Feedback system for divertor impurity seeding based on real-time measurements of surface heat flux in the Alcator C-Mod tokamak

Abstract

Mitigation of the intense heat flux to the divertor is one of the outstanding problems in fusion energy. One technique that has shown promise is impurity seeding, i.e., the injection of low-Z gaseous impurities (typically N2 or Ne) to radiate and dissipate the power before it arrives to the divertor target plate. To this end, the Alcator C-Mod team has created a first-of-its-kind feedback system to control the injection of seed gas based on real-time surface heat flux measurements. Surface thermocouples provide real-time measurements of the surface temperature response to the plasma heat flux. The surface temperature measurements are inputted into an analog computer that "solves" the 1-D heat transport equation to deliver accurate, real-time signals of the surface heat flux. The surface heat flux signals are sent to the C-Mod digital plasma control system, which uses a proportional-integral-derivative (PID) algorithm to control the duty cycle demand to a pulse width modulated piezo valve, which in turn controls the injection of gas into the private flux region of the C-Mod divertor. This paper presents the design and implementation of this new feedback system as well as initial results using it to control divertor heat flux.

Conceptual design study for heat exhaust management in the ARC fusion pilot plant

“…In this situation, active control of divertor conditions is required to maintain the detached state, such as via a feedback control of seed impurities. Experiments performed to-date have used a variety of divertor diagnostics to perform this sensing and control function: Langmuir probes [71], surface thermocouples [72], vacuum ultraviolet spectroscopy [73], tile current shunts [74][75][76]. These experiments have been fairly successful in demonstrating a control system that can accommodate steady-state plasma conditions.…”

Surface heat flux feedback controlled impurity seeding experiments with Alcator C-Mod’s high-Zvertical target plate divertor: performance, limitations and implications for fusion power reactors

“…It requires the identification of a suitable set of sensors that, ideally, directly relate to the quantity that is being controlled Ð in this case, surface heat flux. With this goal in mind, the Alcator Team developed surface thermocouples into a reliable diagnostic for real-time measurements of surface temperature and heat flux [14,29]. The surface thermocouples are simply a coaxial, refractory metal (Mo/W-Re) thermocouple with the thermojunction directly exposed to the divertor plasma.…”

“…The surface heat flux is calculated from the surface thermocouple temperature measurements in two different ways: (1) A digital computation is performed after every plasma pulse and (2) an analog computation is performed in real-time during the plasma pulse using a very simple and accurate 7-node RC-network [14]. The analog computation makes use of the direct relations of voltage and current in electrical diffusion with temperature and heat flux in thermal diffusion; the resistor and capacitor values are scaled to match the thermal diffusivity of the material of interest (here molybdenum).…”

“…A number of techniques have been successfully employed. Sensors have ranged from bolometers [5,7,9] and vacuum ultraviolet spectroscopy [10] chords, to tile current shunts [9,11,12], Langmuir probes [13], and surface thermocouples [14]. All of these systems have had varying degrees of success controlling impurity injection under relatively steady conditions and some have done so while demonstrating maintenance of good core confinement.…”

Evolution of the high-field-side radiation belts during the neon seeding plasma discharge in EAST tokamak

“…At present, many feedback control experiments of divertor detachment have been carried out on advanced tokamaks abroad, such as ASDEX-Upgrade, [7][8][9] JET, [10,11] and C-MOD. [12] Many signals, such as thermal current, electron temperature, ion saturation current at divertor target plates, plasma density, radiation power, and impurity line emissions, were used as reference signals to feedback control the valve actuator of pure deuterium (D 2 ) or impurity gas seeding, for realizing the stable control of divertor detachment. In the Experimen-tal Advanced Superconducting Tokamak (EAST), the stable plasma detachment by active feedback control has also been carried out by high density or impurity seeding, such as neon (Ne) or argon (Ar) mixed with D 2 .…”