Physics of runaway electrons with shattered pellet injection at JET

Runaway electrons created during tokamak disruptions pose a threat to a reliable operation of future larger machines. Experiments using Shattered Pellet Injection (SPI) have been carried out at the JET tokamak to investigate ways to prevent their generation or suppress them if avoidance is not sufficient. Avoidance is possible if the SPI contains a sufficiently low fraction of high-Z material, or if it is fired early in advance of a disruption prone to runaway generation. These results are consistent with previous similar findings obtained with Massive Gas Injection. Suppression of an already accelerated beam is not efficient using High-Z material, but deuterium leads to harmless terminations without heat loads. This effect is the combination of a large MHD instability scattering runaway electrons on a large area and the absence of runaway regeneration during the subsequent current collapse thanks to the flushing of high-Z impurities from the runaway companion plasma. This effect also works in situations where the runaway beam moves upwards and undergoes scraping-off on the wall.

Estimation of CFETR performance and burning fraction in different pellet fueling scenarios by a multi-species radial transport model

Tritium self-sufficiency in future DT fusion reactor is a crucial challenge. As an engineering test reactor, CFETR requires a burning fraction of 3% for the goal to test the accessibility to the future fusion plant. To self-consistently simulate burning plasmas with profile changes in pellet injection scenarios and to estimate the corresponding burning fraction, a one-dimensional (1-D) multi-species radial transport model is developed in BOUT++ frame. Several pellet-fueling scenarios are then tested in the model. Results show that the increased fueling depth improves the burning fraction by particle confinement improvement and fusion power increase. Nevertheless, by increasing the depth, the pellet cooling-down may significantly lower the temperature in the core region. Taking the density perturbation into consideration, the reasonable parameters of the fueling scenario in these simulations are estimated as the pellet radius r_p=3 mm, the injection rate = 4 Hz , the pellet injection velocity =1000–2000 m/s without drift or 450 m/s with high filed side (HFS) drift.

10-Hz beads pellet injection and laser engagement

Laser Inertial Fusion Energy reactor requires repetitive fuel pellet injection and laser engagement to fuse fusion fuel beyond a few Hz. We demonstrate 10 Hz free-fall bead pellets injection and laser engagement with γ-ray generation. Diameter of 1 mm deuterated polystyrene beads were engaged by counter illuminating ultra-intense laser pulses with intensity of 5 x1017 W/cm2 at 10 Hz. The spatial distribution of free-fall beads was 0.86 mm in horizontal, and 0.18 mm in vertical. The system operated beyond 5 minute, 3500 beads supply with achieved frequencies of 2.1 Hz for illumination on bead and 0.7 Hz for γ-ray generation, these frequencies increments three times in relation to the previous 1 Hz injection system. The operation duration was limited by pellet supply. This injection and engagement system can apply for Laser Inertial Fusion Energy research platform.

Shattered Pellet Injection experiments at JET in support of the ITER Disruption Mitigation System design

A series of experiments have been executed at JET to assess the efficacy of the newly installed Shattered Pellet Injection (SPI) system in mitigating the effects of disruptions. Issues, important for the ITER disruption mitigation system, such as thermal load mitigation, avoidance of runaway electron formation, radiation asymmetries during thermal quench mitigation, electromagnetic load control and runaway electron energy dissipation have been addressed over a large parameter range. The efficiency of the mitigation has been examined for the various SPI injection strategies. The paper summarises the results from these JET SPI experiments and discusses their implications for the ITER disruption mitigation scheme.

Pellet triggering of edge localized modes in low collisionality pedestals at DIII-D

Edge localized modes (ELMs) are triggered using deuterium pellets injected into plasmas with ITER-relevant low collisionality pedestals, and the resulting peak ELM energy fluence is reduced by approximately 25-50% relative to natural ELMs destabilized at similar pedestal pressures. Cryogenically frozen deuterium pellets are injected from the low-field side of the DIII-D tokamak at frequencies lower than the natural ELM frequency, and heat flux is measured by infrared cameras. Ideal MHD pedestal stability calculations show that without pellet injection, these low collisionality pedestals were limited by their current density (peeling-limited) rather than their pressure gradient (ballooning-limited). ELM triggering success correlates strongly with pellet mass, consistent with the theory that a large pressure perturbation is required to trigger an ELM in low collisionality discharges that are far from the ballooning stability boundary. For sufficiently large pellets, both instantaneous and time-integrated ELM energy deposition measured by infrared cameras is reduced with respect to naturally occurring ELMs at the inner strike point, which is the position where it is largest for natural ELMs. Energy fluence at the outer strike point is less effected. Cameras observing both heat flux and D-alpha emission often find significant toroidally asymmetric striations in the outboard far scrape-off layer resulting from ELMs that are triggered by pellets. Toroidal asymmetries at the inner strike point are similar between natural and pellet-triggered ELMs, suggesting that the reduction in peak heat flux and total fluence at that location is robust for the conditions reported here.

Post-thermal-quench shattered pellet injection for runaway electron seed depletion in ITER

The possibility of using shattered pellet injection after the thermal quench of an ITER disruption in order to deplete Runaway Electron (RE) seeds before they can substantially avalanche is studied. Analytical and numerical estimates of the required injection rate for shards to be able to penetrate into the forming RE beam and stop REs are given. How much material could be assimilated before the Current Quench (CQ) becomes too short is also estimated. It appears that, if hydrogen pellets were used, the required number of pellets to be injected during the CQ would be prohibitive, at least considering the present design of the ITER Disruption Mitigation System (DMS). For neon or argon, the required number of pellets, although large, might be within reach of the ITER DMS, but the assimilated fraction would have to be very small in order not to shorten the CQ excessively. This study suggests that other injection schemes, based for example on small tungsten pellets coated with a low Z material, may be worth exploring as an option for an upgrade of the ITER DMS.

Model-based electron density profile estimation and control, applied to ITER

In contemporary magnetic confinement devices, the density distribution is sensed with interferometers and actuated with feedback controlled gas injection and open-loop pellet injection. This is at variance with the density control for ITER and DEMO, that will depend mainly on pellet injection as an actuator in feed-back control. This paper presents recent developments in state estimation and control of the electron density profile for ITER using relevant sensors and actuators. As a first step, Thomson scattering is included in an existing dynamic state observer. Second, model predictive control is developed as a strategy to regulate the density profile while avoiding limits associated with the total density (Greenwald limit) or gradients in the density distribution (e.g. neo-classical impurity transport). Simulations show that high quality density profile estimation can be achieved with Thomson Scattering and that the controller is capable of regulating the distribution as desired.