established by a combination of current moments which yields the profile broadening parameter ζ. JET experiments are demonstrated to increase ζ (broaden Jϕ) when NBI is applied to discharges which exhibit either L-mode or H-mode confinement; pellet injection and ICRH or current decay are demonstrated to decrease ζ (peak Jϕ). The time evolution of the parameter ζ is shown to correlate with the changes to the Dα emission seen in H-mode discharges with and without edge localized modes. The broadening of Jϕ by NBI, especially for H-mode pulses, smarts in the edge region and it is identified with the non-inductive bootstrap current component as a consequence of steep density profiles. Associated with current profile broadening is an increase in the edge pressure and the total stored plasma energy W. An analysis of global confinement (variations of W with power P) is made by splitting W into three parts: the fast ion energy, Wf, the edge energy, W1, which is empirically shown to be proportional to the current profile broadening parameter ζ, and the bulk plasma thermal energy, Wp. It is shown that the scaling for Wp in the L- and H-mode confinement regimes is similar.

Arc modulator for the TFTR neutral-beam ion source

A discussion is presented of the protection of the tokamak fusion reactor neutral-beam ion source, located at the Princeton Plasma Physics Laboratory. The system design is based on three operational protection requirements that include provisions for ion-source current pulse matching, ion-source fault current extinction, and metallic fault current extinction. A power circuit configuration satisfying these requirements is illustrated and briefly described. Simplified analytical expressions relating the protection requirements to the circuit parameters are developed. Experimental findings are in good agreement with a computer simulation of the circuit model, although several side effects introduced by the circuit configuration have been observed. The side effects observed include ion-source current overshoot and the existence of a negative ion source current. Modifications to counteract these side effects are briefly described. >

Xenon ion sources for space applications (invited)

In this article, we describe the technology of xenon ion thrusters under development for space-propulsion and plasma-contactor applications. Emphasis is placed on the 1.4-kW Xenon Ion Propulsion Subsystem (XIPS) designed for satellite stationkeeping. The 25-cm-diam XIPS thruster produces 63.5 mN of thrust at a specific impulse of 2800 s, requiring only about one-tenth the propellant mass of conventional chemical-propulsion systems. Using a ring-cusp magnetic field to confine the discharge plasma, the XIPS thruster converts input power into thrust-beam power with an efficiency of over 82%. We also describe a XIPS ion source under development for more demanding propulsion applications such as orbit raising. A 30-cm-diam laboratory-model XIPS thruster scaled from the smaller unit produces over 340 mN of thrust with an input power in excess of 10 kW and a specific impulse of about 4900 s. Under these operating conditions, the XIPS thruster converts input power into thrust-beam power with an efficiency of 91%. With its ion-extraction electrodes and ion-beam neutralizer removed, the XIPS discharge chamber has been shown to be a highly efficient xenon plasma contactor for a variety of applications, ranging from space-science investigations to electrodynamic tethers. We will describe a 1991 flight application in which our 25-cm XIPS plasma contactor will be used to stabilize the electrical potential of the Shuttle Orbiter during firings of a 1.5-A electron beam from its payload bay.

A unique machine for grinding large, off-axis optical components: the OAGM 2500

Design trends in the large optics field have progressed towards systems incorporating large, off-axis aspheric optical elements. These segments may be used in groups to form a single large parent optical surface. Segmented systems are necessary when the mirror size requirements exceed the capacity to produce a monolithic mirror blank. Segmented mirrors also have applications in space, where it is possible to launch a folded mirror array which can be deployed to provide a reflector larger than the diameter of the launch vehicle. The OAGM 2500 has been designed by Cranfield Precision Engineering to generate and measure off-axis elements prior to final polishing. This machine, together with a Neutral Ion Beam Figuring System, will provide Eastman Kodak Company with processes which are highly deterministic yielding rapid, cost efficient fabrication of large complex optical components. The resulting design has led to the manufacture of a machine weighing 130 tons, with a component capacity of 2.5 m × 2.5 m × 0.6 m and overall dimensions 8 m × 6 m × 5 m . Advanced three-axis CNC control for grinding and in situ metrology combined with a multiple path laser interferometer system and metrology frame, is incorporated to achieve the required component accuracy.

