Fast Neutral Particles Research Articles

Theory and operation of a three-gate time-of-flight velocity analyzer

Theoretical considerations and test results are presented for a new-type velocity analyzer for incident fast neutral particles, positive ions, and negative ions. Velocity analysis is carried out by means of a pulsed, three-gate time-of-flight (TOF) technique capable of eliminating alias velocities (harmonics) to sixth order. In addition the design and operation are presented of a four-element ion lens system, with small spherical and chromatic aberrations, suitable for interfacing a large-diameter ion beam from the TOF section with a subsequent mass analyzer.

Application of silicon surface barrier detector for fast neutral particles

A design study of a small-size neutral particle analyzer using a silicon surface barrier detector (SSD) is performed. The SSD is very sensitive to x rays or photons, so that a pair of 45° sector magnets to separate a reionized neutral from x rays and photons will be used for this analyzer. In order to examine the performance, the SSD was applied to measure the species ratio of the prototype neutral beam injector for JT-60. It was confirmed that the energy resolution was 12% at 40 keV and the linear relation between the incident energy of particles and the pulse height was held over the energy range from 16.7 to 100 keV. The species ratio measured by the SSD was in good agreement with that by the Doppler-shift spectrometer. The SSD has a sufficient capability for the energy analysis of fast neutrals.

Tomography of neutral beams

This paper describes a new diagnostic technique to decode information contained in the spectral and spatial distribution of photons emitted by fast neutral beam particles and provide an estimate of the 2-D beam density distribution function in a cross-sectional slice. By scanning Doppler-shifted Hα radiation from the fast beam particles at a number of peripheral locations around the beam axis, tomographic reconstructions of the beam distribution with spatial resolution Δx≂2 cm and time resolution Δt≂10 ms can be made. Using a maximum entropy reconstruction algorithm (making no symmetry assumptions), experimental reconstructions are presented of the 2-D profile of the beam generated by the prototype JET source for each of the Doppler-shifted line radiation components. Applications of this image processing technique in other areas of high-temperature plasma diagnostic development are discussed briefly.

Time-of-flight measurements of charge-exchange neutral particles from the TORTUR II plasma

A disc chopper for time-of-flight analysis of fast neutral particles was constructed for the determination of the ion energy spectrum at lower energies than can be obtained by conventional electromagnetic analyzers. The method has been applied to the TORTUR II Tokamak. The chopper and detection system are described and the measurements are presented. For the interpretation of the results of the measurements, a data analysis procedure was developed. The influence of the kinetic effects near the linear wall was shown to be important in the calculations of the neutral density profile at the plasma edge. The neutral energy spectrum in the lower energy range is strongly influenced by this effect.

High resolution time-of-flight analyzer for charge exchange process.

New time-of-flight energy analyzer for the fast neutral particles are described. The effect of the sweeping field on the ion packet length, the energy spread, and the direction divergency are discussed on the ground of experimental findings. Triple focusing conditions are given by combining the sweeping field with a sector type toroidal electric field. The toroidal field is realized by the application of the Matsuda plates which terminate a cylindrical condenser. A high resolution T-O-F analyzer is installed in the triple focusing geometry, and the time resolution of the analyzer is estimated to be 36,000.

A Pulsed Neutral Particle Source for Active Plasma Diagnosties

A z-pinch gun with a gas puffer was studied as a pulsed neutral particle source for active plasma diagnostics. Typical thermal and drift energies of the gun plasma were 120 eV and 140 eV, respectively. The peak density of fast neutral particles converted from the gun plasma was 6.2×1011cm-3 with an energy of 400 eV at a distance of 1.8 m from the gun. The pulse width was about 4 µsec. It is suggested that this kind of particle source with these values can be used for measuring the spatially-resolved ion temperature of plasmas. The experimental results are analyzed by a collisionless free-expansion model.

On the model of the electron sputtering process of alkali halides

Abstract A model of electron induced sputtering of alkali halides is described. It is assumed that fast neutral particles are emitted when H-centres, taking part in the focused replacement sequence, cross the surface. When the focused replacement sequence is terminated at a certain distance from the surface, the H-centre may diffuse to the surface, giving rise to thermal particles. The model allows calculations of the sputtering yields for thermal, non-thermal and molecular emission. The results of the calculations are in good agreement with the experimental data obtained by different authors.

