Transverse Velocity Spread Research Articles

Transverse kinematics of ions stored in an electrostatic ion beam trap

We present experimental results, as well as numerical simulations, for the transverse velocity distribution of ions stored in an electrostatic ion beam trap. The measurements indicate that the transverse velocity spread is about 1% of the longitudinal velocity, and that the ions fill the whole transverse stable phase space. We also demonstrate that ion losses from the trap due to multiple scattering with molecules from the residual gas is an important factor limiting the lifetime of the beam.

Ｌｉ原子ビームのレーザコリメーション

Laser collimation of a Li atomic beam has been studied to create an intense collimated atomic beam. The gain of Li atomic intensity in the beam has been measured as functions of laser collimation parameters, such as laser-atom interaction length, laser-frequency detuning and laser power density. The Li intensity gain depends non-linearly on the laser-atom interaction length due to the longitudinal velocity distribution of the thermal Li beam (550°C). In the high laser power regime, the dependence of the Li intensity gain on the laser power and detuning disagrees with the theoretical prediction of the scattering force. These findings are explained by considering the saturation broadening of the Li absorption line. By optimizing the laser collimation parameters, 3.2-fold increase of the atomic intensity is attained for the collimated Li beam with the transverse velocity spread of±5m/s.

Preparation and performance of transmission-mode GaAs photocathodes as sources for cold dc electron beams

Photoemission from GaAs cathodes with negative electron affinity (NEA) is applied for producing electron beams with very low longitudinal and transverse velocity spread. GaAs transmission-mode cathodes were activated with Cs and either O2 or NF3 in an extremely high vacuum setup (base pressure below 10−12 mbar). Quantum efficiencies of 20%–25% (at 670 nm) and long dark lifetimes (about 1000 h) could be achieved for both types of activation in a reproducible way. Using a method based on the adiabatic transverse expansion of an electron beam in a spatially decreasing magnetic field, the mean transverse energy (MTE) of the photoemitted electrons was measured systematically, recording longitudinal energy distribution curves. Both the MTE and the longitudinal energy spread strongly depend on the value of NEA and the position of the extracted distribution relative to the bulk conduction band minimum. Electrons with energies above the conduction band minimum are thermalized with the lattice temperature of the cathode, while electrons with energies below this level show a non-Maxwellian distribution with enhanced transverse energies. Thus, when extracting all electrons in a current limited emission mode, the MTE increases with the absolute value of NEA and reaches values up to ≈100 meV. By cutting off the low energy electrons with an external potential barrier, the longitudinal as well as transverse energy spread of the extracted electron ensemble are reduced. The MTE could be reduced down to about 28 meV at room temperature and to about 14 meV at liquid nitrogen temperature. The behavior of the MTE was found equivalent for (Cs, O) and for (Cs, F) activation layers on the same cathode. Conclusions about energy loss and scattering in the emission of photoelectrons from NEA GaAs cathodes are discussed.

A focussing funnel for metastable helium

We have constructed a magneto-optical funnel for He atoms and studied its properties using a laser cooled, highly mono-energetic atomic beam. A simple model of its action allows us to quantitatively understand the observed spot size and “focal length”. We show that for a fast beam, the velocity damping coefficient plays an important role in determining the focal length of the device. The observed spot size is limited mainly by transverse heating processes which impose a transverse velocity spread. The device also permits easy scanning of the focussed spot.

Gyro-TWT with a helical operating waveguide: new possibilities to enhance efficiency and frequency bandwidth

A helical corrugation of the inner surface of an oversized cylindrical waveguide provides, for certain parameters, an almost constant value of group velocity and close to zero longitudinal wavenumber of an eigenwave for a very broad frequency band. The use of such a helical waveguide as an operating section of a gyrotron traveling wave tube (gyro-TWT) allows significant widening of its bandwidth and an increase in the efficiency at very large particle velocity spreads. In this paper, the new concept is confirmed by theoretical analysis and "cold" measurements of the helical waveguide dispersion. Results of a linear and nonlinear theory of the helical gyro-TWT as well as two designs for subrelativistic (80 keV, 20 A) and relativistic (300 keV, 80 A) electron beams are also presented. For both designs, parameters providing a very broad frequency band (about 20%) and high efficiency (above 30%) have been found. When the transverse velocity spread is increased from zero up to a very high value of 40%, simulations showed only a 20%-30% narrowing in the frequency band and a 20% decrease in electron efficiency. The theoretical analysis demonstrates important advantages of the helical gyro-TWT over the "smooth" one in frequency bandwidth, sensitivity to electron velocity spread, and stability to parasitic self-excitation.

