VH-Mode discharges in the DIII-D tokamak
- General Atomics, San Diego, CA (United States)
- California Univ., Los Angeles, CA (United States)
- Kernforschungsanlage Juelich GmbH (Germany)
Introduction. A regime of very high confinement (VH-mode) has been observed in divertor discharges in DIII-D. The VH-mode, first seen following the initial boronization of the DIII-D vessel in 1991, exhibits total energy confinement a factor of 2.5 to 3.5 greater than that predicted by the ITER89-P L-mode scaling relation. Also, confinement of thermal energy alone is greater than 1.6 times that of the JET/DIII-D H-mode scaling and in many cases has exceeded twice that amount. VH-mode is observed during a long ({le}0.8 sec) ELM-free phase of the discharges. At the beginning of the ELM-free period, the plasma appears to be in H-mode, with confinement near that predicted by the JET/DIII-D scaling. In the usual H-mode, confinement is observed to decrease or remain constant over time. In the present discharges, confinement has been observed to remain nearly constant for up to hundreds of milliseconds, after which the behavior sharply deviates from H-mode as the confinement begins to increase over time. This increase in confinement continues until the occurrence of a beta- related ({beta}>2.8I/aB) global MHD event, which rapidly decreases the plasma stored energy with a temperature reduction across the entire profile. Magnetic measurements indicate that at least in some cases, this event includes both an internal n = 1 mode and a more localized high-n mode near the edge. After this event, the plasma relaxes into an ELMing H-mode phase. As a consequence of the boronization, the plasmas in these discharges are unusually clean, with very low radiated power. In previous H-mode discharges, the radiated power increased during the ELM-free, sometimes reaching levels comparable with the input power if the ELM-free period was long enough. Also, Z{sub eff}is constant or decreasing over the length of the discharge, with a central value of {approx}1. It is noted that most of the energy in these discharges is thermal energy, with {le}10% contained in fast ions.
- Research Organization:
- General Atomics, San Diego, CA (United States)
- Sponsoring Organization:
- DOE; USDOE, Washington, DC (United States)
- DOE Contract Number:
- AC03-89ER51114
- OSTI ID:
- 7205692
- Report Number(s):
- GA-A-20917; CONF-920610--8; ON: DE92016257
- Country of Publication:
- United States
- Language:
- English
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Confinement and stability of VH-mode discharges in the DIII-D tokamak
Confinement and stability of VH-mode discharges in the DIII-D tokamak
Related Subjects
700310* -- Plasma Confinement-- (1992-)
BOOTSTRAP CURRENT
BORON
CLOSED PLASMA DEVICES
CONFINEMENT
CURRENTS
DIVERTORS
DOUBLET-3 DEVICE
EDGE LOCALIZED MODES
ELECTRIC CURRENTS
ELEMENTS
H-MODE PLASMA CONFINEMENT
INSTABILITY
PLASMA CONFINEMENT
PLASMA INSTABILITY
PLASMA MACROINSTABILITIES
SEMIMETALS
THERMONUCLEAR DEVICES
TOKAMAK DEVICES