DIII-D Electron Cyclotron Heating Research Articles

DIII-D Electron Cyclotron Heating and Current Drive System Status and Plans

The DIII-D 110-GHz electron cyclotron heating and current drive (ECH/ECCD) system supports a wide range of experiments at the DIII-D National Fusion Facility and is pioneering innovative new techniques for future fusion reactors. The system currently includes eight sockets with four installed gyrotrons, 31.75-mm circular corrugated transmission lines, and four dual real-time steerable launchers plus two top launchers (TLs). After the successful testing of the first TL, this injects the EC waves downward nearly parallel to the resonance layer with double current drive efficiency and a long absorption path, and a second new TL was designed and installed with an upgraded design. Some of the operational limitations encountered for the 110-GHz X-mode second harmonic resonance scenario launched in overdense plasmas were overcome with the alternative use of O-mode second harmonic launch. Two new gyrotrons in a new series manufactured by Communications and Power Industries (CPI) were installed and tested, bringing up the total installed power to 3 MW. The installation of repaired and new gyrotrons will increase the number of installed gyrotrons to 7 in 2023. An addition to the DIII-D building will be completed in early 2022 to provide the space to expand the ECH/ECCD system to ten gyrotron systems plus an extra gyrotron test socket.

Status and Plans for the DIII-D ECH/ECCD System

The DIII-D electron cyclotron heating and current drive (ECH/ECCD) system has been operating with an annual average of 4268 individual gyrotron pulses into the plasma in the last four years and 3-3.6 MW average yearly power availability in the last ten years. This allowed an increase in the number and the range of experiments involving injection of 110 and 117.5 GHz EC power into the DIII-D tokamak plasma. Some of the operational limitations encountered for the 110 GHz X-mode second harmonic resonance scenario launched in overdense plasmas were bypassed with the alternative use of O-mode second harmonic launch and the addition of a new higher frequency gyrotron at 117.5 GHz. A new launcher that will inject the EC waves downward nearly parallel to the resonance layer with higher current drive efficiency and a long absorption path was built and is now installed. Some of the remaining gyrotrons have been successfully in service as long as 14 years and need to be replaced; major plans for commissioning and purchasing a new series of gyrotrons in addition to the repair and refurbishment of the older ones are under way. The number of gyrotrons will increase bringing the generated power up to 10 MW in the next few years.

DIII-D Electron Cyclotron Heating System Status and Upgrades

The DIII-D electron cyclotron heating (ECH) system consists of six 110-GHz gyrotrons with corrugated coaxial 31.75-mm waveguide transmission lines, and steerable launching mirrors. The system has been gradually updated, leading to an increased experimental flexibility and a high system reliability of 91% in the past year. Operationally, the gyrotrons can generate up to a total of 4.8 MW of RF power for pulses up to 5 seconds. The maximum ECH energy injected into the DIII-D is 16.6 MJ. The HE1,1 mode content is over 85% for all the lines, and the transmission coefficient is better than −1.1 dB for all the transmission lines, close to the theoretical value. A new depressed collector gyrotron was recently installed and was injected up to 640 kW of power into the plasma during 2014–2015 tokamak operations. Four dual waveguide launchers, which can steer the RF beams ±20° poloidally and toroidally, are used for real-time neoclassical tearing mode control and suppression. The launchers now have increased poloidal scanning speed and beam positioning accuracy of approximately ±2 mm at the plasma center. Two more gyrotrons are expected to be installed and operational in 2015–2016. The first is a repaired 110-GHz, 1-MW gyrotron that had a gun failure after more than 11 years of operation at DIII-D. The second is a newly designed depressed collector tube in the 1.5 MW class, operating at 117.5 GHz, manufactured by Communications and Power Industries.

