Neutral Beam Ion Research Articles

Theoretical characterization of the potential energy surface for H+N2→HN2. III. Calculations for the excited state surfaces

In a previous paper in this series [J. Chem. Phys. 93, 2384 (1990)] a global potential energy surface (PES) was presented for H+N2→HN2. In this paper, we report additional calculations which characterize PES’s for the excited 2A″ state, for a bound 2 2A′ state, for HN+2, and for the Rydberg states associated with HN+2. It is anticipated that these excited state PES’s will be important in interpreting and designing experiments to characterize the ground state HN2 species via neutralized ion beam techniques.

Fast beam studies of N3 photodissociation

A new fast radical beam apparatus has been used to study the photodissociation cross section of N3 as a function of wavelength. Neutralization of a fast negative ion beam by photodetachment is used to create a beam of cold, mass-selected radicals, with subsequent photodissociation and efficient detection of the neutral fragments. N3 was observed to predissociate throughout the first electronic absorption near 270 nm. High-resolution predissociation spectra were obtained which show rotationally resolved structure. Time-of-flight measurements of the kinetics energy release confirm that the lowest energy dissociation pathway which occurs is the first spin-allowed channel producing (N(2D ) + N2(1Σ+g). The photoelectron spectrum of N−3 was also obtained, yielding an electron affinity of 2.68±0.01 eV for N3.

Application of the intermediate frequency range fast wave to the JIPP TII-U plasma

A series of experiments has been conducted on the JIPP TII-U tokamak since 1989, using the newly constructed 130 MHz radiofrequency system. It has been predicted theoretically that the fast wave in this frequency range interacts weakly with particles. Two mechanisms of wave absorption have been identified in the experiment: electron Landau damping/transit time damping and 3rd harmonic ion cyclotron heating. The former mechanism is intimately connected with fast wave current drive and the latter can provide a new regime of plasma heating or a possible method of controlling the transport of alpha particles. It is found that the efficiency of the 3rd harmonic ion cyclotron heating is improved by using it in combination with neutral beam injection and ion cyclotron range of frequency heating. The heating efficiency obtained is as high as that of conventional heating. The experimental results are also analysed on the basis of a global wave theory which takes into account wave-particle interactions. These mechanisms of interaction are competing with each other; this will also be the case under more realistic reactor conditions.

Particle dynamics and current-free double layers in an expanding, collisionless, two-electron-population plasma

The expansion of a two-electron-population, collisionless plasma into vacuum is investigated experimentally. Detailed in situ measurements of plasma density, plasma potential, electric field, and particle distribution functions are performed. At the source, the electron population consists of a high-density, cold (kTe≂4 eV) Maxwellian, and a sparse, energetic ( (1)/(2) mv2e≂80 eV) tail. During the expansion of plasma, space-charge effects self-consistently produce an ambipolar electric field whose amplitude is controlled by the energy of tail electrons. The ambipolar electric field accelerates a small number (∼1%) of ions to streaming energies which exceed and scale linearly with the energy of tail electrons. As the expansion proceeds, the energetic tail electrons electrostatically trap the colder Maxwellian electrons and prevent them from reaching the expansion front. A potential double layer develops at the position of the cold electron front. Upstream of the double layer both electron populations exist; but downstream, only the tail electrons do. Hence, the expansion front is dominated by retarded tail electrons. Initially, the double layer propagates away from the source with a speed approximately equal to the ion sound speed in the cold electron population. The propagation speed is independent of the tail electron energy. At later times, the propagating double layer slows down and eventually stagnates. The final position and amplitude of the double layer are controlled by the relative densities of the two electron populations in the source. The steady-state double layer persists till the end of the discharge (Δt≂1 msec), much longer than the ion transit time through the device (t≂150 μsec). On the low-potential side, the double layer generates a monoenergetic, neutralized ion beam with no stationary background plasma. The field-aligned double layer contains no trapped or counter-streaming ions, and is current free; i.e., the relative electron–ion drift is zero.

