Abstract
Advanced Tokamak research in DIII-D seeks to develop a scientific basis for steady-state high-performance tokamak operation. Fully noninductive ( f NI ≈ 100%) in-principle steady-state discharges have been maintained for several confinement times. These plasmas have weak negative central shear with q min ≈ 1.5–2, β N ≈ 3.5, and large, well-aligned bootstrap current. The loop voltage is near zero across the entire profile. The remaining current is provided by neutral beam current drive (NBCD) and electron cyclotron current drive (ECCD). Similar plasmas are stationary with f NI ≈ 90–95% and duration up to 2 s, limited only by hardware. In other experiments, β N ≈ 4 is maintained for 2 s with internal transport barriers, exceeding previously achieved performance under similar conditions. This is allowed by broadened profiles and active magnetohydrodynamic instability control. Modifications now underway on DIII-D are expected to allow extension of these results to higher performance and longer duration. A new pumped divertor will allow density control in high triangularity double-null divertor configurations, facilitating access to similar in-principle steady-state regimes with β N > 4. Additional current drive capabilities, both off-axis ECCD and on-axis fast wave current drive (FWCD), will increase the magnitude, duration, and flexibility of externally driven current.
Highlights
There is high confidence that the planned ITER baseline operational scenarios will be capable of achieving fusion gain Q ≈ 10 in a conventional H-mode regime with edge localized modes (ELMs)
Advanced Tokamak research in DIII-D seeks to establish a scientific basis for high-performance steady-state operation in a future tokamak reactor
We describe experimental studies of two scenarios that demonstrate many desirable features as we move toward this goal
Summary
There is high confidence that the planned ITER baseline operational scenarios will be capable of achieving fusion gain Q ≈ 10 in a conventional H-mode regime with edge localized modes (ELMs). In AT experiments in DIII-D, these sources are neutral beam (NBCD), electron cyclotron (ECCD) and fast wave (FWCD) current drive These experiments have demonstrated in-principle steady-state conditions with noninductive current fraction fNI ≈ 100%, bootstrap fraction fBS ≈ 60%, and βN ≈ 3.5 [7]. The neutral beam power is increased to raise βN to a target value, usually about 3.5, and off-axis ECCD is applied to maintain the current profile in a stationary condition During this high-performance phase, both the total and local inductive current approaches zero (Fig. 3). We have assembled a database of AT discharges with fNI = 80–100%, with durations up to 1τCR These experiments have achieved normalized fusion performance up to G = 0.3 and bootstrap fraction fBS = 60%, consistent with the requirements for the ITER Q = 5 steady-state scenario [7]
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