DIII-D Quiescent Double Barrier Regime Experiments and Modeling
Discharges characteristic of the quiescent double barrier (QDB) regime [1] are attractive for development of advanced tokamak (AT) scenarios relevant to fusion reactors [2] and they offer near term advantages for exploring and developing control techniques. We continue to explore the QDB regime in DIII-D to improve understanding of formation and control of these discharges and to explore scaling to steady-state reactors. The formation of an internal transport barrier (ITB) provides a naturally peaked core pressure profile. This peaking in density in combination with the H-mode-like edge barrier and pedestal provide a path to high performance. We have achieved {beta}{sub N}H{sub 89P} {approx} 7 for several energy confinement times ({le} 25 {tau}{sub E}). We discuss here a combination of modeling and experiments using electron cyclotron heating (ECH) and current drive (ECCD) to demonstrate steady state, current-driven equilibria and control of the current distribution, safety factor q, and density profile. Experimental conditions leading to formation of the QDB discharge require establishing two distinct and separated barrier regions, a core region near {rho} {approx} 0.5 and an edge barrier outside {rho} > 0.95, {rho} is the square root of toroidal flux (radial coordinate). A region of higher transport due to a change in polarity of the E x B shearing rate [1] separates the core barrier from the H-mode edge. It is this separation in barriers that so far has required use of counter-NBI to establish QDB conditions. Balanced NBI should also allow this separation of barriers. The edge corresponds to the quiescent H-mode (QH) conditions [3]. In this quiescent edge region, the normally observed transient loss associated with edge-localized-mode (ELM) activity is replaced with a steady particle loss driven by a coherent oscillation residing outside the pedestal region. This edge harmonic oscillation (EHO) [2] typically exhibits 2 or 3 harmonics of a fundamental frequency near 10 kHz. We find this combination of a core ITB and the QH-mode edge to be extremely robust and to produce slowly varying, high performance discharge parameters, Fig. 1, for long durations {approx} 3 s. These conditions are generally limited by the duration of the NBI system and a slow evolution to lower q values as the Ohmic current moves inward on the resistive time scale for diffusion.
- Research Organization:
- General Atomics, San Diego, CA (United States)
- Sponsoring Organization:
- USDOE National Nuclear Security Administration (US)
- DOE Contract Number:
- AC03-99ER54463
- OSTI ID:
- 804354
- Resource Relation:
- Conference: 29th European Physical Society Conference on Plasma Physics and Controlled Fusion, Montreux (CH), 06/17/2002--06/21/2002; Other Information: This is a preprint of a paper presented at the 29th European Physical Society Conference on Plasma Physics and Controlled Fusion, June 17-21, 2002, in Montreux, Switzerland, and to be published in the Proceedings.; PBD: 1 Jul 2002
- Country of Publication:
- United States
- Language:
- English
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CURRENT DRIVE AND PRESSURE PROFILE MODIFICATION WITH ELECTRON CYCLOTRON POWER IN DIII-D QUIESCENT DOUBLE BARRIER EXPERIMENTS
The quiescent double barrier regime in the DIII-D tokamak