Electron gun and collector design for 94-GHz gyro-amplifiers

This paper presents the electrical design of a magnetron injection gun and collector for high peak and average power TE/sub 01/ W-band gyro-amplifiers. The magnetron injection gun design employs an optimized double-anode geometry and a cathode angle of 500 to achieve superior beam optics that are relatively insensitive to electrode misalignments and field errors. A transverse relative velocity spread of 1.6% at a velocity ratio of 1.5 is obtained in simulations for a 6-A 65-kV beam. Cathode edge emission was modeled, found to produce a major effect on velocity spread, and will be eliminated by nonemissive cylinders affixed to the cathode. The collector, which also serves as the output waveguide, is designed to minimize the required collector length while avoiding excessive power loading on the collector surface. An average power density of less than 640 W/cm/sup 2/ on a 3.25-cm-diameter collector is achieved for 91-kW average beam power. Both the gun and collector are currently under construction.

A diode electron gun for a 1MW 140GHz gyrotron

A 40 A diode type magnetron injection gun with a beam power of 3.2 MW for a 1 MW, 140GHz gyrotron operated in the TE22,6 mode has been designed, manufactured, tested and operated in a tube. Due to the diode character without an intermediate anode, the beam parameter space is strongly restricted. The possibility of variable adjusting of the distribution of the cathode magnetic field with the two gun coils increases somewhat the reduced parameter space, however. The gun has been designed to satisfy technical constraints. Measurements of the beam properties have been performed at scaled-down parameters. The measured average values of the beam parameters, such as the velocity ratio, agree well with calculations. From experimental current distribution a transverse velocity spread of ≤ ±15% has been obtained. Operation in a gyrotron at design parameters confirms the numerical results.

Generation of helical electron beams by a non-adiabatic gun

The design of a non-adiabatic gun to produce a 10A, 50kV hollow laminar electron beam for gyrotron applications is reported. The beam is extracted from the emitting ring in a direction parallel to the axial guide magnetic field and then propogates across the radial electric field in the anode gap. The electrons are thereby given a transverse velocity upon passing through the modulation anode region where an electrostatic pumping mechanism takes place, so that a considerable amount of the electron energy is converted to transverse kinetic energy. Such a beam extraction method gives rise to favourable features that are examined throughout the work. The dynamics of hollow electron beams with gyromotion propagating down a cylindrical drift tube are also analysed. Due to the action of the beam's self-space charge field, the transverse velocity spread has an oscillatory behaviour along the drift tube wherein the spatial automodulation period shortens with increasing current. Numerical simulation results indicat...

Inverse magnetron injection gun for a coaxial 1-5MW, 140 GHz gyrotron

A 50 A electron gun with a beam power of 4-5MW for a 1-5MW, 140GHz coaxial gyrotron has been designed, manufactured and tested. An axial rod close to the ground (anode) potential is surrounded by the cathode. The gun is of the diode type with no independent adjustment of the velocity ratio α. Therefore only a low influence of the mechanical tolerances on a can be accepted. The beam properties have been measured using scaled-down parameters. The experimental values of [he velocity ratio agree well with calculation. The measured velocity distribution shows a transverse velocity spread δβ⊥≤15%.

Effusive molecular beam from a capillary array for improved resolution time‐of‐flight and collision spectrometry

AbstractWe report here high mass resolution obtained with an electron impact, Wiley–McLaren type time‐of‐flight spectrometer employing an effusive molecular beam from a capillary array instead of the conventional gas cell. The improvement in resolution is due to the subthermal transverse velocity spread achievable from the well‐directed target beam produced by the capillary array. We also discuss the advantage of using the capillary array in various other spectrometry applications such as translational energy spectrometry and electron spectrometry where the energy resolution is limited by the thermal motion of the target gas.

