Θ-pinch Plasma Research Articles

The dynamic axial compression of FRC with high-speed translated θ-pinch plasma

A novel field reversed configuration (FRC) axial compression experiment with high-speed translated θ-pinch plasma was conducted in the Keda Mirror with AXisymmetry device. The translated north FRC was decelerated and compressed by an oncoming θ-pinch plasma stream, and the dynamic process was revealed by a 2D magnetic probe array. The FRC separatrix length is compressed to one-third of the initial value while the radius expands by ∼57%, resulting in the ∼16% increase in the electron temperature and ∼26% in the density, which matches the calculation from an adiabatic compression model. The good agreement is explained by the fast compression and particle supplementation owing to the compression with plasma. The results reported in this work may contribute to the understanding of electron heating in collision-merged FRC and provide a new compression method for the magneto-inertial fusion concept.

Estimates of the Dependence of the Fusion Neutron Yield on the Initial Plasma Density and Temperature in Fast Pinches

The neutron generation by hot, dense, and fast Z- and Θ-pinch plasma with the parameters achievable at the present development stage of high-power pulsed technology has been calculated. It has been proposed to use a hot plasma with a high density of 1017 cm−3 as an initial state for subsequent compression by Z- and Θ-pinches to achieve the conditions necessary for fusion reactions. Such plasma can be obtained using high-power pulsed plasma accelerators.

Finite-β Effect on the Dynamics of Double Gas-Puff Staged Pinch

The implosion dynamics of a dense θ-pinch plasma driven by a double gas-puff z-pinch is re-examined in the presence of kinetic pressure. A modified snow-plow model has been used to describe the dynamics of such a staged pinch plasma. The inclusion of kinetic pressure introduces the usual plasma β-term. Numerical results demonstrate that the thermonuclear fusion parameters can be achieved in a dense θ-pinch Deuterium-Tritium (D-T) fiber plasma for an optimum choice of density ratio of the test to driver gas at the interface position (α-parameter) of imploding z-pinch plasma and kinetic to magnetic pressure ratio (β-factor). Thus double gas-puff staged pinch device can be a more feasible approach to achieve fusion conditions with an enhanced stability.

Thermonuclear Fusion Conditions in an Impurity-Seeded Multicascade Liner Staged Z-Pinch

The implosion dynamics and fusion parameters of a high density θ-pinch plasma driven by a multicascade liner Z-pinch is investigated. The imploding multicascade liner Z-pinch plasma traps an axial magnetic field, compressing it to several megagauss in an extremely short time. The fast changing magnetic flux then induces an azimuthal θ-current on the fiber surface, with a rise time an order of magnitude smaller than the applied Z-pinch current. Numerical simulation shows that for optimum choice of shell thickness of the multicascade liner system, high-densities and temperatures can be realized for fibers seeded with some high-Z impurities. At peak compression the fuel densities of the order of 1026/cm3 and temperature about 10 keV can be achieved on a time scale of the order of 0.5 ns, yielding the Lawson criterion parameter nτ ≥ 1014 s/cm3 for DT-plasma.

Relative intensities of Delta n=0 transitions within the n=2 and n=3 levels in nitrogenlike Ar XII from a theta -pinch plasma.

In a well diagnosed transient {theta}-pinch deuterium plasma with a small concentration of argon added, the relative intensities of the {Delta}{ital n}=0 transitions within the {ital n}=2 and {ital n}=3 levels were measured at the maximum abundance of the Ar XI ion. The emission spectra of the intrashell transitions were observed in the wavelength range 149--225 A and 434--784 A, respectively. Theoretical predictions for electron collision strengths, oscillator strengths, and radiative decay rates were used from the literature to determine the relative intensities which were compared to the experimental results. Therefore, electron collisional excitation rate coefficients and radiative decay probabilities could be indirectly verified. General overall agreement is obtained with a collisional-radiative model which includes 72 levels, although some transitions deviate.

Excited-level populations in C IV in a recombining Theta -pinch plasma.

Absolute intensities of various transitions from n=3, 4, 5, 6, and 7 levels in C IV ions have been measured in the recombination phase of a Θ-pinch plasma. The discharge vessel was filled with 1.87 Pa (14 mTorr) of acetylene and 0.07 Pa (0.5 mTorr) of hydrogen gas to generate the plasma. The deduced upper-level populations showed inversions for all levels of n=4 and above. An experiment to produce laser gain at 253.0 nm showed only a factor of 6 enhancement in the line intensity, i.e., there was no lasing

Detection of trapped magnetic fields in a theta pinch using a relativistic electron beam

A novel nonperturbative diagnostic is used to infer the existence of trapped magnetic fields in a θ-pinch plasma. A thin beam of 1.5-MeV electrons from a 9-GHz, X-band linear accelerator is injected along the axis of a θ pinch with a period of 6 μs. The electron pulse duration is approximately 1 μs. Without any fill gas, when the θ pinch is fired, only those electrons within ±50 ns of the second Bext =0 are transmitted. The rest of the electrons are deflected by the solenoidal field, which acts as a strong lens. When the plasma is present even these transmitted electrons at Bext=0 are deflected because the plasma can trap some of the field lines. The magnitude and the scale length of these trapped fields can be estimated from the extent of the deflection.

