L. C. Johnson

Plasma Diagnostics with Ionized Barium and Tunable Dye Lasers

Resonance fluorescence and optical pumping techniques using tunable dye lasers permit the measurement of density, ion temperature, electron temperature, and ion drifts, over a wide range of density in barium, or barium doped plasmas.

Measurement of the development and evolution of shock waves in a laser‐induced gas breakdown plasma

Space‐ and time‐resolved interferometric measurements of electron density in CO2‐laser produced plasmas in helium or hydrogen are made near the laser focal spot. Immediately after breakdown, a rapidly growing region of approximately uniform plasma density appears at the focal spot. After a few tens of nanoseconds, shock waves are formed, propagating both transverse and parallel to the incident laser beam direction. Behind the transverse propagating shock is an on‐axis density minimum, which results in laser‐beam self‐trapping. The shock wave propagating toward the focusing lens effectively shields the interior plasma from the incident beam because the lower plasma temperature and higher plasma density in the shock allow strong absorption of the incident beam energy. By arranging the laser radiation‐plasma interaction to begin at a plasma‐vacuum interface at the exit of a free‐expansion jet, this backward propagating shock wave is eliminated, thus permitting efficient energy deposition in the plasma interior.

Conduction heat loss scaling in open field line geometries

A scaling law governing the cooling of a finite‐length column due to electron thermal conduction loss at the column ends, where the magnetic field lines are intercepted by a cold material wall, is derived. In addition to the electron temperature, both the electron number density and the magnetic field strength are allowed to vary along the field lines. It is shown that the cooling of the column is not very sensitive to moderate spatial variations of number density and magnetic field strength and is well represented by a characteristic cooling time given by τ0= (5/2) nL2/K0, where L is the column half‐length and n and K0 are the number density and coefficient of thermal conductivity at the midplane.

Fully‐Ionized Barium Plasma Source

A device for generating a fully‐ionized barium plasma column is described. It is similar to the previously described Q1 device except for the substitution of rhenium ionizing surfaces for tungsten, and barium for the alkali metals. The main motivation for the use of barium is that singly‐ionized barium has its resonance lines in the visible wavelength range, and thus allows the use of spectroscopic techniques for the determination of various plasma parameters. The special problems of barium in a Q device, and the spectroscopic techniques, including range of applicability and achievable accuracy of the measurements, are discussed.

Electrical resistivity of neon plasmas in a Tokamak

In ST Tokamak discharges with hydrogen or helium as the working gas the measured plasma resistance is usually found to be slight but consistently larger (1.1-1.5 times) than the resistance calculated from measured electron temperature and ionic charge radial profiles. The discrepancy has been ascribed tentatively to undetected high-Z material (Mo or W) from the aperture limiter. Recent experiments with neon, with effective ionic charge Z=9-10 in the discharge, do show good agreement between measured and calculated resistance. In other respects the behaviour of the neon discharge differs only slightly and in the expected direction from hydrogen or helium discharges: larger power input, higher electron temperature, and higher radiation losses. Also, measurement of the intensities of the recently discovered resonance lines of Mo XIII and Mo XIV do indicate a molybdenum concentration in the discharges of the ST Tokamak that is adequate to account for the above mentioned discrepancies in the resistance of H and He plasmas.

Observations on a Fully Ionized Barium Plasma

A series of spectroscopic measurements in a fully ionized barium plasma column is described and discussed. The plasma was generated in the Q‐1 device in a manner similar to that used to make alkali‐metal plasmas. The observations show that the density contours and the temperature profiles in the body of the plasma were strongly influenced by conditions at the end plates. The results also indicate that the losses at the ends of the column dominated over the losses in the body of the plasma. Evidence is presented for the existence of internal circulating currents along the confining magnetic field lines in the absence of externally applied potentials. A verification of the Thompson effect in a current‐carrying, fully ionized plasma is also presented.

Optical Measurements of Effects of Collisional Drift Waves on Plasma Density in Barium Plasmas

Effects of the collisional drift wave and of the presence of a Langmuir probe on plasma density in a Q‐device barium plasma are investigated by means of independent measurements of local ion densities with the method of resonance fluorescence.

Electrical Conductivity of a Partially Ionized Gas

Electron velocity distribution functions for a partially ionized gas in a weak, steady electric field are obtained by solving the Boltzmann‐Fokker‐Planck equation numerically. As in the case of a fully ionized gas, the electron‐electron interactions in a partially ionized gas affect the velocity distribution function and, consequently, the electrical conductivity. Electrical conductivities are calculated for a variety of assumed electron‐molecule collision frequencies. The results differ by only a few percent from those obtained using an approximation suggested by Frost. A simple procedure, requiring no numerical integrations, is given relating electron temperature to electrical conductivity for a partially ionized gas.

Electron Temperatures from He I Intensity Measurements

Data are presented relating the intensity of the λ 5876 A line of He I to electron temperature for use in electron thermometry of plasmas in the range 2 eV ≲ Te ≲ 10eV and 1010cm−3 ≲ ne ≲ 1014cm−3.