Magneto-optical preparation of a slow, cold and bright Ne ∗ atomic beam

An efficient experimental scheme for the generation of a monochromatic, slow and bright atomic beam is reported. It combines the good efficiency of a Zeeman-slower for medium slow final velocities with an optimized atomic funnel design. The two severe problems of a Zeeman-slower, as there is the high final divergence and the vanishing efficiency for very low final velocities, are overcome. The total efficiency in flux of the complete device has been measured to more than 10% of the thermal atomic beam. Using a beam of metastable neon atoms (Ne ∗) a beam with a transverse velocity spread of σ x < 0.34 m/s and a mean longitudinal velocity of 19 m/s with a spread of σ v < 1.5 m/s is prepared. By this preparation technique a purely metastable atomic beam is obtained, all Ne atoms in the 3s[3/2] 2-state.

Proton beam diagnostics using optical spectroscopy

A non-intercepting diagnostic technique has been used for measuring the transverse velocity spread of an intense proton beam using optical spectroscopy. Protons in a beam can undergo one-electron transfer reactions with the background gas to form fast hydrogen atoms. The beam velocity distribution can be determined from spectral measurements of the Doppler broadening of the H α emission from these atoms. The transverse velocity spread is measured by placing the spectrometer axis normal to the beam direction. Details of this technique and examples of its application are described.

Erratum: A generalized formulation of free-electron lasers in the low-gain regime including transverse velocity spread and wiggler incoherence [J. Appl. Phys. 52, 599 (1981)

Erratum: A generalized formulation of free-electron lasers in the low-gain regime including transverse velocity spread and wiggler incoherence [J. Appl. Phys. <b>52</b>, 599 (1981)

A generalized formulation of free-electron lasers in the low-gain regime including transverse velocity spread and wiggler incoherence

It is shown that in the small-signal, low-gain regime, an analytic expression for the gain can be derived which is valid for a general electron distribution function and general longitudinal variation of the transverse magnetic field pump. This expression is used to evaluate the effects on the gain curve due to transverse momentum spread in the beam as well as inaccuracies (incoherence) in the magnetic pump phase and amplitude. The restricting criteria for the neglect of these effects are derived.

Filamentation of a heavy-ion beam in a reactor vessel

A heavy-ion beam driver for inertial confinement fusion is subject to filamentation instability over a broad range of beam and plasma background conditions. The case of a beam injected into a gas-filled reactor vessel, where finite pulse length and propagation distance play an important role in limiting mode growth, is analyzed. The effects of transverse thermal spread, spherical convergence to the pellet, and finite magnetic decay rate of eddy currents are included in this treatment. It is concluded that a cold beam will be severly disrupted unless the product of magnetic plasma frequency and propagation time is not large compared with unity. If this condition is not met, mode growth may still be limited to about six e folds by adding transverse velocity spread such that the pulse tail is in a state of pinch equilibrium. However, this approach causes much of the pulse to be lost by thermal expansion.

Measurement of Momentum Cooling Rates with Electron Cooling at NAP-M

In this paper we report the measurements of the damping rates for the momentum spread of a proton beam by means of electron cooling at NAP-M. Until recently, a systematic investigation of cooling of betatron oscillations was carried out; but, in the longitudinal plane, it had been possible to measure only the behavior of the longitudinal drag force F˝. But obviously a direct observation of the momentum cooling rate λ˝ would be more significant. We measured the momentum cooling rate λ˝ versus: (a) proton beam longitudinal spread θ˝p, (b) electron beam Larmor velocity spread ⊥L, (c) electron beam transverse velocity spread θ˝e, and (d) electron beam longitudinal spread θ˝e. Also we measured the cooling rate,' after sweeping the electron beam through the proton beam spread, and compared the result with those obtained with the standard technique to observe any improvement. All measurements have been made at an energy of 65 MeV and with a typical electron beam density of 0.38 A/cm2.

Beam-plasma instability of a finite temperature electromagnetoplasma

The beam-plasma instability of the collisional magnetoplasma in the presence of a weak longitudinal electric field has been studied. The dispersion relation and the growth rate for the whistler mode instability have been derived and computed. The earlier known results are recovered when the parallel electric field (E0) is reduced to zero. The effect of a parallel electric field is to increase the growth rate. However, the effective electron collision frequency quenches the instability caused by the electrostatic field and the beam. The reversal in the direction of electric field decreases the growth rate. Further, it has been shown that the whistler mode instability can also be excited when the beam particles have no transverse velocity spread. This study may be useful to explain the observations of v.l.f. triggered emissions in the magnetospheric and ionospheric plasma.