Fishbone activity in JET

During auxiliary heating experiments in JET, periodic bursts of MHD oscillations resembling ‘fishbones’ have been observed in the signals of several diagnostics. The bursts have repetition times of 10–40 ms and oscillation frequencies ranging from 1 kHz to more than 20 kHz. While the amplitude of these oscillations increases at high poloidal and toroidal beta, they are also observed in plasmas with modest values of beta. Moreover, they occur in discharges with either neutral beam injection or ion cyclotron resonance heating, which is consistent with the idea that they are due to an instability driven by energetic ions. However, there is little evidence in JET that the bursts have a significant impact on fast ion containment or energy confinement. The paper describes the characteristics of the bursts and gives an analysis of the regimes over which they occur. In addition, the evidence for the interaction of the oscillations with high energy particles is considered and discussed in the light of theoretical models of the instability which is believed to be responsible for the bursts.

A search for the low-lying triplet state in ammonia by photoionization of a neutralized ion beam

A neutralized ion beam photoionization technique has been used in an attempt to detect the lowest triplet state in ammonia. Although triplet ammonia molecules are a likely product of the neutralization reaction, only highly excited Rydberg states were observed by photoionization. The absence of the triplet state is most likely due to its rapid predissociation rate.

Impurity control in JET using fueling

Deuterium gas fueling during high power Neutral Beam Injection and Ion Cyclotron Resonance Frequency heating inhibits impurity density increase in the plasma core in Be-gettered and Be-limiter operation in JET. By careful control of the amount of gas injected, the resulting increase in electron density and decrease in peak temperature can be made small, so that the neutron yield is unchanged. A large reduction in [density n(0)/influx φ], for both impurity and electron density, is observed due to gas injection. No indication of an increase in edge density diffusivity is found. The conventional impurity shielding is not sufficient to account for the observed large decrease in n(0)/φ for impurities and deuterium. The experiments discussed here point to the presence of shielding processes associated with changes of flows in the scrape-off-layer.

On the production of hard X-rays in solar flares

The problem of producing the hard X-ray burst at the onset of solar flares may be thought of in terms of the problem of producing the non-thermal electrons which emit the X-rays via bremsstrahlung. Electron acceleration to relativistic energies without similar ion acceleration is difficult to achieve, even in an ad hoc theoretical model. Yet from global energetic considerations, it is not feasible to accelerate the electrons as a minor constituent of the total energetic particle population. Therefore, it is necessary to invoke a more sophisticated process for the electron acceleration. In this paper we describe a mechanism for achieving this via an initial acceleration of a neutralized ion beam. When such a beam impacts the chromosphere, the electrons start to scatter while the ions continue downwards, rapidly setting up an electric field which is either cancelled by the inflow of background chromospheric electrons or results in the runaway acceleration of beam electrons. In the former case the result is simply heating, whereas in the latter case much of the ion kinetic energy is transferred into electron kinetic energy. The final electron energy may be similar to the typical energy of the ions. The electrons that are accelerated are those in the neutral beam that experience an electric field greater than the critical Dreicer field. Thus there will be a low-energy cut-off to the electron spectrum which overcomes the well-known energetics problem at low energies with certain other spectral forms.

Confinement and stability in JET: recent results

The versatility of the JET device allows a wide range of tokamak operating regimes to be explored and plasmas bounded both by material limiters and by a magnetic separatrix have been investigated extensively. This has permitted the confinement and MHD stability properties of plasmas heated to temperatures above 10 keV by neutral beam injections or ion cyclotron resonance heating to be studied in detail. The results of analyses of transport and confinement in the L- and H-mode regimes in JET are discussed and the properties of H-mode plasmas produced by both major forms of heating are compared. Several aspects of the MHD stability of such plasmas, particularly at high toroidal beta beta psi , and at the density limit, are reviewed.

ATF neutral particle analysis system

A two-dimensional scanning neutral particle analyzer has been installed on the Advanced Toroidal Facility (ATF). This TFTR-type E∥B spectrometer provides an extended energy range, mass resolution, and flexibility in viewing orientation. Characteristics of the E∥B analyzer and the associated data acquisition and control systems are discussed. Data representative of those taken during neutral beam injection and ion cyclotron heating on ATF are presented to illustrate the analyzer capabilities.

