Abstract Through predictive modeling validated by a series of experiments on DIII-D, the vertical stability of low β diverted plasmas with strong negative triangularity (NT) ( δ∼−0.6 ) is assessed. As a result of their unique magnetic geometry, NT plasmas feature larger Shafranov shifts and more elongated inner flux surfaces than positive triangularity counterparts, typically leading to enhanced vertical instability growth rates. However, coupling with the non-conformal vessel DIII-D wall reduces these growth rates to controllable values, providing a path forward for stabilizing strongly NT plasma with a diverted geometry. These discharges are used to validate GSdesign (part of the TokSys code suite) stability calculations in strong NT plasmas on DIII-D, with errors of no more than ∼20% observed between modeled and experimentally measured growth rates. Additions of diagnostic noise and power supply tuning to the TokSys model are needed to accurately capture the time dependence of DIII-D NT discharges, assisting with the design of control schemes specific to the DIII-D poloidal field coils. Finally, implications of these results on a future NT reactor are briefly described.
Since the successful first plasma generation in the middle of 2008, three experimental campaigns were successfully made for the KSTAR device, accompanied with a necessary upgrade in the power supply, heating, wall-conditioning and diagnostic systems. KSTAR was operated with the toroidal magnetic field up to 3.6 T and the circular and shaped plasmas with current up to 700 kA and pulse length of 7 s, have been achieved with limited capacity of PF magnet power supplies. The mission of the KSTAR experimental program is to achieve steady-state operations with high performance plasmas relevant to ITER and future reactors. The first phase (2008–2012) of operation of KSTAR is dedicated to the development of operational capabilities for a super-conducting device with relatively short pulse. Development of start-up scenario for a super-conducting tokamak and the understanding of magnetic field errors on start-up are one of the important issues to be resolved. Some specific operation techniques for a super-conducting device are also developed and tested. The second harmonic pre-ionization with 84 and 110 GHz gyrotrons is an example. Various parameters have been scanned to optimize the pre-ionization. Another example is the ICRF wall conditioning (ICWC), which was routinely applied during the shot to shot interval. The plasma operation window has been extended in terms of plasma beta and stability boundary. The achievement of high confinement mode was made in the last campaign with the first neutral beam injector and good wall conditioning. Plasma control has been applied in shape and position control and now a preliminary kinetic control scheme is being applied including plasma current and density. Advanced control schemes will be developed and tested in future operations including active profiles, heating and current drives and control coil-driven magnetic perturbation.
In this paper we present results from recent experiments at DIII-D which measured the plasma stability and confinement performance product, {beta}{tau}{sub E}, in one previously studied and three new plasma shapes. One important goal of these experiments was to identify performance vs shape trends which would identify a shape compatible with both high performance and the planned effort to decrease the power flux to the divertor floor using a closed ``slot`` divertor geometry. power flux to the divertor floor using a closed ``slot`` divertor geometry. The closed divertor hardware must be designed for a reduced set of plasma shapes, so care must be taken to choose the shape that optimizes {beta}{tau}{sub E} and divertor performance. The four shapes studied form a matrix of moderate and high elongations ({kappa} {congruent} 1.8 and 2.1) and low and high triangularities ({delta} {congruent} 0.3 and 0.9). All configurations were double-null diverted (DND), held fixed during a shot, with neutral beam heating. The shapes span a range of X-point locations compatible with the envisioned closed divertor. We find that from shape to shape, a shot`s transient normalized performance, {beta}{sub N}H, where {beta}{sub N} {equivalent_to} {beta}/(I{sup p})/aB{sub T} and H {equivalent_to} {tau}{sub E}/{tau}{sub E}{sup ITER-89P}, increasesmore » strongly with triangularity, but depends only weakly on elongation. However, the normalized performance during quasi stationary ELMing H-mode, to which these discharges eventually relax, is insensitive to both triangularity and elongation. The moderate elongation, high triangularity DND shape is shown to be near optimum for future studies on DIII-D.« less
This paper presents a summary of DIII-D boundary and diverter experiments directed at solving the scientific and technical problems of high power, long-pulse tokamak diverters. Single-null poloidal diverters were studied in the standard DIII-D open diverter configuration and in interaction with the toroidally symmetric ring biasing electrode and exhaust gas collection plenum. New results are reported for: scrape-off layer (SOL) transport; SOL density and unpurity influx; SOL density and diverter bias; ELM particle and energy content; ELMing SOL transport; open radiative diverter; passive exhaust gas collection; diverter bias and exhaust gas collection; diverter bias and H-mode threshold. The paper concludes with future plans for diverter research at DIII-D.