Modified perveance law for neutral-beam ion sources

The perveance law for ion sources is modified based on experimental data gained during the conditioning phase of three multiaperture ion sources for neutral-beam injection into TEXTOR. The ion sources are modified JET PINIs with a three-grid accel-decel extraction system designed for 55 kV, 88 A, 4.8 MW, operated in hydrogen. Results show that best agreement of the perveance law with the experimental data is achieved when the measured ion species mixture is used and, additionally, when the total width of the first acceleration gap, usually determined by the geometrical distance d1 between grid 1 and grid 2 and thickness D1 of grid 1, is enlarged by a third distance term r2, taking the effect of distortion of the electrical field lines at the decel grid into account. At perveance matched conditions, i.e., minimum beam divergence, the optimum value is found to be r′2 =0.8r2, where r2 is the aperture radius of the decel grid. The species fractions were measured by optical and magnetic spectrometry.

Spectroscopy of triatomic hydrogen

The visible emission spectra of H3, D2H, H2D and D3 were studied. Triatomic hydrogen molecules were produced by neutralization of fast ion beams in alkali vapors. In addition to the well-known 0-0 bands, weak vibrational bands of the electronic 3p2A″2 → 2s2A′1 transition of D3 and D2H were observed and analyzed. From the decay of the emission along the molecular beam lifetimes of excited states were obtained. For all observed states, 3p2A″2, 3s2A′1 and 3d, lifetimes strongly depend on the isotopic mixture. Many lifetimes measured were considerably shorter than ab initio predictions of the radiative lifetimes. Thus radiationless decay is important for all these excited states. Predissociation of the 3p2A″2 state increases with the rotational quantum number. Possible decay channels are discussed.

Spectroscopy of triatomic hydrogen

The strongest emission band of triatomic hydrogen near 7100 A was observed after neutralization of mass-selected ion beams of H 3 + , H 2 D + , D 2 H + and D 3 + . For D 3 improved molecular constants were determined by inclusion of higher rotational lines. Lifetimes of the upper states of the 7100 A band were measured. They strongly decrease for the lighter isotopomers because of predissociation. New emission bands of D 3 observed between 4200 A and 4600 A are assigned to transitions between n=4 and n=2 states

Observation of dissociative and radiative states of N2H by neutralized ion beam techniques

The N2 H(D) radical has been studied experimentally by measurement of the kinetic energy release in its unimolecular dissociation following formation by electron transfer from metal atoms to high velocity, mass-resolved ion beams and theoretically by ab initio techniques. Calculations of the dissociation coordinate of the ground state radical at the MP4/6-311G**//MP3/6-311G** level of theory indicate that the radical is unstable with respect to N2 and H by 0.6 eV but separated from the dissociation products by a 0.4 eV barrier. One dimensional tunneling lifetimes are determined to be 7.0×10−12 s for N2 H and 3.6×10−10 s for N2 D. Neutralization of the ion by Zn targets produces predominantly radicals in the 2 A′ ground state with dissociative lifetimes τ<0.5 μs, in agreement with the calculations. Mg targets produce the radical in a mixture of the 2A′ ground and 2A″(π) excited states with a branching ratio dependent on the internal energy of the precursor ion. A higher excited state of the radical, suggested to be an n=3 Rydberg level, is produced with K targets and is inferred to undergo radiative transitions, probably containing some discrete structure, to the lower 2A′ and 2A″(π) states in the wavelength range of 2700–4500 Å. Observations of these transitions may constitute the first spectroscopic observation of the radical.

Plasmoid propagation in a transverse magnetic field and in a magnetized plasma

The propagation of plasmoids (neutralized ion beams) in a vacuum transverse-magnetic field has been studied in the University of California, Irvine laboratory for several years [Phys. Fluids 24, 739 (1981); 25, 730, 2353 (1982); 26, 2276 (1983); J. Appl. Phys. 64, 73 (1988)]. These experiments have confirmed that the plasmoid propagates by the E×B drift in a low beta and high beta plasmoid beam (0.01<β<300), where β is the ratio of beam kinetic energy to magnetic field energy. The polarization electric field E arises from the opposite deflection of the plasmoid ions and electrons, because of the Lorentz force, and allows the plasmoid to propagate undeflected at essentially the initial plasmoid velocity. In these experiments, plasmoids (150 keV, 5 kA, 50–100 A/cm2, 1 μ sec) were injected into transverse fields of Bt=0–400 G. Anomalously fast penetration of the transverse magnetic field has been observed as in the ‘‘Porcupine’’ experiments [J. Geophys. Res. 91, 10,183 (1987)]. The most recent experiments are aimed at studying the plasmoid propagation dynamics and losses in the presence of a background, magnetized plasma which is intended to short the induced polarization electric field and stop the beam. Background plasma was generated by TiH4 plasma guns fired along Bt to produce a plasma density, np =1012 −1014 cm−3 . Preliminary results indicate that the beam propagation losses increase with the background plasma density; compared to vacuum propagation, roughly a 50% reduction in ion current density was noted 70 cm downstream from the anode for np∼1013 cm−3 . Principal diagnostics include magnetically insulated Faraday cups, floating potential probes, calorimeters, microwave interferometer, and thermal-witness paper.

