Figure 1

Shown are (a) the NSTX machine components used for CHI start-up in NSTX and fast camera fish eye images showing (b) discharge evolution from near the injector region and (c) later during the discharge. The bright region at the top of image (b) is the top of the CHI plasma that has extended to approximately the middle of the vessel at a time during the discharge when $I_p$ is below the peak value. As $I_p$ increases to near the peak value, the discharge further elongates vertically to fill the vessel as shown in (c). The bright ring-shaped region at the top of this image is referred to as an absorber arc, a condition when part of $I_{\mathrm{inj}}$ bridges the upper divertor gap. Both gas and microwaves are injected in to a 100 L toroidal cavity beneath the lower divertor plates. The gas, which is ionized by the microwaves, emerges from the gap between the lower divertor plates, which eases the requirements for breakdown when the main capacitor bank discharge is initiated.Reuse & Permissions

Figure 2

Shown are (a) $I_p$ and $I_{\mathrm{inj}}$ traces, (b) fast camera images, and (c) the electron temperature and electron pressure profiles, at 13 and 15 ms from a CHI discharge in NSTX. The highest measured electron temperatures (not shown here) in transient CHI discharges during the closed flux phase was 30 eV. In this discharge about 2 kA of $I_{\mathrm{inj}}$ produces 120 kA of toroidal current resulting in a current multiplication of 60. The best attained current multiplication (not shown here) was 70. During these discharges, the NSTX CS was disconnected from its power supply. The small bright glow is light from a tungsten filament. The images at 11 and 13 ms show an elongated dark region, which is surrounded by a brighter region, like usual tokamak photos. The darker region is probably where most of the toroidal current flows, but because the neutral interaction is strongest in the edge region and because the image brightness is a function of neutral density and electron temperature, the brightness or darkness cannot be directly correlated to the electron current density in these visible camera images. Such images are used as control room diagnostics for discharge development. It is also useful to note that an absorber arc has not been produced in this discharge.Reuse & Permissions

Figure 3

Equilibrium reconstructions show the shape evolution of the CHI produced plasma in response to decaying current, which is further evidence of decoupling from the injector leaving behind an inductively decaying plasma. In this discharge the increase in $I_{\mathrm{inj}}$ around 7.5 ms is due to the presence of an absorber arc. Overdriving the injector by using a larger capacitor bank (45 mF, charged to 1.8 kV) caused a large amount of unused capacitor bank energy to be dissipated as an absorber arc, which probably contributed to an increase in the resistivity of these yet unoptimized discharges.Reuse & Permissions

Figure 4

(a) Shown in the top are fast camera fish eye images during the first 40 ms of a 400 ms discharge. (b) Also shown are $I_p$ and $I_{\mathrm{inj}}$. The small bright image that appears just below and to the left of the plasma ring (in time frame at 40 ms) is light from an electrical filament inside NSTX. The numbers at the bottom of the camera images are maximum scale values for the images.Reuse & Permissions