Neutral Atom Beam Research Articles

On the possibility of measuring the q-profile in dense plasmas by means of molecular hydrogen beams

A method of measuring the q-profile in a plasma is analyzed in detail for its applicability to fusion plasmas. Radially injected neutral hydrogen molecules will be ionized in the plasma and will rotate according to the local magnetic field. After dissociation the information on the velocity of the molecular ion at the time of dissociation can be carried by the neutral atom to a neutral particle analyzer. This paper shows how this information can be used to calculate the local field. The production and penetration of molecular beams are discussed as well as the requirements to be met by the beam and analyzer. Also discussed are the possibility and influence of neutral atom beams and their possible relevance to the same type of measurement. It is shown that the Shafranov shift can be determined by suitable choices of the injection and detection geometries. The method seems to be especially suited to measuring the time variation of local q-values. Although the penetration of the particles depends on the plasma conditions, the quantity measured by the method discussed only depends on the local field ratio. It is shown that the method could also be applied in large plasma experiments the size of JET, and that an accuracy in measuring q of better than or about 10% may be achieved.

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.

Formation of cathode plasma in a magnetically insulated high-current diode to generate negative hydrogen ions with current densities up to 200 A/cm2

High-power pulsed beams of neutral atoms may be useful for CTR as alternative ways to drive the target. They can be obtained by stripping high-current negative ion beams. Experiments are described on cathode plasma formation with sufficient negative hydrogen ion concentration to obtain H− current density up to 200 A/cm2. The plasma was formed by a surface discharge on a dielectric initiated by a bipolar prepulse or by laser illumination. It is shown that the electron current density flowing through the cathode plasma needs to be greater than 104 A/cm2 to create H− continuously in the outer region of the plasma and thereby produce H− current densities up to 200 A/cm2.

Development of H− and D− ion sources for plasma heating

The large size of future, and possibly present day, tokamaks requires high neutral atom beam energies for current drive and heating. These injectors must also have a high electrical efficiency and this can be achieved using negative ion sources to create the beam. These large tokamaks, such as ITER or NET require a neutral beam power of around 50–100 MW at beam energy of 1 MeV of D0. This power is achieved by operating several sources in parallel, each one of which would extract a 28-A beam of D− ions of which 57% are converted to D0 in a gas cell neutralizer and can be transmitted to the torus. The injector for one port consists of four units, each of which is a fully independent unit. This article describes the details of the injector unit which is based on volume plasma sources of D− ions.

Two-dimensional focusing of neutral atomic beams by resonant laser radiation

A theoretical investigation is reported of a two-dimensional focusing of an atomic beam by a centrally symmetric optical field formed by two pairs of mutually perpendicular laser beams. Conditions under which this optical field acts as a converging laser lens are identified. The errors of such a laser focusing system are determined.

Diode-laser deceleration and collimation of a rubidium beam

We report the use of frequency-chirped diode lasers to decelerate a thermal atomic beam of Rb and, in a separate experiment, the use of frequency-stabilized diode lasers to collimate the beam with optical molasses. For this we used a 100-mW cw laser-diode array injection locked to light from a low-power, frequency-narrowed, single-stripe diode laser. Also, a novel scheme for imaging the neutral-atom beam profile has been developed for monitoring the collimation experiments in two dimensions. We have increased both beam brightness and intensity by a factor of more than 20.

Neutral atomic beam source by a spark-discharge method

A spark-discharge method has been successfully applied to the generation of neutral metal atoms. The neutral atomic source developed by the method has advantages in simplicity, low cost, and high intensity. Resonance ionization spectroscopy of the Fe and Al atoms was carried out by using the spark-discharge method.

The 5dnf J=4 and 5 autoionising Rydberg series in barium: experiment and MQDT analysis

Accurate values for the energies and linewidths of the 5dnf J=5, 4 (n=4-50) autoionising Rydberg series of neutral barium have been collected using either a single-mode CW ring dye laser or a Nd:YAG pumped pulsed dye laser and a beam of neutral atoms in the metastable 5d2 1G4 and 5d2 3F states. The interactions between series within the configuration converging to the 5d3/2 and 5d5/2 ionisation limits as well as the continuum interactions (for J=5) have been analysed using multichannel quantum defect theory (MQDT) in the 'phaseshifted reaction matrix' formalism. Good agreement between experiment and MQDT calculations is obtained; however, not all interaction parameters could be determined from the available data.

Hyperfine structure in the 6snh (l = 5) Rydberg series of barium

The hyperfine structure in the odd-parity 6snh (9 ≤ n ≤ 40) Rydberg series of barium has been investigated, using a single cw ring dye laser and a beam of neutral atoms in the metastable 5d2 1G4 state. Owing to small fine-structure splitting, this hyperfine structure is dominated by the Fermi contact interaction of the 6s electron already at n = 12. The hyperfine-structure analysis results in a determination of the complete fine structure of the 6snh series as well as in values for the singlet–triplet mixing in the 6snh J =5 states. The 6snh series turn out to be perturbed by levels of the 5d4f configuration; J = 4 and J = 5 levels of this configuration have been observed. The Rydberg series perturbations are analyzed, using multichannel quantum-defect theory in the shifted R-matrix formalism. Far from the 5d4f perturbers, the 6snh configuration is almost purely jj coupled.