Weakly bound metal complexes by neutralized ion beam techniques: NiL and NiL2 (L = CO, D2O, and ND3)

Weakly bound metal complexes by neutralized ion beam techniques: NiL and NiL₂ (L = CO, D₂O, and ND₃)

Observation of the ND4 Sch�ler band in a neutralized ion beam experiment

The emission of the ND4 Schuler band was observed after neutralization of a mass-selected ND4+ ion beam. Thus the assignment of this band to the ammonium radical was confirmed. The lifetime of the upper state (3p2F2) was determined to be 4.2 ns, which is much shorter than ab initio values of the radiative lifetime, showing that this state decays predominantly by predissociation. The Schuster band was not observed, neither after neutralization of ND4+, nor of ND3+.

Observations of the plasma environment during an active ionospheric ion beam

Data obtained during an active rocket‐borne ion beam injection experiment (ARCS 3), which was carried out in the polar cap ionosphere over Sondre Stromfjord, Greenland, are presented and discussed. The experiment consisted of a main instrumented sounding rocket payload and a separated subpayload from which two Ar+ ion beam generators were operated. These generators produced neutralized ion beams which were aligned generally perpendicular and antiparallel to the ambient geomagnetic field lines, as the two spinning payloads moved apart along (0–1 km) and across (0–0.5 km) field lines. Ion flux data obtained aboard the main payload show certain characteristics which are directly attributable to known beam‐injection geometry parameters, and others which are not so easily understood. Specifically, when the beam is injected transverse to the geomagnetic field, strong ion fluxes near 90° magnetic pitch angle are observed, as expected. The particle energies in these cases were different from those expected, however, with separate ion flux components appearing near 100 and 15 eV/q. The expected beam energy was near 200 eV/q, so that subpayload charging to levels on the order of 100 V is inferred to have taken place during transverse beam injections. During antiparallel injections a component of ∼200 eV/q ions is observed to be moving up the field line, in addition to two other components: one transverse component at ∼200 eV/q and a second transverse component near 15 eV/q. In this case, no spacecraft charging is inferred. The mechanism giving rise to the 15‐eV/q transverse ion components, observed during both transverse and antiparallel injections, is an open question. The existence of ion fluxes at magnetic pitch angles other than those anticipated during both transverse and antiparallel beam injections indicate ion emissions from the subpayload which are much less beamlike than expected based on preflight laboratory measurements and that beam‐plasma interactions tend to isotropize the beam very near the emitting payload. In addition, photometric, electric field, and ion flux data are presented which support the picture of a spatially asymmetric and highly localized region of strong beam‐plasma interactions surrounding the beam‐emitting subpayload.

Long-range cross-field ion-beam propagation in the diamagnetic regime.

It is shown that high-kinetic-$\ensuremath{\beta}$ neutralized ion beams with transverse width less than an ion gyroradius and density larger than the background plasma density can propagate uninhibited through a magnetized plasma over many ion gyroradii. The model is confirmed by computer simulations using a two-dimensional hybrid code.

Molecular states formed by ion beam neutralization of [C2H2]+ and [C2H3]+ on metal targets

AbstractA combination of charge‐stripping and beam‐scattering techniques has been used to study the molecular states formed when a fast beam of [C2H2]+ and [C2H3]+ in several isotopic forms are neutralized by electron transfer from metal target atoms (K, Na, Mg and Zn). For [C2H3]+ the isotopic compositions and relative abundances of product states were found to be insensitive to the method of ion preparation (electron impact and chemical ionization). Ground state neutrals are formed in partial abundance when Mg or Zn is used as a target atom. With low ionization potential targets (K and Na) excitel dissociative states of C2H2 and C2H3 are formed as major beam constituents. For these states decomposition products have been identified and fragmentation energies measured. The excited states of C2H2 and C2H3 lie alout 6.8 eV and 2.9 eV, respectively, above their stable ground states. The discussion focuses on the possible identity of the excited states and their structural relations to the precursor ions.

