Theta-pinch Plasma Research Articles

Electron densities from Thomson scattering and helium-neon laser interferometry

Electron density measurements from Thomson scattering were made in a theta-pinch plasma. Rayleigh scattering from nitrogen substituted for the plasma in the system was used as a calibration method for these density measurements. A HeNe laser interferometer determined electron line densities independently. The large discrepancy between experimental and theoretical Rayleigh scattering cross sections and, therefore, electron densities from Thomson scattering reported by Skowronek et al. could not be confirmed.

Magnetohydrodynamics of nearly cylindrical configurations with fixed internal currents

A theta pinch plasma with perturbing magnetic fields produced by fixed internal currents is considered. The constant pressure model is used and stability is calculated for k=0 modes, which are neutrally stable in the unperturbed situation. The approach is very similar to that used for the high-beta stellarator. The field perturbations produced by internal windings are found to be stabilizing whereas they are destabilizing for the high-beta stellarator with external windings.

The Spectrum ArIX and Extended Spectral Classification in ArV to ArVIII and ArX

Spectroscopic observations of a theta-pinch plasma are analysed, resulting in the classification of nearly 200 spectral lines of ArV to ArX. The ArIX data are of particular interest as the observed lines mainly belong to the 2p53d-2p54f, 5f, 6f, and 2p53p-2p54d, 5d, 4s, 5s and 2p53s-2p54p transition arrays, for which no information previously existed. 80 new ArIX energy levels are established. Theoretical calculations of atomic energy levels and oscillator strengths aided the identification of spectral lines.

Intense relativistic electron beam interaction with a cool theta pinch plasma

Experimental results are presented for the heating of a 4 m long plasma confined by a uniform magnetic field of 4–5 kG by an intense relativistic electron beam. Beam parameters were 0.5–1 MeV, 25–80 kA, 60–70 nsec pulse duration, and electron density of 2–5×1011/cm3. The initial plasma density ranged from 5×1013/cm3 to 4×1015/cm3 and the electron temperature was 1–3 eV. The lower density cases were partially ionized with Te≫Ti, and the higher density cases were highly ionized with Te≈Ti. In all cases, the energy coupled from the beam to the plasma was greater than can be explained by binary collisions between beam electrons and the plasma particles. Beam energy transferred to the plasma ranged from 2–7%/m, and was uniform over the 4 m length of the plasma. Over most of the density range tested, 5×1013/cm3 to 1.5×1015/cm3, the plasma heating cannot be explained by classical processes. These results are found to be explained quantitatively by the use of a full nonlinear treatment of the electron-electron two-stream instability in the kinetic regime.

Laser scattering measurement of nonthermal density fluctuations in a theta-pinch plasma

Nonthermal density fluctuations of scale length less than λD are observed in a theta‐pinch plasma by laser scattering measurements. Results show the thermal spectrum modulated approximately at harmonics of ωp. For the plasma ne≃1016 cm−3 and Te≃5 eV.

Some effects of end stoppering on a theta-pinch plasma

Experimental data are presented indicating improved homogeneity and stability of the luminous plasma in a hard core ϑ-pinch device equipped with quartz end-stopper disks.

Heating a dense theta-pinch plasma with strong fast-rising magnetic fields

Results of experimental investigations on ultra-rapid heating of a dense plasma in a linear θ-pinch and switching of megampere currents in the supply system based on the application of an inductive energy accumulation method are presented. Magnetic fields of up to 50 kG with rise-times of approximately 0.2 μs were generated in a solenoid 7 cm in diameter and 50 cm long. The following plasma parameters were obtained: ne = (2−5) × 1016 cm−3, k(Te + Ti) = 1−1.5 keV.

End loss from a collision dominated theta pinch plasma

The plasma parameters and end loss in a collision-dominated theta pinch plasma have been investigated photographically, interferometrically, with axially arrayed diamagnetic probes and with a local pressure sensitive diagnostic. The compressed plasma (nemax∼1017 cm−3, Te=Ti∼45 eV) is highly stable for a period of approximately 9 μsec; the end-loss process is examined over this time interval. The development of a plasma throat region has been identified at an axial position of approximately one coil diameter within the theta pinch coil ends. The existence of inward propagating rarefaction waves is indicated from the plasma temperature time history. The plasma end-loss rate derived from local pressure measurements indicates an initial period of rapid particle loss followed by a period of gradual decay in the loss rate. The experimentally observed end-loss time is in excellent agreement with the prediction of a recent numerical magnetohydrodynamic computation.

Measurements of He I Line Profiles with Forbidden Components Induced by Fluctuating Fields in a Small Theta Pinch Plasma

Spectral line profiles of He I 4922 A line (2 1 P-4 1 D, 1 F) and 4471 A line (2 3 P-4 3 D, 3 F) with their forbidden components from a small linear theta pinch plasma have been measured and the plasma satellites have been observed on the wing of the line profiles. The observed profiles have broadened wings, and they are interpreted as the Doppler broadening due to fast neutral particles produced by the charge transfer with hot ions in the plasma of the neutral particle density of ∼10 14 cm -3 comparable with the plasma density. The low and high frequency fluctuating fields with the average strength of several kV/cm are deduced from the plasma satellites in the observed profiles.

