Discovery of stationary operation of quiescent H-mode plasmas with net-zero neutral beam injection torque and high energy confinement on DIII-D

Abstract

Recent experiments in DIII-D [J. L. Luxon et al., in Plasma Physics and Controlled Nuclear Fusion Research 1996 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] have led to the discovery of a means of modifying edge turbulence to achieve stationary, high confinement operation without Edge Localized Mode (ELM) instabilities and with no net external torque input. Eliminating the ELM-induced heat bursts and controlling plasma stability at low rotation represent two of the great challenges for fusion energy. By exploiting edge turbulence in a novel manner, we achieved excellent tokamak performance, well above the H98y2 international tokamak energy confinement scaling (H98y2 = 1.25), thus meeting an additional confinement challenge that is usually difficult at low torque. The new regime is triggered in double null plasmas by ramping the injected torque to zero and then maintaining it there. This lowers E × B rotation shear in the plasma edge, allowing low-k, broadband, electromagnetic turbulence to increase. In the H-mode edge, a narrow transport barrier usually grows until MHD instability (a peeling ballooning mode) leads to the ELM heat burst. However, the increased turbulence reduces the pressure gradient, allowing the development of a broader and thus higher transport barrier. A 60% increase in pedestal pressure and 40% increase in energy confinement result. An increase in the E × B shearing rate inside of the edge pedestal is a key factor in the confinement increase. Strong double-null plasma shaping raises the threshold for the ELM instability, allowing the plasma to reach a transport-limited state near but below the explosive ELM stability boundary. The resulting plasmas have burning-plasma-relevant βN = 1.6–1.8 and run without the need for extra torque from 3D magnetic fields. To date, stationary conditions have been produced for 2 s or 12 energy confinement times, limited only by external hardware constraints. Stationary operation with improved pedestal conditions is highly significant for future burning plasma devices, since operation without ELMs at low rotation and good confinement is key for fusion energy production.

E × Bflow shear drive of the linear low-nmodes of EHO in the QH-mode regime

“…Very recently, a new QH-mode regime was observed in the DIII-D tokamak at low rotation [13], where a low-n broadband electromagnetic turbulence replaced the coherent EHOs in the pedestal region. Here, a reduced E × B flow shear in the edge pedestal was linked to this change of edge mode behaviors.…”

“…Very recently, a new kind of QH-mode regime with increased pedestal pressure and width at low rotation has been discovered serendipitously in the DIII-D tokamak [13]. The low rotation was achieved by slowly ramping down the external injected torque during the plasma discharge through balancing the co-I p and counter-I p oriented external torque injection from neutral beams.…”

“…In the fourth section, we propose a new model of kink/ peeling mode in the helical coordinate system [3], derive the linear eigenvalue equation, and calculate dispersion relation using the energy principle. In the fifth section, we conduct a linear eigenvalue analysis of a typical QH-mode discharge measured in DIII-D [13]. Finally, discussions and a summary are presented in the last section.…”

“…In this section, we conduct a linear eigenvalue analysis using DIII-D experimental data in a typical QH-mode discharge with toroidal rotation velocity ramping down to zero [13].…”

Control of power, torque, and instability drive using in-shot variable neutral beam energy in tokamaks

“…Figure 2 compares low torque beam waveforms for a modulation case and the variable energy case. The modulation case is from an experiment on QH-mode development at zero beam torque [18]. As seen in figure 2, however, this zero net-torque condition is reached with a power modulation of 2-6 MW (figure 2(a)) and torque modulation of −1 to +2 NM.…”