Computer Method for New Elliptical Abel Inversion Applied to Holographic Plasma Diagnostics

A new and simple elliptical Abel inversion for calculating spatial distributions of electron density from elliptical shaped plasma is presented. In this numerical method inversion matrix elements were calculated analytically and examined using a test function. This method was applied to an end-on holographic interferogram of a θ-pinch plasma for the first time.

The Measurement of Branching Ratios of Spontaneous Radiative Transition Probabilities for Be-Like Ions N IV and O V

In an experiment to measure emission line intensities from the Belike ion Ne VII (Lang 1983), the intensity ratio I(2s3s3S−2s3p3P)/I(2p23P−2s3p3P) was found to be a factor six larger than predicted from the ratio of the theoretical spontaneous radiative transition probabilities (A values). Indeed, to reconcile the Ne VII experiment and theory in several respects it was suggested also that in Be-like C III the theoretical ratios I(2s3s1 S−2s3p1P)/I(2p21D−2s3p1P) was about a factor three too low.The same apparatus was used to measure A value ratios in Be-like N IV and 0 V. Briefly the 40 kJ θ-pinch plasma source was viewed radially using a m Ebert and a l m grazing incidence photoelectric spectrometer. The m Ebert was intensity calibrated from 7300 Å to 3600 Å using a tungsten filament lamp, and from 3800 Å to 2500 Å using a deuterium lamp, both lamps having been calibrated by NPL. As described fully by Lang (1983), the branching-ratios method was used to calibrate the front exit slit of the grazing incidence instrument in the wavelength range 88 Å to 500 Å. Only Li-like line pairs of those listed in that paper were used with the addition of the C IV doublets 2s2S−3p2P (312.4 Å) and 3s2S−3p2P (5801.33 Å, 5811.98 Å). The H-like ion He II 1–4 transition (243.0 Å) and 3–4 transition (4685.7 Å) were used to cross-check the calibration. As shown in the figure, the He II result agrees excellently with the calibration as defined from the Li-like lines.

Multiple-pass CO2 laser heating of a ϑ-pinch plasma

A 20-cm-long 3.6-cm-diam ϑ-pinch plasma is placed within a coupled optical cavity of a 40-J CO2 TEA laser as an active optical element. Ruby-laser scattering is used to measure the plasma temperature for the non-laser-heated, the single-pass, and the multiple-pass laser-heated plasmas. At times in the pinch cycle when holography shows a favorable density profile, multiple-pass heating equivalent to over five passes of the single-pass energy is observed.

THE MEASUREMENT OF ELECTRON TEMPERATURE OF PLASMA BY LASER SCATTERING AND THE ENERGY LOSS OF θ-PINCH PLASMA

In this paper, the measurement of electron temperature of the θ-pinch plasma by means of Thomson scattering of ruby laser is described. The experimental data of clectron temperature in θ-coil midplane are analysed using the one-fluid model. The experimental results show that the electron temperature is much less than the theoretical value in the ease of adiabatic compression, and the experimental data are in good agreement with the theory when the loss of the electron thermal conduction is considered. The conduction coefficient calculated according to the one-fluid model is larger than the Spitzer's classical value by a factor of 2-3. After about 3 μs, the electron temperature dropped sharply, the thermal conduction theory holds no longer. A possible explaination is that the end loss becomes dominant, and thus the plasma collapses rapidly.

Stimulated Brillouin backscatter detection from a ϑ-pinch plasma

Detection of prompt stimulated Brillouin backscatter is reported from an experiment in which a high-power pulsed CO2 laser irradiates an independently produced ϑ-pinch plasma. Analysis of the backscatter signal indicates that significant SBS gain is limited to a time period near peak incident power. The restriction on SBS gain is attributed to a varying temperature gradient.

10.6μm interferometry and Faraday rotation measurements on a θ-pinch plasma

The initial stages of a 40 kJ theta pinch in hydrogen at 13 mTorr have been investigated using a 10.6 mu m TEA CO2 laser. Electron density profiles measured across the tube using single-fringe interferometry show two imploding density fronts with a region of increased ionization at the tube axis when the first front approaches the axis. Electrons are swept up mainly by the outer front. 10.6 mu m Faraday rotation measurements of the magnetic field on the axis during the implosion stages are described. During the initial stages, the magnetic field is reversed with magnitude increasing to 0.4+or-0.01 Wb m-2 but changes direction at t=410 ns when the first density front approaches the axis.

The use of intense ion beams for heating long ϑ-pinch plasmas to thermonuclear temperatures

Motivated by recent advances in the production of intense ion beams, we consider the possible utility of such beams for heating a long thin ϑ-pinch plasma to thermonuclear temperatures. It is found that ion beams offer an alternative to previously proposed schemes utilizing CO2 laser beams and intense relativistic electron beams.

