J. Doody

Structural analysis of high-field-side RF antennas during a disruption on the Advanced Divertor eXperiment (ADX)

The Advanced Divertor and RF tokamak eXperiment (ADX) is a compact, high-field device proposed by the MIT Plasma Science and Fusion Center and collaborators [1], which will address critical gaps in world fusion research on the pathway to fusion energy. In addition to developing and testing new divertor concepts at reactor level magnetic field strengths and power densities, ADX will test new antenna concepts for Lower Hybrid Current Drive (LHCD) and Ion Cyclotron Range of Frequency (ICRF) heating systems. In particular, ADX will be purpose-built to allow antennas to be positioned on the high magnetic field side of the torus, i.e., on the inner wall. With antennas placed at this location, plasma-wall interactions are greatly reduced and favorable RF wave physics projects to dramatic improvements in current drive efficiency and current profile control as well as very effective scenarios for RF heating and flow drive [2][3][4]. Initial designs for a high field side LHCD and ICRF antennas have been completed and are analyzed to determine the loads induced during a full-current plasma disruption. While locating antennas at the inner wall is beneficial from an RF standpoint, it exposes them to a higher toroidal field which, when combined with the eddy currents caused by a disrupting plasma, will lead to higher loads. Using COMSOL Multiphysics [5], a model of the ADX vessel and coils is created to predict the magnetic fields, eddy currents and loads acting on the antennas during a disruption. Structural models are then run to predict the stresses and to provide guidance for design improvement, such as determining where structural reinforcements may be necessary.

New alcator C-Mod rotated 10° 4-strap ICRF antenna

We have developed a design for a new rotated 4-strap ICRF antenna. The design is based on a modification of the existing C-Mod antennas with the antenna rotated 10° such that the entire structure is perpendicular to total magnetic field. This rotation is implemented in an attempt to reduce ICRF impurity production. The rotation results in an antenna with less surface area than the previous antennas and therefore a higher power density will be obtained for a given input power of 3 MW. The power density would reach 15MW/m2, near the world record power density obtained in Tore Supra. We will describe the RF of this new design. The RF analysis was accomplished using the CST computer code. The fields were studied to minimize E field breakdown in the feed system and to accomplish the symmetry required for reduced impurity production in the four straps.

High Power Water Load for Lower Hybrid Current Drive at 4.6 GHz on Alcator C-Mod

We have developed a high power water load capable of absorbing 250 kW for 5 S at 4.6 GHz. These loads are required for testing and calibrating the lower hybrid current drive systems. The design and analysis of the load was done with the aid of the computer codes CST for RF and Algor and Comsol for water fluid considerations. The load consists of a stainless steel jacket with a Teflon insert. The Teflon is used as a RF impedance matching device, a water seal and is designed to manage water flow over the Teflon-water interface. This design distributes the absorption uniformly at the interface. The flow requirements are 22 gallons/minute with less than 2 psi drop. A shrink fit application to eliminate conventional seals was utilized on the Teflon insert. Initial tests resulted in 300 kW being absorbed for 500 ms with less than -26 db of reflection.

Power systems analysis and design for ADX

The Advanced Divertor eXperiment (ADX) [1] is a compact, high field (6.5 T with possible upgrade to 8 T), high power density tokamak being proposed to test new advanced divertor concepts at reactor-level conditions. Development and testing of advanced Lower Hybrid Current Drive (LHCD) and Ion Cyclotron Range of Frequency (ICRF) concepts including high-field-side launch capability is also an important goal for ADX [2], [3]. Where possible the design of the ADX experiment will make extensive use of existing power systems at MIT that presently support Alcator C-Mod, which includes a 225 MVA alternator/flywheel (2 GJ stored energy) and 32 MVA (peak) substation. Analysis of existing alternator/flywheel, substation, and power system supply capabilities and their application to support ADX operation up to 6.5 T will be discussed. Power system supplies, magnet voltages and currents and operating requirements based on the current point design will be presented. Potential power system upgrades that would support ADX operation at 8 T and 2 MA plasma current will also be described.

RF, disruption and thermal analyses of EAST antennas

CAS IPP and MIT PSFC are collaborating on Experimental Advanced Superconducting Tokamak (EAST), the first tokamak with superconducting toroidal and poloidal magnets and a testbed for technologies proposed for the ITER project. Presented in this paper are RF, disruption and thermal analyses of EAST antennas. All were performed by COMSOL commercial software package Version 5. Analyzed are the I port 4 strap and B port 2 × 2 strap antennas, which are currently installed on EAST. RF analysis over the Ion Cyclotron Range of Frequencies (ICRF) gets insight into the coupling mechanism to optimize antenna plasma coupling. A lossy dielectric model was created which loads the antenna. The Scattering parameters (Sparameter) were extracted. Peak electric field parallel to the magnetic field of the straps, coaxes and other components were determined. Parametric analysis of the operation frequencies on the electric field are also performed. Disruption analysis addresses the impact of the magnetic field and plasma. Temporal currents of poloidal field and plasma as well as the spatial toroidal field were imported into the electromagnetic (EM) model. The structural analysis afterwards determined the stress due to antenna loads generated during the disruption. The loads resulted from the reaction of circulating eddy currents in the antennas with the toroidal and poloidal magnetic fields. Thermal analysis, a fluid - heat transfer - structural multiphysics analysis, performed for the strap and Faraday rod by applying heat loads from the plasma, ripple trapped particles and RF heating for steady state, are also presented. Finally, benefits of a future field-aligned 4 strap antenna were discussed.