Microinstability threshold of an axially focused neutralized ion beam

The response to electrostatic perturbations, φ=φ̂(r,z)ei(lθ−ωt), is investigated for the Migma Exyder, a proposed disk-shaped device where ions are trapped in the periphery and focused towards the axis [Nucl. Instrum. Methods A 271, 214 (1988)]. The ions are modeled as a relativistic monoenergetic distribution with a small spread in the impact parameter (the distance of closest approach to the axis) and with the axial extent ΔZ much shorter than the radial extent R0. It is found that odd l modes do not give rise to a low-frequency response. For l=0, an energy principle for low frequency ion–ion interactions is formulated, and it is shown that the extremizing perturbation is located in a narrow region surrounding the axis. The threshold is proportional to the total number of stored particles, N=4ε0(π−2)γ(γ2−1) ×mic2R0/q2i, where γ is the usual relativistic factor, and this stability condition therefore allows for a high density on axis in highly focused systems. Compared to a cylindrical shape, a disk-shaped plasma gives an improvement in the density by the factor R0/ΔZ. The threshold in particle number for the relativistic ion–ion two-stream instability scales as γ3 and thus with relativistic effects the instability is further suppressed. Inclusion of the electron response increases the threshold even further. Preliminary investigations indicate that finite frequency l=0 modes can give a lower threshold than the zero frequency case, at least for nonrelativistic beam energies. These finite frequency instabilities can arise from coupling between positive and negative energy modes. Finite l modes arise if electrons are magnetized at the axis of the device. A balance between the electron field energy and the cross field electron convection establishes discrete modes. When these modes resonate with the ion bounce motion or azimuthal drift motion, bands of instability arise with relatively low density thresholds compared to the l=0 case.

A reinterpretation of the observed metastability of NeH

The metastability of 20NeH and 20NeD previously identified by neutralized ion beam studies in this laboratory has been confirmed by the observation of metastability for 22NeH and 22NeD in a similar study. A lifetime of τ≥10 μs is determined for the metastable states which are tentatively identified as Rydberg levels of intermediate n(n≤14) and possibly high l. The discrepancy between our previously measured dissociative kinetic energy release for 20NeD and that of Peterson et al. [J. Chem. Phys. 91, 6880 (1989)] is resolved.

Measurement of an autoionizing triplet state in D 2 by photoexcitation of a neutralized ion beam

Neutralized ion beam techniques have been extended to observe the laser ionization spectrum of the c 3Π u state of D 2. The autoionizing triplet→ nd Rydberg series has been observed. From the break in the continuous ionization signal the ionization energy of the c 3Π u state is found to be 3.675±0.020 eV.

Production and neutralization of negative ions and beams (Report on the 5th International Symposium, Upton, NY, USA, 30 October – 3 November 1989)

Over the past decade, the importance of the production, extraction and neutralization of negative ion beams has grown greatly. This was recorded in the proceedings of the triennial Brookhaven symposia since 1977, which were a major focus for discussion of this field. The most recent meeting in this series was from 30 October to 3 November 1989. This review presents the major highlights of this meeting.

Energetic particle effects on global magnetohydrodynamic modes

The effects of energetic particles on magnetohydrodynamic (MHD) type modes are studied using analytical theories and the nonvariational kinetic-MHD stability code (nova-k) [Workshop on Theory of Fusion Plasmas, (Societa Italiana di Fisica, Bologna, 1987), p. 185]. In particular, the problems of (1) the stabilization of ideal MHD internal kink modes and the excitation of resonant ‘‘fishbone’’ internal modes and (2) the alpha particle destabilization of toroidicity-induced Alfvén eigenmodes (TAE) via transit resonances are addressed. Analytical theories are presented to help explain the nova-k results. For energetic trapped particles generated by neutral beam injection or ion cyclotron resonant heating, a stability window for the n=1 internal kink mode in the hot particle beta space exists even in the absence of core ion finite Larmor radius effect. On the other hand, the trapped alpha particles are found to resonantly excite instability of the n=1 internal mode and can lower the critical beta threshold. The circulating alpha particles can strongly destabilize TAE modes via inverse Landau damping associated with the spatial gradient of the alpha-particle pressure.

A study of the astronomically important cyclopropenyl system by neutralized ion beam techniques

The cyclopropenyl cation and radical, proposed procursors of interstellar cyclopropenylidene, C 3H 2, have been studied using neutralized ion beam techniques. The study indicates the presence of low-lying stable and dissociative forms for the C 3H 3 radical. The possible origins of these states are discussed. The relative H and D atom loss rates of the monodeuterated radical, C 3H 2D, are investigated both by collisional ionization and by dissociative excitation. In neither case is evidence found to indicate the preferential loss of H atoms from the radical. Fragmentation energies for H (D) loss from the dissociative state of C 3H 3 (C 3D 3) have been measured.

