W. E. Quinn

Effects of Ionization and Magnetic Initial Conditions on a Magnetically Compressed Plasma (Scylla)

The effects of strong preionization and the application at steady bias magnetic fields on the operation of the Magnetic compression device Scylla are studied. It is shown that both strong preionization and a bias field B/sub o/ antiparallel to the main compression field B/sub z/ are necessary to produce d-d neutrons during the first half-cycle of B/sub z/. Other aspects of the plasma activity are also shown to depend strongly upon the sign of B/sub o/. Application of bias fields with weak preionization leads to production of hard x rays, which occur on the half-cycle of the discharge preceeding that of neutron emission. When hard x rays are produced the plasma is not hydromagnetic. The hard x rays are extinguished when there is strong preionization, leading to a hydromagnetic plasma. In the case of the hydromagnetic plasma it is concluded that the antiparallel B/sub o/ is most effective early in a given half-cycle and affects the plasma primarily during its preheating, ionization phase, rather than during the later adiabatic-compression phase. An interpretation is given in terms of a plasma sheath that has special properties when it separates magnetic fields of opposite signs. (auth)

Plasma End Losses and Heating in the ``Low‐Pressure'' Regime of a Theta Pinch

A neutron‐producing plasma with ion energy ∼3–4 keV has been produced at filling densities 10–50 μHg without negative bias magnetic fields in a 570‐kJ theta pinch. Axial interferograms, taken with a ruby‐laser‐illuminated Mach—Zehnder interferometer show that a stable compressed plasma core exists throughout the magnetic half cycle with no ionized impurities outside the core, and no drift toward the wall. The interferograms give peak plasma densities of 2 to 5 × 1016 cm‐3, and also indicate a loss of particles as a function of time. Plasma containment times (e‐folding times of N) before peak compression are 6 to 30 μsec. The observed loss rates are approximately in agreement with predictions of free flow through an orifice whose radius is equal to an ion Larmor radius. Soft x‐ray measurements yield ∼300 eV electron temperature for all filling pressures. Absolute intensities of the soft x‐ray emissions show the impurity level to be <0.1%. The ion energy for the low‐pressure regime deduced from pressure bal...

Helical Field Experiments on a Three‐Meter Theta Pinch

Small helical fields have been added to the main compression field of a 3‐m long theta pinch, both by grooving the main compression coil and by adding helical windings driven by capacitor banks. The equilibrium and stability of the theta‐pinch plasma column and the interference force produced by a combination of helical fields have been studied and are found to be in agreement with theory.

Plasma Experiments with a Three‐Meter θ Pinch

Measurements are reported on the plasma produced in an 830‐kJ theta pinch (scylla IV) with a 3‐m compression coil in which the compression field is extended to 120 μsec by means of “crowbar” switches. Double‐exposure holographic interferometry, side‐on streak photographs, and neutron‐emission measurements establish the following comparison with the previous version of the experiment: (1) The plasma confinement time and neutron emission have been extended from 3 to 10 μsec. (2) The scaling of confinement time and plasma temperature from the 1‐m device is accounted for by simple end‐loss and shock‐compression considerations. (3) An m = 1 “wobble”of the plasma column is observed in the present experiment and is interpreted in terms of plasma rotation propagating from the ends at approximately the Alfven speed.

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.

Collisionless flow and end loss from a high‐energy theta‐pinch plasma

End‐loss experiments on the high‐energy (Te+Ti=3.3 keV, ne=1.5×1016 cm−3) 5 m Scylla IV‐P theta pinch are reported. The evolution of the theta‐pinch plasma parameters in the presence of axial losses and the behavior of the exhausting plasma near the ends of the device have been investigated. The measured decay of the theta‐pinch plasma electron temperature agrees with code predictions based on classical axial thermal conduction losses. However, the axial ion heat flux is found to be unmeasurably small in the collisionless ion plasma. Energy‐line‐density measurements at the coil midplane also agree with code predictions and provide evidence of inward traveling rarefaction‐like waves. At the theta‐pinch ends, the exhausting plasma is comprised of a collimated plasma core which remains radially confined for tens of centimeters, strongly convects magnetic fields, and contains the bulk of the ejected plasma. This collimated core is surrounded by a plasma annulus that expands rapidly to the walls after leaving ...

Faraday Rotation Measurements on the Scylla IV Theta Pinch

Faraday rotation measurements have been combined with Mach‐Zehnder interferometer measurements to yield the value of the internal magnetic field Bi in the Scylla IV plasma as a function of time and radius. Since the Faraday rotation is proportional to ∫ Bi(l)ne(l) dl and the Mach‐Zehnder interferometer provides ∫ ne(l) dl, the data give the average value of the internal magnetic field Bi(r) which exists over the plasma length l at the radius r. The measurements of Bi(r) are combined with those of the external magnetic field Be to determine the average plasma β as a function of radius, β = 1 − Bi2(r)/Be2. The Faraday rotation measurement employs a 1.5‐mm diam beam of visible radiation (6328 A) from a He‐Ne gas laser as a plasma magnetic‐field probe and a calcite Wollaston prism as the polarization analyzer with silicon diode detectors. In the low‐pressure plasma regime (10‐25 mTorr) the β values in the central plasma‐core (∼5‐mm diameter) range in value between 0.9 and 1.0 with initial reversed bias magnet...

Feedback stabilization of an l = 0, 1, 2 high-beta stellarator

Feedback stabilization of the Scyllac 120° toroidal sector is reported. The confinement time was increased by 10–20 μs using feedback to a maximum time of 35–45 μs, which is over 10 growth times of the long-wavelength m = 1 instability. These results were obtained after circuits providing flexible waveforms had been used to drive auxiliary equilibrium windings. The resultant improved equilibrium agrees well with recent theory. It was observed that normally stable short-wavelength m = 1 modes could be driven unstable by feedback. This, instability, caused by local feedback control, increases the feedback system energy consumption. An instability involving direct coupling of the feedback l = 2 field to the plasma l = 1 motion was also observed. The plasma parameters were: temperature, Te Ti 100 eV; density, ne 2 × 1016 cm−3; radius, a 1 cm; and β 0.7. Beta decreased significantly in 40 μs, which can be accounted for by classical resistivity and particle loss from the sector ends.

SCYLLAC FEEDBACK STABILIZATION EXPERIMENTS

For feedback stabilization experiments the magnetic field in a 120

Injected Magnetic Compression Experiment

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