We report the achievement of a world unique capability of high power co/counter steerable off-axis neutral beam injection on a major tokamak, which widens the broad pressure and current profile parameter space for high beta steady-state advanced tokamak (AT) scenarios on DIII-D, while retaining the ability to balance the injected torque for low rotation studies. The unique steering capability of co/counter off-axis neutral beam (CCOANB) is being used to validate physics-based energetic particle and thermal transport models that are utilized in designing next-step facilities based on the steady-state AT approach. Prior to meaningful validation, however, a careful assessment of the transmitted power and energetic ion population produced by this novel heating and current drive system is critical. In this work, the off-axis beam injection is assessed through visible imaging (Fig. 1), neutron measurements and rotation profile measurements at balanced torque (Fig. 2), and used to broaden the pressure profile in steady-state advanced tokamak scenarios.
DIII-D has undergone a major upgrade and successfully injected high power off-axis neutral beam power (~4 MW) using the CCOANB in both co-current and counter-current directions. The total off-axis neutral beam power is approximately 7.3 MW at nominal operating voltage of 75 kV. Achieving an off-axis and co/counter steering capability necessitated significant modifications to the ion sources, internal neutral beam components and adaptor seal at the tokamak vessel connection. DIII-D is equipped with four neutral beam lines injecting through midplane ports at toroidal angles of 30, 150, 210 and 330 degrees. Each beamline houses two ion sources individually capable of ~ 2 MW injected power at ~ 80 kV, labeled left (LT) and right (RT) as viewed from behind facing the torus. For the CCOANB upgrade to the 210 degree beamline, each ion source has been modified to achieve a stronger vertical focus by modifying the accelerator grid modules to aim towards the beam centerline in a manner similar to Ref. 1, but the width of the ion source plates have not been reduced to retain high power. Tilting of the ion source plates is required for the beam to pass through the smaller effective aperture when the beam is off-axis. Minimal losses of neutral beam power have been achieved by optimizing the strong focus ion sources and optimization of the gradient grid voltage, enabling maximum power transmission by minimizing the amount of power “scraping off” on internal beamline components. Tilting of the ion source has been guided by fast visible imaging in Ref. 2, as shown in Fig. 1 (a,b), and resulted in neutral beam injection along the design centerline, as shown in Fig. 1 (c,d), with empirical characterization of each beam’s divergence derived from the beam vertical profiles, as shown in Fig. 1 (e,f). These measured beam characteristics are used in the parameterization of the beam in the NUBEAM Monte-Carlo heating and current drive package.
Through exclusive power injection of each source into MHD quiescent plasmas across a range of neutral beam voltage, perveance and plasma current in the same manner as Ref. 3, we conclude that a modest reduction of transmitted power (compared to on-axis, standard focus) has occurred. Prior to the beamline modification the 210LT and 210RT sources operating at 75, 81 kV and 2.60, 2.56 $μ$-perv respectively, each produced 2.0 MW. After commissioning, optimizing the neutral beam aiming, and implementing the off-axis injection geometry in NUBEAM, an approximate 15% lower than expected neutron yield has been observed. We attribute this reduced neutron yield to interaction with internal beamline components and reionization in the drift duct, as indicated by thermocouple measurements and photoemission detected by photodiodes in the drift duct.
Good ability to balance the neutral beam torque has been demonstrated by injecting the new (210 degree) off-axis counter injected beam against the existing (150 degree) off-axis co-injected beam in 2.0 T, 1.0 MA, MHD quiescent L-mode plasmas. L-mode at 1.0 MA has been chosen to minimize the edge intrinsic torque, retain good beam ion confinement with off-axis injection and minimize MHD. Verifying the ability to operate with balanced injection is critical for achieving “low torque” and “low rotation” operation for physics studies and in ITER demonstration discharges. This torque balancing exercise has been performed in a matrix using both tangential and perpendicular injection (2x2) and a three point scan in counter-injected voltage, as shown in Fig. 2. In these conditions, the intrinsic rotation profile has a positive edge offset with a slightly hollow core rotation, and therefore we do not expect to achieve a flat zero velocity profile at 0.0 Nm. Nevertheless, at zero injected torque the observed velocity shear is very weak and much smaller than a single co-injected off-axis neutral beam injecting +0.62 Nm of torque, confirming that low toroidal rotation shear can be achieved by balancing torque and controlling the beneficial effects of $E \times B$ shear on confinement. This submission supports DIII-D papers by C.S. Collins, J.M. Park and B.S. Victor.
This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences, using the DIII-D National Fusion Facility, a DOE Office of Science user facility, under Awards DE-FC02-04ER54698, DE-AC02-09CH11466, DE-AC05-00OR22725, DE-FG02-07ER54917, DE-SC0020337
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(2) M.A. Van Zeeland, et. al., Plasma Phys. Control. Fusion 52, 045006 (2010)
(3) W.W. Heidbrink, et. al., Nucl. Fusion 52, 094005 (2012)
|Country or International Organization||United States|
|Affiliation||Princeton Plasma Physics Laboratory|