Absorption of fast waves excited by a phased four-loop antenna array in the JFT-2M tokamak

The absorption characteristics of fast waves excited by a phased four-loop antenna array have been investigated experimentally in JFT-2M. The frequency of the fast waves is 200 MHz, which corresponds to approximately the tenth harmonic of the ion cyclotron frequency of hydrogen. The fast wave power is absorbed mainly by the bulk thermal electrons. It is shown that the absorption efficiency of the excited fast waves is improved with increasing density and temperature, and with decreasing phase velocity of the fast wave. The results are consistent with the theoretical predictions obtained from ray-tracing calculations. The power deposition profile is obtained through synchronous detection of the electron cyclotron emission modulated by a periodic heat source. In this modulation experiment with a limiter plasma on JFT-2M the electron thermal diffusivity is 2-3 m2·s−1 and the convection velocity is 20-40 m·s−1 at n̄e = 2 × 1019 m−3 and Ip = 230 kA. The resultant power deposition profile has a peak at the plasma centre and agrees well with that calculated with the ray-tracing code. The absorption efficiency calculated from the power deposition profile is 0.3-0.4 at n̄e = 2 × 1019 m−3 and Te0 = 0.7 keV, which agrees roughly with that estimated from the initial rise of the plasma stored energy. The electron heating efficiency estimated from the absorption efficiency is (4–5) × 1019 eV·m−3·kW−1 and the incremental confinement time is 8-10 ms, which is slightly longer than that in L-mode plasmas heated by neutral beam injection and/or ion cyclotron heating in JFT-2M.

Rydberg states of triatomic hydrogen(H3): application of neutralized ion beam techniques

Rydberg states of triatomic hydrogen(H3): application of neutralized ion beam techniques

Cross-section measurements for electron-impact ionization of atoms.

Absolute electron-impact cross sections have been measured from 0 to 200 eV for single ionization of 16 atoms (Mg, Fe, Cu, Ag, Al, Si, Ge, Sn, Pb, P, As, Sb, Bi, S, Se, and Te) with an estimated accuracy of \ifmmode\pm\else\textpm\fi{}10%. Combined with our recent measurements of He, Ne, Ar, Kr, Xe, F, Cl, Br, I, Ga, and In [Wetzel et al., Phys. Rev. A 35, 559 (1987); Hayes et al., ibid. 35, 578 (1987); Shul, Wetzel, and Freund, ibid. 39, 5588 (1989)], a set of 27 atomic single-ionization cross sections has now been measured with the same apparatus. In addition, cross sections are reported for double ionization of ten atoms and triple ionization of eight atoms. The measurements are made by crossing an electron beam with a 3-keV beam of neutral atoms, prepared by charge-transfer neutralization of a mass-selected ion beam. The critical measurement of absolute neutral beam flux is made with a calibrated pyroelectric crystal. The magnitudes of the single-ionization-peak cross sections decrease monotonically across rows of the periodic table from group IIIA (Al,Ga,In) to group VIIIA (Ar,Kr,Xe), varying much more than predicted by various empirical formulas and classical and quantum-mechanical theories.

Zeff measurements and low-Z impurity transport for NBI and ICRF heated plasmas in JIPP T-IIU

A visible bremsstrahlung detector array system for Zeff measurements and a charge exchange recombination spectroscopy system for fully ionized impurity profile measurements have been installed on JIPP T-IIU to study impurity transport in neutral beam injection (NBI) and ion cyclotron resonance frequency (ICRF) heated plasmas. More impurities are sputtered during ICRF heating than during NBI and/or Ohmic heating. The contribution of carbon to Zeff is 80-90% for NBI heated plasmas and 60% for NBI + ICRF heated plasmas. With carbon coating of the vacuum vessel, the contribution of carbon to Zeff becomes 80-90% also for NBI + ICRF heated plasmas. From the radial profile and the time evolution of fully ionized carbon after the ICRF pulse is turned on, a diffusion coefficient Da of 1.0 m2·s−1 and a convective velocity va(a) of 13 m·s−1 for the carbon impurity at the plasma edge are obtained.