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The conjecture that magnetic helicity (linked flux) is conserved in magnetized plasmas for time scales that are short compared to the resistive diffusion time is experimentally tested in the CTX spheromak [Phys. Rev. Lett. 45, 1264 (1980); 51, 39 (1983); Nucl. Fusion 24, 267 (1984)]. Helicity is created electrostatically by current drawn from electrodes. The magnetized plasma then flows into a conducting flux conserver where the energy per helicity of the plasma is minimized and a spheromak is formed on a relaxation time scale of many Alfvén times. The magnetic field strength of the equilibrium is subsequently increased and sustained. The amount of helicity created by the magnetized coaxial plasma source, the helicity content of the spheromak equilibrium, and the resistive loss of the helicity are measured to determine the balance of helicity between source and spheromak with a ±16% uncertainty. In CTX the amount of energy that must be rapidly dissipated within the conducting boundary while conserving helicity in the process of sustaining the spheromak is experimentally controllable, and has varied from 1.8 times the spheromak magnetic energy to greater than 10 times. The relaxation, or minimization of the energy-to-helicity ratio, determines the gross structure (the normalized spatial profile) of the spheromak, while the conservation of helicity itself determines the magnitude and time dependence of the magnetic fields of the spheromak equilibrium. Helicity balance tests are done by individually varying the sign and magnitude of the source voltage and flux, and by observing sustainment of spheromaks with fields opposing those of the source. A threshold for helicity injection from the source is measured and related to the source and entrance region size. During times short compared to resistive diffusion time scales the helicity is shown to be conserved with a ±12% uncertainty using no free parameters. For longer times the resistive dissipation Its value is independently measured and appears to be related to the expected classical resistive decay. Absolutely calibrated bolometer measurements are consistent with excess source energy heating the spheromak plasma during the sustainment by electrostatic helicity injection.

The Helicity Injected Torus [T. R. Jarboe, Fusion Technol. 15, 7 (1989)] is a low aspect ratio tokamak that is formed and sustained by coaxial helicity injection with no transformer. Toroidal plasma currents of over 200 kA have been achieved with electron temperatures in the 100 eV range and electron density between 1019 and 1020 m−3. The major radius is 0.3 m and the minor radius is 0.2 m. New results from equilibrium and stability analysis of the external magnetic diagnostics and new results from the Transient Internal Probe (TIP), an internal magnetic field diagnostic, are presented. A mechanism for the transfer of current drive on the open to the closed flux regions is presented.

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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.