A. T. Young

In situ measurement of rovibrational populations of H2 ground electronic state in a plasma by VUV laser absorption

Abstract Absorption spectroscopy employing laser-generated VUV has been employed to measure the populations and temperatures of H2 in individual rovibrational levels of the ground electronic state in a low-pressure plasma discharge. Vibrational states up to ν=5 and rotational states up to J=8 were observed. This is the first quantitative in situ measurement of vibrational levels with ν ⪢ 3 of the X1Σg+ state of H2 in a plasma.

In situ density and temperature measurements of vibrationally excited hydrogen molecules in ion source plasmas

The role of highly vibrationally excited hydrogen molecules has been postulated to be of great importance in H− ion sources. However the the difficulty of making in situ measurements has led to a paucity of direct determinations of these species within the plasmas of these sources. Recently, vacuum‐ultraviolet (VUV) laser absorption spectroscopy has been used to measure the H2 rovibrational populations up to v‘=5 and J‘=8 in a medium‐power hydrogen plasma. This work extends those measurements to v‘=8 and to J‘=13. The populations of the vibrational levels still appear to be almost Boltzmann. The theoretically predicted plateau is not observed up to the detection limit. The dependence of several vibrational levels on discharge current and filling pressure is shown.

VUV laser absorption spectrometer system for measurement of H0 density and temperature in a plasma

A system to determine the density and temperature of ground‐state hydrogen atoms in a hydrogen plasma by measurement of the absorption of Lyman‐beta or Lyman‐gamma radiation is described. The Lyman‐series radiation is generated by resonant four‐wave sum‐frequency mixing in mercury vapor. A wide range of hydrogen atom densities can be measured by employing these two transitions. A sample measurement on a H− ion‐source discharge is presented. Extensions to Lyman‐alpha and other vacuum‐ultraviolet wavelengths are discussed.

Atomic hydrogen density measurements in an ion source plasma using a VUV absorption spectrometer

A system to determine the density and temperature of ground‐state hydrogen atoms in a plasma by vacuum ultraviolet laser‐absorption spectroscopy is described. The continuous tunability of the spectrometer allows for analysis at any of the Lyman transitions. The narrow bandwidth of the laser source allows for the accurate determination of the hydrogen absorption line shape and, hence, the translational temperature. The utility of the system is exemplified by data obtained on an ion source plasma. The measurements show the quality of the data as well as illustrating the behavior of these sources under varying discharge conditions.

Laser and spectroscopic diagnostics of H− ion source plasmas

This paper reviews the results obtained in this laboratory in experimental studies of volume H− ion sources. Two techniques are discussed, emission spectroscopy and vacuum ultraviolet laser absorption spectroscopy. Time‐resolved detection of Balmer‐alpha emission has been used to study the excited‐state hydrogen atom population. Vacuum ultraviolet laser absorption spectroscopy has been used to measure the ground electronic state atomic and molecular hydrogen populations. The Balmer‐alpha emission exhibits a modulation in the intensity at a frequency of twice the exciting rf radiation. On the other hand, the laser absorption measurements show that the density of ground‐state hydrogen atoms does not vary significantly with the phase of the exciting rf. Measurements of the vibrational population summed over all rotational states give a vibrational temperature of 6200 K. However, there is no evidence of the ‘‘plateau’’ predicted in the distribution for v≥5.

Photoemission starting of induction rf‐driven multicusp ion sources

It has been demonstrated that pulsed and continuous wave, rf‐driven hydrogen discharges can be started with photoemission. The extracted H− current from a photoemission‐started plasma has been investigated and does not differ significantly from that of a filament‐started plasma. The minimum pressure for photoemissive starting was found to be higher than that of filament starting, 17 mTorr compared to 7 mTorr, respectively, in this particular configuration.

Optimization design study for an elliptical wiggler at the Advanced Light Source

This paper describes a design study with the objective of optimizing spectral performance of an elliptical wiggler to be installed at the Lawrence Berkeley Laboratory Advanced Light Source (ALS). This device is to produce circularly polarized radiation in the energy range of 50 eV–10 keV. A figure of merit, which is a function of flux density and degree of circular polarization, is introduced as the objective function for optimization. An optimum set point for a particular photon energy is characterized by values of peak vertical field, horizontal deflection parameter, and vertical aperture. Optimum performance is evaluated for the nominal ALS operating energy of 1.5 GeV.

Measurement of atomic and molecular hydrogen in a tandem magnetic multicusp H− ion source by VUV spectroscopy

The populations of ground electronic state atomic hydrogen and ground electronic state, vibrationally–rotationally excited hydrogen molecule in a negative hydrogen ion source discharge have been measured using vacuum ultraviolet (VUV) laser absorption spectroscopy. These populations have been measured under a variety of discharge conditions in two different regions of a tandem chamber ion source. Preliminary results of the measurements in the driver region and filter region are given. It is observed that the atomic hydrogen density decreases as one goes from the driver to the filter region. This indicates that the surfaces directly adjacent to the filter region are net sinks for hydrogen atoms. In contrast, the molecular vibrational population distribution shows only a small difference between the two regions, indicating that these surfaces are not net sinks for the excited molecules.

Nanosecond‐length electron pulses from a laser‐excited photocathode

A photocathode made from polycrystalline lanthanum hexaboride (LaB6) has produced nanosecond‐length electron pulses when excited by an excimer laser at 308 nm. Peak currents in excess of 1 A have been observed, with quantum yields of 4×10−5 being measured. A method for extracting the electrons from an emission‐limited cathode, plasma extraction, has been demonstrated. This technique uses a low‐power continuous discharge to provide the electric field needed to extract the photoelectrons. This technique may be useful in producing high‐repetition‐rate short‐pulse ion sources.

VUV laser diagnostics of H− ion sources

Vacuum ultraviolet laser absorption spectroscopy has been employed to measure the populations and temperatures of ground electronic state H‐atoms and vibrationally‐excited H2 molecules in a volume H− ion source. Measurements of both species have been made under a variety of discharge conditions. Vibrational levels to v=8 have been measured, with the vibrational population distribution well described by a temperature of 4150 K.