Measurements of He I Line Profiles with Forbidden Components Induced by Fluctuating Fields in a Small Theta Pinch Plasma

Spectral line profiles of He I 4922 A line (2 1 P-4 1 D, 1 F) and 4471 A line (2 3 P-4 3 D, 3 F) with their forbidden components from a small linear theta pinch plasma have been measured and the plasma satellites have been observed on the wing of the line profiles. The observed profiles have broadened wings, and they are interpreted as the Doppler broadening due to fast neutral particles produced by the charge transfer with hot ions in the plasma of the neutral particle density of ∼10 14 cm -3 comparable with the plasma density. The low and high frequency fluctuating fields with the average strength of several kV/cm are deduced from the plasma satellites in the observed profiles.

Possibility of Medium Energy Neutral Beam Injection into Stellarator Reactor

The energy necessary for neutral particles to penetrate into the plasma center may be of the order of 0.6∼1 MeV in the case of a fusion reactor. The conversion efficiency to convert such a high energy ion beam into a neutral beam is extremely small. We study here the case where fast neutral particles are injected so that the ionized fast ions are trapped in helical ripples of the stellarator field. It is shown that the trapped ions can drift into the plasma center even if the energy is of the order of 100 keV.

A disk chopper for time-of-flight analysis of neutral particles

A disk chopper for time-of-flight analysis of fast neutral particles (νthermal∼107 cm/sec) is described. The open time of the disk chopper is of the order of 1 μsec, and the closed time is of the order of 100 μsec.

A large area thermal detector for fast neutral particles

A neutral particle detector is described which consists of a set of thermocouple elements evaporated on to a thin mica disc supported at its edge. This design provides a high sensitivity over a large area whilst still having a reasonable short time constant (4.5 s). The device has been used to measure beam powers of up to 10 mW, whilst minimum detectable signal is below 0.1 mu W. Response is linear over this entire range. The unit is easily constructed, is quite rugged and its response is independent of background temperature.

Diagnosis of a hydrogen plasma by beams of helium atoms of different energy

The underlying theory is described for a method of diagnosing a hydrogen plasma by means of beams of helium atoms of different energy. The range of measured density is 1014 to 1016 cm−3 with a length of plasma section probed ≈ 10 cm. The highest accuracy (≈ ± 20%) is attained in the middle of the range. The accuracy in measuring electron temperature from 10 to 50 eV is no worse than 10–30%. Higher temperatures can be determined with an accuracy of the same order. Methods have been developed in recent years for active diagnosis of a high-temperature plasma using beams of fast neutral particles [1–5]. These methods, in spite of involving somewhat unwieldy apparatus, promise to permit the study of a plasma in the range of parameters difficult to investigate by traditional methods (probes, microwave equipment, and so on). In addition, they have relatively high timewise and spatial resolutions and are noncontact methods in practice.

Scattering with Charge Exchange to the StateH(2p)for the SystemsH++ He andH++ Ar

Studies have been made of differential scattering with charge exchange to the $2p$ state of hydrogen for the systems ${\mathrm{H}}^{+}$ + Ar and ${\mathrm{H}}^{+}$ + He. Fast hydrogen atoms in the state $\mathrm{H}(2p)$ resulting from the collision process were detected by making coincidence measurements between the $\mathrm{Ly}\ensuremath{-}\ensuremath{\alpha}$ photon emitted in the decay of the $2p$ state and the resultant fast neutral particle. The probability of transfer to the state $\mathrm{H}(2p)$, ${P}_{2p}$, is presented for ${\mathrm{H}}^{+}$ + Ar for the condition $\ensuremath{\Theta}T=20$ keV deg, where $\ensuremath{\Theta}$ is the scattering angle and $T$ the incident-ion kinetic energy, which represents an impact parameter of 0.53 a.u. For the ${\mathrm{H}}^{+}$ + He system a comparison between experimental results for ${P}_{2p}$ and the many-state calculation of Sin Fai Lam shows the theory to overestimate ${P}_{2p}$ by an order of magnitude for $\ensuremath{\Theta}T=20$ keV deg as well as for measurements where the impact parameter varied from 0.05 to 0.25 a.u.