Plasma equilibrium and stability in the Scyllac toroidal sector experiments

Experiments are reported on helical plasma equilibrium and stability in the Scyllac toroidal θ-pinch sectors (120°) which have major radii of 2.375 and 4.0 m with coil arc lengths of 5.0 and 8.4 m, respectively. In these experiments the outward toroidal drift force was compensated by a combination of ℓ = 1 helical and ℓ = 0 bumpy fields which are generated by shaping the inner surface of the compression coil or by driven ℓ = 1 windings. Time-resolved measurements were made of the gross plasma-column motion, the plasma radius, the magnetic flux excluded by the plasma, the external magnetic field, the plasma density, the electron and ion temperatures, and the plasma β at axial locations of minimum and maximum plasma radius. These data are used to study the approach to the theoretically predicted toroidal equilibrium (including axial pressure equilibrium). The plasma column remained in stable equilibrium for 7 – 10 μs in the 8-m sector compared with 4 – 7 μs in the 5-m experiment, at which times the onset of a terminating m = 1, k ≈ 0 sideways motion occurred. The results show that the plasma achieved axial pressure equilibrium (nkT = const) in 4 – 6 μs, while maintaining equilibrium in the toroidal plane for 10 μs or longer. The measurements of the plasma radius, β and magnetic field in the various experiments have confirmed in detail the stable toroidal equilibrium observed in the streak photographs during the first 4-10 μs of the discharge. The observed toroidal equilibria of the high-β, θ-pinch plasma are in quantitative agreement with MHD sharp-boundary theory and confirm the theoretical scaling of the equilibrium field between the 5-m and the 8-m sector experiments.

Transverse heating of a cold dense helium plasma by a pulsed CO2 laser beam

We present experimental evidence for the transverse heating of a dense (3.2 × 1017 cm−3) cold (≃4 eV) helium θ-pinch plasma by the focused beam of a TEA CO2 laser having a pulse duration of 180 nsec and a peak power of approximately 5 MW. The laser light was incident on the plasma during the first pinch compression, when streak photographs taken with a high-speed camera indicate a stable quiescent plasma lasting about 1.5 μsec. Time-resolved spectroscopic measurements were made to determine the plasma density and temperature before and after irradiation by the CO2 laser. Maximum temperature increases of 0.8 eV and density increases of 0.5 × 1017 cm−3 were observed for a laser energy of 0.7 J incident on the plasma. The theoretical description of the interaction considers the quasistatic temporal response of the plasma to the absorption of laser energy by inverse bremsstrahlung, and includes deviations from local thermodynamic equilibrium (LTE) before, during, and after irradiation. The absorbed energy is consumed in expansion, heating, and further ionization of the plasma. Highly excited atomic or ionic states are assumed to be in Saha equilibrium with each other and with the ionization continuum, whereas the ground states of successive ionization stages are assumed to be in coronal equilibrium. Thermal conduction losses are negligible. The theory correctly predicts the observed temperature increases but not the observed density changes, since inertial effects are neglected in order to render the model tractable.

Trapping and absorption of an axially directed CO2 laser beam by a θ-pinch plasma

A deuterium-filled θ pinch was axially irradiated in the first half-cycle by a CO2 laser to demonstrate the effects of beam trapping and absorption by a plasma. Peak electron densities were 5×1016 cm−3 and the nonirradiated axial temperature was about 1.3 eV. Detection of the laser radiation transmitted through the plasma showed that beam trapping occurred when the density profile had a minimum on axis, and the maximum laser-induced temperature increase of 2 eV occurred for irradiation at the time of maximum density, in agreement with inverse bremsstrahlung predictions. Furthermore, the plasma emission histories indicated that the column underwent a laser-induced elongation of up to twice its original length.

Axial structure in a theta-pinch plasma

Axial irregularities in a θ-pinch plasma are observed during the initial breakdown and main discharge phases. The structure is associated with a gas-breakdown effect. Wavelengths and growth times are consistent with the development of instabilities associated with thermal waves.

Comparison of Langmuir Double Probe and Laser Scattering Measurements of Plasma Parameters

Langmuir double probe and Thomson scattering measurements of density and temperature were made on a plasma with a view to estimating the reliability of probe measurements. The plasma used was the afterglow of a small θ-pinch plasma for which density and temperature were of the order of 1015 cm−3 and 1 eV, respectively. There was close agreement between probe measurement and scattering measurement of the electron temperature while the density as measured by the probe was lower by a factor of about 4 as compared to the scattering measurement.

Heating of Θ-pinch plasmas by pulsed CO2 lasers

The problem of heating a uniform Θ-pinch plasma by a pulsed CO2 laser is considered. The effort is concentrated on determining the final temperature of the plasma while taking into account its radial expansion against the confining magnetic field. It is shown how the final temperature scales with the pinch-tube filling factor and the ratio of the laser energy to the initial internal energy of the plasma. The results indicate that for large filling factors it is convenient to heat the plasma quasistatically with the use of laser pulses longer than the time it takes for an acoustic wave to traverse the plasma radius. For small filling factors, rapid heating becomes more efficient than quasistatic heating when the laser energy is much larger than the initial internal energy of the plasma.