The production of ions for single-ion traps

A new method of obtaining ions for electrodynamic traps at high vacuum is described, in which an oven produces both a beam of neutral atoms and an electron beam with which to ionize the atoms within the confinement volume of a trap.

Soft x-ray W/Be multilayer and its application to a diffraction grating Yuichi,Utsumi

Tungsten/beryllium multilayers were coated onto lamellar diffraction gratings using a neutral atom beam sputtering technique. Boundary roughness of the multilayer was estimated to be 6.0 Å as a standard deviation. Diffraction efficiency of the multilayer-coated grating was measured using monochromatized synchrotron radiation.

Laser Optics of Neutral Atomic Beams

In ordinary optics one exploits the interaction of radiation with matter to manipulate beams of light. One can also turn the tables and control beams of charged or neutral material particles through their interaction with light. There are deep analogies and similarities between these two sorts of optics, even though they involve very different kinds of interactions between matter and radiation. Perhaps one can exploit every such interaction for some kind of “optics”—controlling beams of photons or massive particles. In this article we will describe recent developments in the laser optics of atomic beams—how experimenters are using the radiation pressure of laser light to manipulate neutral atoms.

Tungsten–beryllium multilayer mirrors for soft x rays

Multilayer structures of tungsten and beryllium were synthesized onto flat silicon single-crystal substrates by the neutral atom beam sputtering technique. Structures of constituent tungsten and beryllium thin films were evaluated. The standard deviation of the interface roughness of the multilayer was estimated to be ~2.5 A. Reflectivities of multilayer mirrors were measured at a grazing incidence of 5.0 degrees . The observed reflectivity of 30% at 1055 eV was in good agreement with the calculated value considering the interface roughness and oxygen contamination for a tungsten-beryllium structure having a period of 77.0 A.

Atomic oxygen simulation and analysis

A technique has been developed for creating in the laboratory suprathermal, neutral atomic beams by means of grazing-incidence collisions between ion beams and metal surfaces. Residual ions are deflected by applied electric fields. The resulting neutral beams are pure, well-focused and mono-energetic. Energies from a few eV to tens of keV are obtainable. The technique permits the beams to be used in conjunction with electron and photon irradiation for studies of synergistic effects. Of particular interest is the creation of a low-energy, neutral, atomic-oxygen beam, which is of importance to space-related research.

Laser optics of beams of neutral atoms

Laser optics of beams of neutral atoms

Photodetachment of Negative Ion Beams in the Presence of a Background Gas

To suppress space charge blowup in an ion beam passing through a photoneutralizer, it is necessary to introduce some background gas. An analysis is presented of the neutralization of a high-energy, >200-keV negative deuterium ion beam, exposed to photodetachment while in the presence of deuterium. With a gas thickness of <0.01 Torr.cm, the neutral fraction in the output beam is found to be about the same as that gotten from the photoneutralizer operating in vacuum. Neutral atom beam injection for plasma heating is discussed.

Lazernaya optika puchkov neitral'nykh atomov

Lazernaya optika puchkov neitral'nykh atomov

Use of the Kalman filter in signal processing to reduce beam requirements for alpha-particle diagnostics

In principle, a probing beam of neutral atoms can be used to measure the velocity distribution of fast alpha particles confined in an ignited fusion plasma. In this article we discuss beam modulation techniques and a compatible signal processor based on a Kalman filter that together can provide, in one hypothetical case, a reduction of a factor of 10 in the beam current required, as compared with that required for a straightforward measurement with a dc beam.

High kinetic energy (1–10 eV) laser sustained neutral atom beam source

Development of high energy (1–10eV) neutral atom beams has relied primarily on the use of charge exchange or DC/RF discharge techniques. A new source is described which uses a laser sustained plasma technique for producing high intensity (⪢0 15/cm 2s) and high translational energy (⪢1 eV) atomic beam species for use in gas-surface scattering experiments. Laser sustained plasmas have demonstrated temperatures of 10 000 K in xenon, 18 000 K in argon, and a predicted temperature of 30 000 K in helium. Combining this plasma with hydrodynamic expansion techniques should produce atomic beam velocities greater than 10 km/s for many species. Initial experiments with xenon using 70 W of CO 2 laser power have demonstrated beam kinetic temperatures of 8–9000 K with Mach numbers of 4–6 resulting in peak velocities for xenon of 1.5 km/s (1.5 eV). Extrapolation of these results to helium predicts that velocities in excess of 10 km/s are possible but will require laser powers in excess of 1.3 kW.

Cooled Atomic Beams for Frequency Standards

During the recent past much effort has been spent on cooling of neutral atomic beams. As a result, two successful experimental methods for laser cooling of atomic beams were developed with post-cooling temperatures in the mK regime. We discuss in which ways these cooled atomic beams and new methods for further cooling and handling of these slow atomic beams could improve present and future frequency standards.