Neutral and plasma shielding model for pellet ablation

The neutral gas shielding model for ablation of frozen hydrogenic pellets is extended to include the effects of (1) an initial Maxwellian distribution of incident electron energies; (2) a cold plasma shield outside the neutral shield and extended along the magnetic field; (3) energetic neutral beam ions and alpha particles; and (4) self-limiting electron ablation in the collisionless plasma limit. Including the full electron distribution increases ablation, but adding the cold ionized shield reduces ablation; the net effect is a modest reduction in pellet penetration compared with the monoenergetic electron neutral shielding model with no plasma shield. Unlike electrons, fast ions can enter the neutral shield directly without passing through the cold ionized shield because their gyro-orbits are typically larger than the diameter of the cold plasma tube. Fast alpha particles should not enhance the ablation rate unless their population exceeds that expected from local classical thermalization. Fast beam ions, however, may enhance ablation in the plasma periphery if their population is high enough. Self-limiting ablation in the collisionless limit leads to a temporary distortion of the original plasma electron Maxwellian distribution function through preferential depopulation of the higher energy electrons.

Studies of Hypervalent Radicals By Neutralized Ion-Beam Techniques

ABSTRACT The advantages of obtaining both neutral fragment scattering distributions and reionized mass spectra for investigating highly reactive and/or metastable species produced by the neutralization of a high velocity precursor ion beam is outlined. Practical applications of these techniques are illustrated by results obtained recently for the neon hydride and the methanium (CH5) radicals. In these studies a unique metastability for the ground state of NeH is identified and the previously observed metastability of CH5 is shown to arise from the formation of high Rydberg levels of the radical rather than the ground state as first proposed. Future prospects for obtaining absorption spectra of unusual neutral species by combining optical spectroscopic techniques with those of neutralized ion beams are discussed.

Observation and characterization of the CH5 radical by neutralized ion beam techniques

The CH5(CD5) radical has been produced by charge transfer neutralization of fast ion beams using Na and Zn targets. The results obtained with Na targets are in excellent agreement with previous similar studies using alkali metal targets. In particular, no evidence for metastability was observed. Alternatively, results obtained with Zn targets show clear evidence for production of metastable states of the radical with lifetimes in the 1–10 μs range. Measurement of the kinetic energy released upon dissociation indicates that some neutral radicals produced in CH+5(CD+5)/Zn reactions have energies very close to the ionization limit. It is proposed that the observed metastability may be explained by the production of high Rydberg levels of the radical having sufficiently long radiative lifetimes.

Current neutralization of ion beam rotating and propagating in plasma

The current-neutralization fraction of a rotating and propagating light ion beam (LIB) injected into a low density plasma is investigated numerically. The beam space charge is essentially neutralized by a redistribution of the background plasma electrons in a time duration equal to the inverse of electron plasma frequency. When the density of the background plasma is comparable with that of the beam, incomplete current neutralization occurs because the strong magnetic field induced by the intense ion beam restricts the return plasma current.In the simulation, the ion beam and the background plasma are treated as the fluids coupled with Maxwell's equations and Ohm's law, including the effect of the magnetic field on electrical conductivity. The calculations assume that the ion beam is injected in an unsteady fashion into the uniform plasma. It is found that the return current strongly depends on the density of the background plasma. The beam deceleration and the acceleration of the beam head and tail are also considered.

Experimental evidence for the imino methyl radical (HCNH˙) by neutralized ion beam spectroscopy

Experimental evidence for the imino methyl radical (HCNH^˙) by neutralized ion beam spectroscopy

Recent progress of studies on intense particle beam at Nagaoka—ETIGO Project

Recent experimental and theoretical research on intense pulsed particle beams involving the ETIGO Project at the Technological University of Nagaoka is reviewed. Experimentally, we review the following: (1) studies on local divergence angle in parameter space, (2) development of self-magnetically insulated “Plasma-Focus Diode,” (3) beam focusing by z-discharged plasma channel, (3) measurement of ablation process of beam-target interaction, (4) time-resolvable measurement of energy and species, (5) inductive postacceleration of highly neutralized ion beam, (6) construction of new pulse-power machine, and pulse-compression experiment by plasma erosion opening switch. A brief summary is also given on the development of 2.5-dimensional particle-in-cell computer simulation code.