DIII-D Electron Cyclotron Heating System Status and Upgrades

DIII-D Electron Cyclotron Heating System Status and Upgrades

Calorimetric Measurements of the Radio-Frequency Power and of the Transmission Line Losses on the DIII-D ECH System

The measurement of the power injected by the electron cyclotron heating (ECH) system in the DIII-D tokamak is a critical requirement for analysis of experiments, for tuning the gyrotrons for maximum power and efficiency, for tracking long-term operational trends, and for providing a warning of problems with the system. The ECH system at General Atomics consists of six 110-GHz, 1-MW-class gyrotrons. The radio-frequency (rf) power generated by each gyrotron is determined from calorimetry, using the relevant temperature and flow measurements from the cooling circuits of the cavity, matching optics unit, and dummy loads (DLs). The rf pulse length and time dependence are measured using an rf monitor at the first miter bend in the transmission line. The cavity power loading measured directly gives the generated rf power using a previously determined relationship between cavity loading and rf production. The direct measurement of the efficiencies of four of the transmission lines was performed using a high-power DL placed alternately in two positions of each DIII-D waveguide line, at accessible points close to the beginning and the end of each line. Total losses in the transmission lines range from 21.2 to 30.7%. Experimental results are compared to theoretical predictions of the performance of the components and waveguide lines.

Intrinsic rotation in DIII-D

In the absence of any auxiliary torque input, the DIII-D plasma consists of nonzero toroidal angular momentum, in other words, it rotates. This effect is commonly observed in tokamaks, being referred to as intrinsic rotation. Measurements of intrinsic rotation profiles have been made in DIII-D [J. Luxon, Nucl. Fusion 42, 614 (2002)] H-mode discharges, with both Ohmic heating (OH) and electron cyclotron heating (ECH) in which there is no auxiliary torque. Recently, the H-mode data set has been extended with the newly configured DIII-D simultaneous co- and counter-directed neutral beam injection (NBI) capability resulting in control of the local torque deposition, where co and counter refer to the direction relative to the toroidal plasma current. Understanding intrinsic rotation is important for projection toward burning plasma performance where any NBI torque will be relatively small. The toroidal velocity is recognizably important regarding issues of stability and confinement. In DIII-D ECH H-modes the rotation profile is hollow, co-directed at large minor radius and depressed, or actually counter-directed, nearer the magnetic axis. This profile varies with the ECH power deposition profile to some extent. In contrast, OH H-modes have a relatively flat co-directed rotation profile. There is a scaling of the DIII-D intrinsic toroidal velocity with W∕Ip, as seen in intrinsic rotation in Alcator C-Mod [J. Rice, Nucl. Fusion 39, 1175 (1999)], where W is the total plasma thermal energy and Ip is the magnitude of the toroidal plasma current. This common scaling resulted in a dimensionless similarity experiment between DIII-D and Alcator C-Mod on intrinsic rotation, obtaining a single spatial point match in the toroidal velocity normalized to the ion thermal velocity. The balanced NBI capability in DIII-D is a useful tool to push scaling studies to higher values of the plasma normalized energy, notwithstanding the details of torque deposition for co-NBI versus counter-NBI. There are theories which address intrinsic rotation, both extensions of neoclassical theory and related to turbulent transport. At this time, the comparisons with theory are qualitative.

Progress toward long-pulse high-performance Advanced Tokamak discharges on the DIII-D tokamak

Significant progress has been made in obtaining high-performance discharges for many energy confinement times in the DIII-D tokamak [J. L. Luxon et al., Plasma Physics and Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159]. Normalized performance (measured by the product of βNH89 and indicative of the proximity to both conventional β limits and energy confinement quality, respectively) ∼10 has been sustained for >5 τE with qmin>1.5. These edge localized modes (ELMing) H-mode discharges have β∼5%, which is limited by the onset of resistive wall modes slightly above the ideal no-wall n=1 limit, with approximately 75% of the current driven noninductively. The remaining Ohmic current is localized near the half-radius. The DIII-D electron cyclotron heating system is being upgraded to replace this inductively driven current with localized electron cyclotron current drive (ECCD). Density control, which is required for effective ECCD, has been successfully demonstrated in long-pulse high-performance ELMing H-mode discharges with βNH89∼7 for up to 6.3 s. In plasma shapes compatible with good density control in the present divertor configuration, the achieved βN is somewhat less than that in the high βNH89=10 discharges.