Effect of electric fields and fluctuations on confinement in a bumpy torus

Abstract

In order to understand the relationships between confinement and space potential (electric field) and between confinement and density fluctuations, plasma parameters in the ELMO Bumpy Torus Scale (EBT-S)[in Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Tokyo, 1974), Vol. 2, p. 141; Plasma Phys. 25, 597 (1983)] have been measured systematically for a wide range of operating conditions. Present EBT plasma parameters do not show a strong dependence on the potential profile, but rather exhibit a correlation with the fluctuations. The plasma pressure profile is found to be consistent with the profile anticipated on the basis of the flute stability criterion for a marginally stable plasma. For a heating power of 100 kW, the stored energy density is found to be restricted to the range between 4.5×1013 eV-cm−3 and 7×1013 eV-cm−3. The lower limit remains constant regardless of heating power and pertains to plasmas lacking an equilibrium and/or stability. The upper limit increases with heating power and is found to result from the onset of instabilities. In between the two limits is a plasma that is in an equilibrium state and is marginally stable. Operational trajectories exist that take the EBT plasma from one limit to the other.

Summary of ELMO Bumpy Torus (EBT-S) experiments from 1982 to 1984

“…The data of Fig. 4(c) show that a similar trend also holds for the stored energy density [10], where the higher (lower) temperatures correspond to the lower (higher) ambient pressures. The central stored energy density was nearly the same for the ring killer and ring modifier experiments, in the C-mode, and at low magnetic field.…”

“…Such a transition was not observed. The product of central n e T e is only 50% larger in the T-mode than in the nonequilibrium C-mode [10], as will be discussed later.…”

“…It is important that the hot electron interchange mode be stable because the interchange mode prohibited lowering the pressure to obtain higher bulk plasma temperatures. [10,17] Since the hot electrons in EBT-S did not have sufficient energy density to modify the vacuum magnetic field so as to form an MHD-stable configuration, the plasma might have been expected to be flute unstable. Surprisingly [10,17], only broadband turbulence [n e fi/n e )2 (peak to peak) « 5% with frequencies up to 500 kHz] was observed during normal operation.…”

“…In contrast to this, central potential contours in the T-mode (lower ambient pressure) were nested, implying that particles were confined. However, the stored energy in the Tmode was only twice as large as that in the nonequilibrium C-mode, which suggests that plasma in the T-mode suffered from some form of anomalous transport [10] (see Section 2.1).…”

“…(3) The effect of plasma fluctuations on plasma performance has been studied [10]. Broadband, low frequency density fluctuations seem to be correlated with lower plasma density and lower stored energy, making it appear likely that transport was affected by turbulence.…”