1 - Nuclear Sciences and Applications - IAEA

416 MAUELetal.
Small tokamak plasmas illustrating high beta instabilities with m ~ 4 have
also been made in the CDX device at Princeton Plasma Physics Laboratory. The
plasma parameters of CDX are R = 0.59 m, a = 0.04 m, Bt = 0.4 T, 5 < qa < 10,
~ 2 x 10 19 m- 3 , and EJ3p < 1 [3]. These plasmas are formed and
maintained solely by a low energy (~ 400 V), high current (~ 125 A) electron
beam which provides substantial heating power to the plasma [4],
2. High-/} and low-? instabilities in the HBT pulsed tokamak
The HBT diagnostic set includes several arrays of internal and external
magnetic probes, a CO2 interferometer, a 16-channel IR tomography array, a 10point
Thomson scattering system, a tunable monochromator for visible line
emission detection and several Rogowski coils to measure the plasma current
and various external coil currents. The poloidal beta, f3p, the cylindrical safety
factor (q* = 2Tta 2 Bt/fjQRIp), and the central safety factor (jqeff(P) =BtAr/RABo)
are measured magnetically throughout the shot [1]. The average beta is
determined from the equilibrium relationship and is presented here normalized to
the Troyon-Sykes critical beta (fic = 3.5 x 10~ 8 IlaBt) [5,6], The value of
determined from the equilibrium relationship for a circular cross-section plasma
is found to be in close agreement with Thomson scattering profile measurements
and free boundary equilibrium calculations described in Section 3.
Figure 1 illustrates the evolution of an ohmically sustained discharge (
~ 10 eV, ~ 1.5 x 10 14 cm~^) in which the plasma current is steadily
increased as the toroidal field decays (Figs, la and lb). The plasma is observed
to become unstable as q* approaches 2 and //Jc exceeds 1.0 (Figs, lc and
le). The instability is characterized by the rapid onset of toroidally nonaxisymmetric
fluctuations of the internal magnetic field and a gross inward
motion of the plasma column as measured by two multi-channel magnetic probes
(Figs, li and Id). The two internal probes, toroidally separated by A