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Equilibrium, stability, and confinement properties of compact toroids produced in field-reversed theta-pinch experiments (FRX) are reported. Two experimental facilities, FRX-A and FRX-B, have been used to study highly elongated compact toroid plasmas confined in a purely poloidal field geometry. Spatial scans and fill pressure scaling of the equilibrium plasma parameters are presented. Plasma conditions range from Te∼150 eV, Ti∼800 eV, nm∼1×1015 cm−3 to Te∼100 eV, Ti∼150 eV, nm∼4×1015 cm−3. Typical confined plasma dimensions are: major radius R∼4 cm, minor radius a∼2 cm, and total length 35–50 cm. The plasma configuration remains in a stable equilibrium for up to 50 μsec followed by the destructive n = 2 rotational instability. The stable period prior to the onset of the rotational mode is up to one hundred times greater than characteristic Alfvén transit times of the plasma. This stable period increases and the mode growth rate decreases with increased a/ρi (where ρi is the ion gyroradius). Agreement of experimental and theoretical mode frequencies for the instability is observed. Preferential particle loss has been proposed as a likely cause of rotation. The particle inventory at the onset of the instability is consistent with this hypothesis. The particle loss rate is also consistent with the predicted anomalous transport near the separatrix. Contributions to rotational instability from diffusion, end-shorting, equipartition, and compression are also discussed.

In June 1992 a NASA sponsored sounding rocket was flown through the Arecibo heater beam to study the structure of the heated volume. The rocket carried an instrument payload and traversed the 5.1-MHz reflection height at 268.5 kin. Data from the plasma density probe are presented in this paper. The rocket passed through several regions of disturbed plasma both above and below the reflection level. In these regions, over 180 deep filamentary density depletions were detected. Measured perpendicular to the magnetic field, these depletions or filaments have a mean width at half maximum of 7 m which is roughly equal to twice the ion gyroradius (0 +) and a mean depletion depth of 6%. The ratio of parallel to perpendicular scale for these structures exceeds 20,000, and the spacing between the filaments is around 15 m. A power spectrum of the rocket data clearly shows the spectral content of the filaments and also reveals peaks at longer wavelengths which we interpret as the spacing between the bunches and between sets of filaments within a given bunch. We believe that previous scintillation and satellite measurements emphasized these longer wavelengths. The power spectrum mea. sured by the rocket instrumentation falls off as k -4 for wavenumber k larger than 0.4/m and remains above the system noise for structure down to 1 m. It is clear that VHF backscatter from these structures can be explained by our data, as can many features of heater-related, field-aligned irregularities found in the literature. 17,367 17,374 KELLEY ET AL.' DENSITY DEPLETIONS AT THE 10-M SCALE Perkins, F. W., and M. V. Goldman, Self-focusing of radio waves in an underdense ionosphere, J. Geophys. Res., 86, 600, 1981. Perkins, F. W., and E. J. Valeo, Thermal self-focusing of electromagnetic waves in plasma, Phys. Rev. Lett., 3oe, 1234, 1974. Robinson, T. R., The heating of the high latitude ionosphere by high power radio waves, Phys. Rep., 179(2), 79, 1989. Rose, G., B. Grandal, E. Neske, W. Ott, K. Spenner, J. Holtet, K. M•eide, and J. Troim, Experimental results from the HERO project: In situ measurements of ionospheric modifications using sounding rockets, J. Geophys. Res., 90, 2851, 1985. Szuszczewicz, E. P., and J. C. Holmes, Observations of electron temperature gradients in mid-latitude E, layers, J. ) M. Sulzer, Arecibo Observatory, Arecibo, PR 00613. (email: sulzer@naic.edu) (