Material End Plugging of StraightθPinches

The interaction of a theta-pinch plasma with cold-material end plugs is investigated with a numerical hydrodynamic and heat flow model. A very appreciable improvement in energy confinement is obtained for the 5-m system considered compared to the same system plugs. The improvement is due to the elimination of convective energy loss, a reduction of thermal conductive loss when Z > 1 end plugs are used, and some recompression of the plasma by ablated plug material.

Numerical study of a CO_2 laser beam focused on a theta-pinch plasma in the axial direction

Using two different empirical density profiles for the end region of a theta-pinch plasma, one with a maximum density on the axis (radiation-dispersing profile) the other with a pronounced axial minimum (radiation- trapping profile), the trajectory of the CO(2) laser beam (10.6 microm) focused axially on such a plasma was studied numerically. This calculation is used to evaluate the optical influence of the plasma, since the maximum power density in the focal plane can be reduced by several orders of magnitude owing to the presence of the plasma. This influence can be substantial even for very subcritical electron densities (n(e) << 10(19) cm(-3)). In cases of large dispersion, the characteristics of a multifocal lens capable of producing perfect focusing are found, and it is shown that the solution is not unique. The radial distribution of the laser beam power density is also calculated and shows numerous irregularities and discontinuities due to the nonuniform beam dispersion.

Long wavelength m=1 magnetohydrodynamics of a theta pinch

The treatment of the magnetohydrodynamic problem of a sharp-boundary theta pinch plasma column in the presence of perturbing magnetic fields is extended to include long wavelength m=1 motion. The long wavelength m=1 instability, which is a serious problem for fusion devices based on the theta pinch, is explained in a simple way. Plasma equations of motion which are suitable for modeling of complicated experimental situations are given. The high-beta stellarator is discussed.

Measurement of the total collisional ionization rates of Ne VI, VII and VIII

The total collisional ionization rates of neon VI, VII and VIII have been measured in a time-evolving theta-pinch plasma. The method used was to calculate the expected emission from the plasma using the measured time- and space-dependent electron density (1016-1017 cm-3) and temperature (120-400 eV). The semi-empirical ionization rates of Kunze and Lotz were used in the calculation and were adjusted, by multiplying with a constant, so that the predicted photon emission matched the observed line emission. The multiplicative factors, with estimated errors, required for best fit are: for neon VI, 0.15+or-0.1; for neon VII, 0.4+or-0.2; and for neon VIII, 0.7+or-0.25.

逆磁界を閉じ込めたテータピンチプラズマの軸方向の振舞

逆磁界を閉じ込めたテータピンチプラズマの軸方向の振舞

Double-pulsed time differential holographic interferometry

A unique method for double-pulsed holographic interferometry has been developed and employed in a meter-long theta-pinch experiment. Q-switching of two orthogonally polarized ruby laser beams, using the same lasing medium but different laser cavities, allows pulse separations of 0.2-100 microsec. The double-pulse operation with short pulse separation avoids many temporal aberrations seen in single-pulse systems and allows time differential interferograms of transient phenomena such as seen in theta-pinch plasmas. Orthogonal polarization of the two beams permits background fringe construction without the need for a mechanically introduced planar phase shift.

Spectroscopic Observation of Fluctuating Fields in a Theta Pinch Plasma with No Bias Field

Spectroscopic Observation of Fluctuating Fields in a Theta Pinch Plasma with No Bias Field

Motion of the Double Theta-Pinch Plasma

Macroscopic radial motion of a long annular plasma produced by a double theta pinch is studied by experiments and some of the results are compared with calculations using a simple vacuum field model. It is shown that the radial motion is roughly divided into two parts, slow motion of the equilibrium position r 0 and small radial oscillation r 1 . The equilibrium position r 0 is controllable by changing the ratio of the current in the ordinary theta-pinch coil to that in the inverse one. The resistance of the plasma has effect to damp both r 0 and r 1 motions, and the damping constant of r 0 is about twice that of r 1 . It is also shown that the resistive damping of r 0 can be suppressed selectively by the power crowbarring of the current in the inverse theta-pinch coil, while r 1 damps to vanish by the resistance of the plasma.

Theoretical modelling of the staged theta pinch

A theoretical model is used to predict the effect of full, partial, or no staging on the location and temperature of a theta-pinch plasma. Staging means a combination of separated implosion heating and adiabatic compression phases in the pinch cycle. The model involves numerical integration of fluid equations for electrons, particle orbit equations for ions,and self-consistent turbulent heating and transport for both species.

Neutron Emission from a Turbulently Heated High-Voltage Theta-Pinch Plasma

Spatial and ten poral origins of three distinct neutron emission peaks observed in the University of Maryland turbulently heated high-voltage theta pinch machine have been identified. They are the result of fusion reactions involving reflected hot deuterium ions, pinched plasma, and energetic ions colliding with deuterium absorbed on the vacuum vessel wall, respectively. Studies of the wall-interaction neutron peak further reveal that the particle end loss in the reversed-field case is substantially slower than in the parallel-field case.

On the relaxation of heavy ion energy anisotropy in a theta pinch plasma

Line profiles from impurity ions in a small theta pinch have been analyzed. Isotropization of the distribution function is fast compared to binary collision times and occurs preferentiallly perpendicular to the magnetic field in the beginning.