Differential Angular Distribution of H andH+Dissociation Fragments of FastH2+Ions Incident onH2Gas

Angular distributions of the fast H and ${\mathrm{H}}^{+}$ dissociation fragments produced in single collisions of ${\mathrm{H}}_{2}^{+}$ with ${\mathrm{H}}_{2}$ have been measured in the energy range 5-80 keV. The observed distributions are discussed in terms of three dissociation reactions, ${\mathrm{H}}_{2}^{+}$ \ensuremath{\rightarrow} ${\mathrm{H}}^{+}$+H, $\mathrm{H}_{2}^{}{}_{}{}^{+}\ensuremath{\rightarrow}{\mathrm{H}}^{+}+{\mathrm{H}}^{+}+e$, and $\mathrm{H}_{2}^{}{}_{}{}^{+}+e\ensuremath{\rightarrow}\mathrm{H}+\mathrm{H}$, whose relative proportions vary strongly over the energy range of the investigation. Distributions of the component of velocity of the dissociation fragments transverse to the ${\mathrm{H}}_{2}^{+}$ beam direction are deduced from the measurements. The H-atom transverse velocity distributions are approximately independent of ${\mathrm{H}}_{2}^{+}$ projectile energy and are dominated by H atoms from the H+H dissociation reaction. The ${\mathrm{H}}^{+}$ transverse velocity distributions vary in width as the ${\mathrm{H}}_{2}^{+}$ ion energy is varied. At low energies, where the ${\mathrm{H}}^{+}$ production is dominated by the ${\mathrm{H}}^{+}$+H reaction, the transverse velocity spread is narrower than at high energies where the ${\mathrm{H}}^{+}+{\mathrm{H}}^{+}+e$ reaction dominates the ${\mathrm{H}}^{+}$ production. A theory of the dissociation mechanism which predicts the angular distribution of dissociation fragments is developed. The theory is based upon Born-approximation calculations of the dissociation cross section versus internuclear axis orientation and internuclear spacing of the ${\mathrm{H}}_{2}^{+}$ ions prior to the collision. These calculations, presently available only for the case of an H-atom target and for the $1s{\ensuremath{\sigma}}_{g}\ensuremath{\rightarrow}2p{\ensuremath{\sigma}}_{u}$ dissociative transition of the incident ${\mathrm{H}}_{2}^{+}$ ion, were used to deduce the angular distribution of dissociation fragments in the laboratory coordinate system. Fairly comprehensive agreement is found between the calculated distribution and the observed ${\mathrm{H}}^{+}$ distribution for an ${\mathrm{H}}_{2}$ target at 10 keV where the H+${\mathrm{H}}^{+}$ dissociation mode dominates the ${\mathrm{H}}^{+}$ production. The theoretical model is used to discuss qualitatively the observed shapes and widths of the ${\mathrm{H}}^{+}$ and H transverse velocity distributions and to relate qualitatively the shapes of these distributions to the distribution of ${\mathrm{H}}_{2}^{+}$ ion internuclear spacings prior to the collision.

Bipotential cathodes for reduction of grid current

The useful operating range of triodes may be extended into the positive-grid region by preventing electrons from reaching the grid. To do so, electron emission must be suppressed from the cathode area under the grid, and the emission that does take place must be guided through the grid openings. Both of these requirements are met by the use of passive-base emitting areas and by clean metal nonemitting areas of high work function. Calculation of electron trajectories in the absence of space charge shows that the two potentials on the cathode surface provide transverse fields which focus the electrons between the grid wires. For best focusing, there is an optimum height of the nonemitting stripe relative to the emitting surface. Measurements on large scale models of planar triodes show that these results can be achieved with the use of appropriate materials. Striking reductions in electron current to the positive grid are obtained with titanium or platinum nonemitting stripes. Pinhole camera studies of the current distribution and transverse velocities at the anode plane reveal the nature of the focusing action and conditions in the grid plane. Additional benefits of this construction are reduction of transverse velocity spread and more uniform electron transit times. Bipotential cathodes have been incorporated in practical size metal-ceramic triodes with the expected improvements in performance.