Magnetic fluctuations have been reduced to approximately 1% during discharges on the Sustained Spheromak Physics Experiment by shaping the spatial distribution of the bias magnetic flux in the device. In the resulting quiescent regime, the safety factor profile is nearly flat in the plasma and the dominant ideal and resistive MHD modes are greatly reduced. During this period, the temperature profile is peaked at the magnetic axis and maps onto magnetic flux contours. Energy confinement time is improved over previous reports in a driven spheromak.

The motional Stark effect (MSE) diagnostic on DIII-D has been expanded to take advantage of a change in the neutral beam geometry, adding 24 new MSE channels viewing a beam injected counter to the plasma current. When data from these channels are used with those from two older MSE arrays viewing a different beam, the overall radial resolution improves near the magnetic axis at least a factor of 2, and the uncertainty in calculations of vertical magnetic field and radial electric field decreases in the edge at least a factor of 4. The new design uses two optical systems mounted on the same vacuum port with a common shutter and shielding.

Characterization of the plasma density and temperature at the last closed flux surface (the separatrix) of a tokamak requires accurate knowledge of the location of the separatrix. In this paper we discuss the effect of inaccuracy in the separatrix location on the measured parameters in DIII-D [Luxon et al., International Conference on Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1986), p. 159] An uncertainty in the separatrix position of ±0.5 cm, as expected in this device using magnetic reconstruction to determine the location of the separatrix, leads to unacceptably large uncertainty in the plasma parameters. Several techniques to improve the accuracy obtained from magnetic reconstruction are discussed. A new technique that is based on a characterization of the electron temperature profile is proposed. A comparison of the separatrix location defined in this manner with that obtained using magnetic reconstruction techniques suggests a systematic error in the reconstruction when the plasma is far from the walls and magnetic diagnostics. Determination of the perpendicular transport coefficients is given as an example of the improved statistics obtained using the new technique of defining the separatrix position.

In thermal-barrier experiments in the tandem mirror experiment upgrade, axial confinement times of 50 to 100 ms have been achieved. During enhanced confinement we measured the thermal-barrier potential profile using a neutral-particle-beam probe. The experimental data agree qualitatively and quantitatively with the theory of thermal-barrier formation in a tandem mirror.

Globally coherent magnetic fluctuations often observed during the driven phase after spheromak formation in the Sustained Spheromak Physics Experiment 1 (SSPX) can be reduced to small amplitude by programming the magnetic flux=ψ gun and the discharge current=I gun in the formation gun. Scanning the edge normalized current=λ edge =λ gun =µ 0 I gun /ψ gun above and below the minimum energy eigenvalue 2 =λ FC of the flux conserver provides a variation in the internal q=safety factor profile producing the expected q=m/n=poloidal/toroidal mode spectrum. By driving the edge with the proper λ gun , the system can be operated with the poloidal/toroidal mode spectrum between the m/n=1/2 and 2/3 modes producing low magnetic fluctuation amplitudes and high electron temperature=T e > 350 eV. Transport and confinement parameters calculated using Thomson scattering-measured T e and n e profiles coupled with the equilibrium code internal current profiles show a reduction in electron thermal diffusivity as T e increases. This scaling behavior is more classical-like than Bohm or open field line transport models 3 where thermal diffusivity increases with T e . Electron diffusivity is calculated to be less than 10 m 2 /s, approaching levels seen in tokamaks.