Spheromaks with lifetimes of 1 ms are produced in the CTX experiment. This paper describes the diagnostics and measurements on plasmas which, for CTX-produced plasmas, are the hottest and longest-lived discharges using a solid copper flux conserver. These spheromaks are formed using a static hydrogen background gas filling the entire vacuum system before the discharge. The density rapidly decays in 150–300 μs from an initial value of (1–3) × 1014 cm−3 to a steady-state plateau with a value of (1–4) × 1013cm−3,determine d by the pressure of the gas fill. A multi-point Thomson scattering system measures the radial profiles of electron temperature and density. Peak temperatures of over 40 eV are observed, and the average temperature increases in time by Ohmic heating from 15 eV to over 30 eV. Equilibrium models for the magnetic field structure are used to calculate values of peak local beta (8–13%), volume-averaged beta (3–8%), and engineering beta (10–25%). The operation with a filling gas results in a reduction of the impurity radiation power as measured by spectroscopy. Improved vacuum practices, discharge cleaning and the use of the static gas fill have resulted in discharges in which the radiation power loss is not dominating the energy balance late in time. Particle loss and the associated ionization and heating of the neutral particles required to maintain the density plateau appear to be the major energy loss processes in the spheromak.
The first spheromaks with Thomson-scattering-measured electron temperatures of over 100 eV are described. The spheromak is generated by a magnetized coaxial plasma source in a background gas of 30 mTorr of H2, and it is stably confined in an oblate 80 cm diam copper mesh flux conserver. The open mesh design allows rapid impurity transport out of the spheromak. The peak temperature, measured using multipoint Thomson scattering, is observed to rise from approximately 25 eV to over 100 eV in about 0.2 msec due to Ohmic heating from the decaying magnetic fields. Density (∼5×1013 cm−3) and magnetic fields (approximately 2 kG) are measured using interferometry and magnetic probes.
Experiments and theory at Los Alamos have contributed to advances and increased understanding of spheromak physics. Application of the relaxation principle and the concept of helicity injection has led to new, improved formation methods and to the ability to sustain spheromaks for long times against resistive decay. Use of oblate flux conservers has provided gross stability of the spheromak, even in the presence of bias magnetic fields. Magnetic diagnostics have seen oscillations caused by rotating non-resonant internal kink modes. The stability thresholds of these modes agree with the measured equilibrium of the spheromak, confirming that those equilibria depart significantly from the minimum-energy state. Reduction of impurities and use of background filling gas have created resistively decaying spheromaks with non-radiation-dominated confinement.
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