J. L. Pack

Longitudinal electron diffusion coefficients in gases: Noble gases

Values of the ratio of the longitudinal diffusion coefficient to mobility DL/μ for electrons in He, Ar, Kr, and Xe are derived from current waveforms obtained during earlier measurements of electron mobility. The electric field to gas density ratios E/N cover the wide range of 10−3 to 20 Td, thereby bridging previous experiments at low E/N to recent experiments at high E/N. Here 1 Td=1×10−21 V m2. The corresponding DL/μ values range from 0.0066 eV for thermal electrons at 77 K to 10 eV. In addition to the well‐known peak in DL/μ for Ar at E/N between 0.01 and 0.1 Td caused by the Ramsauer minimum in the momentum transfer cross section, we find previously unreported low‐energy peaks in DL/μ vs E/N in Kr and Xe and previously unreported pronounced leveling‐off in DL/μ at E/N≳8 Td in Ar, Kr, and Xe. Calculations of transport coefficients using numerical solutions of the Boltzmann equation and cross section sets in the literature give good agreement with experiment from E/N producing thermal electrons up to a...

Measurement of Time Varying Optical Absorption

Time sampling techniques have been applied to the study of optical absorption transients. The method is described in terms of its use for the study of the decay of the concentration of metastable atoms following a pulsed discharge. Light from a capillary source is passed through the discharge tube. The transmitted intensity of one of the spectral lines which is absorbed by the metastable atoms is measured using an interference filter and a gated photomultiplier. The photomultiplier gate occurs at twice the discharge frequency so that alternate pulses of the photomultiplier output are reduced by absorption. The component of the photomultiplier output at the discharge frequency is proportional to the absorption and can be measured with a narrow band amplifier and synchronous detector. Since the time resolution of the system is determined by the width of the photomultiplier gate, the response of the synchronous detector can be made slow enough to average the absorption signal over many decay periods. Fluctuations in the number of electrons leaving the cathode of the photomultiplier limit the useful sensitivity of the present system to approximately two parts in 104. This represents a hundredfold improvement in sensitivity over that available with previous techniques. The results of some studies of helium metastables are presented in order to illustrate the detail which can be obtained with this technique.

Plasma Production by a High‐Power Q‐Switched Laser

Studies have been made of the confinement in a magnetic mirror of plasmas produced by focusing a Q‐switched laser beam on aluminum disks, foils, and spheres. High speed photographs show that, in the absence of a confining magnetic field, the plasma generated from ball targets is roughly symmetrical whereas one component of the plasma generated from flat targets is asymmetric and moves toward the laser with a velocity of about 107 cm/sec. A second component which is more nearly symmetric expands with a velocity of approximately 2 × 106 cm/sec. The expansion velocities are not appreciably reduced in a 14–6‐14 kG mirror field. However, the duration of the luminosity from a slower constituent of the plasma increases from 1 μsec to about 5 μsec and microwave interferometer measurements show an electron density of about 5 × 1011 cm−3 at times as late as 50 μsec. Electrostatic probes verify that the maximum expansion velocity corresponds to an ion kinetic energy of about 1.4 keV. Time integrated spectra of the p...

Linear and Nonlinear Behavior of Laser Produced Aluminum Plasmas

Framing camera and energy absorption studies of aluminum plasmas produced from single spheres 50‐130 μm in radius indicate linear and nonlinear behavior.

High average power pulser design for copper halide laser systems

A circuit using two thyratrons is described which provides alternating polarity, high‐current pulses at pulse repetition rates up to 20 kHz, suitable for operating copper halide lasers. The circuit is a modification of a Blumlein configuration in which two networks are charged in parallel and discharged in series, providing a voltage quadrupling effect when used with resonant charging. By triggering the thyratrons sequentially the current is reversed on alternate pulses, which greatly reduces axial cataphoretic effects and extends the laser tube operating lifetime. The circuit can deliver up to 5 kW average power at 15 kHz.

Long‐lived lead‐vapor lasers

Lead‐laser operation has been demonstrated at 7229 A in all‐hot sealed‐off quartz discharge tubes for 50‐h cumulative operating times without tube degradation or failure. The maximum output energy of 230 μJ or 2.0 μJ cm−3 was obtained at ∼1.5 kHz, and the discharge efficiency was 0.12%. Average‐power levels of 750 mW at 6 kHz were maintained under burst mode excitation conditions for extended periods at temperatures of ∼1050 °C. Lead‐laser emission at ∼500 °C was also obtained using lead iodide as the starting material.

Axial cataphoresis effects in continuously pulsed copper halide lasers

It has been observed in a continuously pulsed copper halide laser that laser performance deteriorates rapidly due to axial cataphoresis effects. Dissociated copper ions are pumped cataphoretically toward the cathode by the applied electric field, causing nonuniform laser discharges and preferential copper condensation near the cathode. These effects can be eliminated experimentally by alternating the polarity of the applied voltage either mechanically or electronically. This improvement permits long‐lived copper halide laser operating at average power levels in the 1–10‐W range.

Retracting Pedestal Apparatus that Presents a Single Solid Target to a Focused Q‐Switched Laser Beam

An electromechanically operated retracting pedestal system has been developed for injection of a single solid target as small as a few tens of microns in radius into a vacuum system. After the pedestal has withdrawn completely from the system, the falling target, which originally was positioned on top of the pedestal, is ionized by a focused, high power, Q‐switched laser beam. Since the plasma is produced in a relatively large volume free of mechanical obstructions, it can be easily studied or used as a source of ions. The pedestal is withdrawn with a velocity in excess of 10 m‐sec−1, whereas the target falls with a velocity given solely by the acceleration due to gravity. The laser beam which ionizes the target is triggered by an optical tracking system, consisting principally of an He–Ne laser whose beam is collinear with that of the pulsed laser. The device can be loaded rapidly in vacuum from a reservior with a capacity of hundreds of targets, and fired every few minutes. Application to experiments on...

Voltage and current measurements for fast pulsed high current discharges

A method for correcting for the inductance in a fast pulsed discharge circuit yields accurate discharge voltage and power input waveforms. An economic fast current shunt is described.

MICROWAVE DIAGNOSTICS OF LASER PRODUCED ALUMINUM PLASMAS.

A focused laser beam has produced plasmas from single aluminum spheres 50‐150 μ in radius. Microwave studies determined the expansion velocity at 300 nsec and the average electron density between 1 and 6 μsec.