Charge-Transfer Cross Sections forH+,Li+, andNa+onN2

Measurements of the total cross section for charge transfer ${\ensuremath{\sigma}}_{10}$ of ${\mathrm{H}}^{+}$, ${\mathrm{Li}}^{+}$, and ${\mathrm{Na}}^{+}$ on ${\mathrm{N}}_{2}$ are reported here in the energy range of 25-100 keV. The method used was direct detection of fast neutral particles that were formed in single charge-transfer collisions. Data for ${\mathrm{H}}^{+}$ on ${\mathrm{N}}_{2}$ are in excellent agreement with previously published results with ${\ensuremath{\sigma}}_{10}$ increasing with decreasing energy, reaching a value of 1.55 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$/molecule at 5 keV. From data for ${\mathrm{Li}}^{+}$ and ${\mathrm{Na}}^{+}$ on ${\mathrm{N}}_{2}$, ${\ensuremath{\sigma}}_{10}$ is observed to increase with increasing energy. The cross section for ${\mathrm{Na}}^{+}$ has a maximum of 1.1 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$/molecule at about 50 keV, while the cross section for ${\mathrm{Li}}^{+}$ is still increasing at 100 keV, although an irregularity is observed at approximately 30 keV.

Neutral injection into closed line magnetic traps

The build-up of a plasma by fast neutral particle injection into a closed-line magnetic trap is studied. By contrast with the mirror-machine case, the slow plasma formed by charge exchange and ionisation is contained within the system. This slow plasma increases the efficiency of fast neutral particle trapping and, through ‘plasma pumping’, lowers the density of slow neutrals. Hence for given initial conditions of equivalent beam current, base pressure and volume a higher fast ion density can be achieved in a closed-line trap than in an open one. This higher fast ion density is accompanied in most cases by a still larger density of slow plasma. However with a sufficiently intense neutral beam the slow plasma can be heated by energy transfer to temperatures of thermonuclear interest.

Spectroscopic measurement of plasma rotation in a penning discharge

Balmer lines emitted from a Penning discharge in hydrogen gas show a Doppler shift indicating rotation around the axis of the discharge column. This effect is explained as a result of the charge exchange collisions of the drifting plasma ions with the neutral atoms, and the fast neutral particle thus produced being excited by the plasma electrons. Accordingly we identify this rotation with the drift motion of the plasma. The velocity of rotation vrot is obtained as a function of radius from the measured Doppler shifts for two values of the confining magnetic field strength Bz The results show that for plasma layers with radial distances r smaller than or equal to the radius of the hollow anode cylinder, that means inside a tube which contains radial currents Ir, the rotational speed vrot can be expressed by the following very simple formula: vrot = (IrBz / nimin0 ?coll2?rL)1/2. A derivation of this expression is given.

The production of intense neutral and negative ion beams

The basic concepts underlying the production of ion beams are reviewed. The properties of the plasma from which the ions are extracted are discussed with respect to their effect on the ion beam. The influence of the positive ion beam emittance and electron attachment on the production of fast neutral particle beams, and beams of negative ions is then examined. It is shown that the production of negative ions is best achieved by direct extraction from a plasma, whereas neutral particle beams must be formed by neutralization of energetic primary positive ion beams.

Fast Neutral Particles from Arc Cathode

The rotating drum method was used to measure the velocity distribution in the vapor jet which emerges from the cathode region in a magnetically stabilized high-vacuum arc discharge maintained between cooled copper electrodes. To obtain sufficient detection sensitivity part of the cathode was made of radioactive copper. The number distribution of copper atoms deposited on the drum as a function of their velocity perpendicular to its surface was measured by a Geiger-M\uller counter. An average of about 4\ifmmode\times\else\texttimes\fi{}${10}^{4}$ cm/sec was found for the atomic beam velocity. It is suggested that misinterpretation of earlier observations has led to apparent velocities of up to ${10}^{6}$ cm/sec.