Design of the C-Mod Advanced Outer Divertor

Operational requirements and research considerations make a high-temperature, toroidally continuous outer divertor an important aspect for future operation of the Alcator C-Mod tokamak. Leading edge melting of tiles, nonuniform heat loads, large electromagnetic forces, and localized impurity sources limit the performance of bulk plasmas. These issues can be addressed by the installation of a well-aligned, toroidally continuous outer divertor. In addition, future proposed long pulse operation of C-Mod will cause the temperature of the outer divertor to reach bulk temperatures as high as 500-600°C. This future operational requirement combined with the strong temperature dependence of plasma surface interactions (especially fuel retention), makes a controllable, high-temperature outer divertor desirable and necessary. The design and development of the C-Mod Advanced Outer Divertor (AOD) is discussed.

Structural Analysis of High-Field-Side RF Antennas During a Disruption on the Advanced Divertor eXperiment (ADX)

The Advanced Divertor and RF tokamak eXperiment (ADX) is a compact, high-field device proposed by the MIT Plasma Science and Fusion Center and collaborators, which will address critical gaps in world fusion research on the pathway to fusion energy. In addition to developing and testing new divertor concepts at reactor level magnetic field strengths and power densities, ADX will test new antenna concepts for lower hybrid current drive (LHCD) and ion cyclotron range of frequency (ICRF) heating systems. In particular, ADX will be purpose-built to allow antennas to be positioned on the high magnetic field side of the torus, i.e., on the inner wall. With antennas placed at this location, plasma-wall interactions are greatly reduced and favorable RF wave physics projects to dramatic improvements in current drive efficiency and current profile control as well as very effective scenarios for RF heating and flow drive. Initial designs for the high-field-side LHCD and ICRF antennas have been completed and are analyzed to determine the loads induced during a full-current plasma disruption. While locating antennas at the inner wall is beneficial from an RF standpoint, it exposes them to a higher toroidal field which, when combined with the eddy currents caused by a disrupting plasma, will lead to higher loads. Using COMSOL Multiphysics, a model of the ADX vessel and coils is created to predict the magnetic fields, eddy currents, and loads acting on the antennas during a disruption. Structural models are then run to predict the stresses and to provide guidance for design improvement, such as determining where structural reinforcements may be necessary.

RF, Disruption, and Thermal Stress Analyses of EAST I and B Port Antennas

Chinese Academy of Sciences Institute of Plasma Physics and Massachusetts Institute of Technology Plasma Science and Fusion Center have been collaborating on experimental advanced superconducting tokamak (EAST). Presented in this paper are RF, disruption, and thermal stress analyses of EAST antennas. Analyzed are I port four-strap and B port 2 × 2 strap antennas, which are currently installed on EAST. As for RF analysis, scattering parameters are checked to make sure that the antennas are loaded, and then electric field parallel to magnetic field are checked to find out if they are below the permissible level. As for disruption analysis, mechanical stresses for both straps with the support box and Faraday screen are obtained. As for thermal stress analysis, temperature and thermal stress for a typical strap and a Faraday tube were presented. All analyses were performed by COMSOL commercial finite-element analysis software package version 5 or 5.2.

Engineering upgrades to the accelerator-based in-situ materials surveillance diagnostic on Alcator C-Mod

This paper presents an overview of the engineering upgrades being made to optimize the AIMS diagnostic on the Alcator C-Mod tokamak, a novel, particle accelerator-based diagnostic that can nondestructively measure the evolution of material surface compositions inside magnetic fusion devices. Three major AIMS subsystems are presented: the RFQ deuteron accelerator; the particle detectors; and the Alcator C-Mod tokamak. The combined results of the upgrades will enable AIMS to routinely map critical plasma-material interaction quantities, such as net erosion/redeposition and fusion fuel retention, over large areas of PFC surfaces between plasma shots and after the run day.

Novel vacuum vessel & coil system design for the Advanced Divertor eXperiment (ADX)

The Advanced Divertor eXperiment (ADX) [1] is a compact, high field (> 6.5 tesla), high power density tokamak, proposed by the Plasma Science and Fusion Center (PSFC) and collaborators, designed specifically to develop and test advanced divertor configurations that can accommodate the extreme plasma heat exhaust densities anticipated in next-step plasma fusion devices. ADX will also develop and test advanced technologies for Lower Hybrid Current Drive (LHCD) and Ion Cyclotron Range of Frequency (ICRF) heating, including the ability to deploy RF launch structures on the high-field-side for the first time. This potential game-changing innovation is expected to provide efficient heating and high efficiency, off-axis current drive while minimizing impurity production via plasma-launcher interactions [2, 3]. This combination of advanced divertors and innovative RF systems places unique demands on ADXs vacuum vessel (VV), which must have an integrated design that can incorporate the required poloidal field coil set and embedded infrastructure for RF feeds to the high-field-side vacuum vessel wall. Much of the ADX poloidal field (PF) coil system, toroidal field (TF) magnet and structural design is based on the successes of the C-Mod tokamak program, with the capability to operate at up to 8 tesla on axis - a rigid vacuum vessel providing structural support for the PF coils, and a liquid nitrogen cooled, demountable TF magnet. However, five separate axisymmetric structural shells and one inner cylinder are bolted together to form the VV in a novel configuration for ADX. This unique design accommodates the poloidal coil configurations required to produce the proposed advanced divertor shapes while at the same time providing flexibility for implementing alternative coil configurations. This paper describes ADXs vacuum vessel, coil system design and in-vessel components.