Abstract Existence of operational window in both edge safety factor and line averaged plasma density for suppression of ELMs using n=4 Resonant Magnetic Perturbations in low input torque plasmas has been observed in EAST experiment, in which q95 and plasma normalized beta (βN) close to that required in ITER high-Q operation. Here, n is toroidal mode number of the magnetic perturbation. In contrast to previous reports from other tokamaks, there is not only an upper density limit but also a lower one for accessing ELM suppression. Modelling results using the MARS-F code show that the RMP with linear plasma response has a peak at an intermediate density and decays as the density increases or decreases, which results in a minimal RMP field penetration threshold at the intermediate density. In this experiment, the observed lower density limit operationally manifests a sensitivity of the q-profile: different densities alter the edge current profile, which change the alignment of the eigenmode structure with the RMP coil configuration, causing a reduction of the resonant field in both low- and high-density cases, and hence making field penetration more difficult. The modelled window of the strongest resonant plasma response in terms of [⟨ne⟩, q95] agrees well with the observed ELM suppression in EAST. Peeling-ballooning modes stability analysis using the ELITE code shows that plasmas gradually approach peeling instability boundary caused by increase of edge bootstrap current as the plasma density decreases, which is consistent with the observation that ELMs come back again in lower density plasmas for fixed q95. These results indicate that linear modelling with full toroidal geometry can well predict the optimized RMP configuration for ELM suppression and reveal the important roles of pedestal plasma current, which need to be carefully considered in the application of high n RMPs for ELM suppression in future ITER.
Abstract Locked modes are known as one of the major causes of disruptions, and investigating their impact on runaway electron (RE) generation is essential for the development and implementation of disruption mitigation systems. The effect of multiple mode islands (including the 2/1 mode and the 3/1 mode islands) on the RE suppression is simulated in the J-TEXT tokamak. The O -points of the 2/1 and 3/1 mode islands are initially phase-locked in the poloidal direction. NIMROD and DREAM simulations show that the RE suppression is significantly dependent on the relative phase between the O -points of multi-mode islands and the Massive Gas Injection (MGI) port. When the relative phase is 0 ∘ or 180 ∘ , the loss of RE seeds reaches its maximum in the NIMROD simulation. In contrast, the efficiency of RE mitigation is lower when the relative phase is 90 ∘ or 270 ∘ . This distinct behavior can be attributed to differences in the degree of magnetic-field stochasticity, which arise from impurity penetration and the evolution of magnetic perturbations. The transport coefficients calculated from the stochastic magnetic fields are incorporated into DREAM to evaluate the runaway current evolution during the current quench phase. In the cases with relative phases of 0 ∘ and 180 ∘ , the transport-induced losses are sufficiently strong to overcome the avalanche generation, resulting in a negative net RE generation rate and effective suppression of RE current. These findings demonstrate that the topology of multi-mode islands prior to disruption can significantly modulate the RE loss during disruption. For the locked-mode disruptions, effective runaway current mitigation may be achieved through the optimization of the relative phases between MGI and locked-mode islands.
Abstract Large plasma sources are employed in negative ion based heating neutral beam injectors for fusion applications. A review of the ITER beam source plasma properties is presented, in comparison with negative ion sources of comparable size, based on the latest experimental measurements from SPIDER and supporting numerical simulations. Spatial investigation of plasma parameters using several diagnostics, including the beam itself, retarding field energy analyzers, optical emission spectroscopy, and movable and fixed Langmuir probes, is key to understanding the source physics, to support the operation and improve the source performance. The influence of multiple RF-driver configuration on uniformity and the improvement flexibility it provides, as well as the implications of control parameters like filter field and plasma grid bias, are examined. Similarities and differences with other giant negative ion sources are discussed to highlight the key physical processes leading to optimal operation and viable paths for possible improvements to the ITER source design.
Abstract We measured single event effects (SEEs) in 40 nm and 65 nm Static Random Access Memories (SRAMs) exposed to the D–T fusion neutron environment of the JET tokamak during the DTE-3 campaign (September–October 2023), while also characterizing the neutron spectrum seen by the devices. Using consolidated JET neutron production data, the measured SEE rates show good agreement with predictions based on the measured spectra and independently determined SRAM sensitivity parameters from thermal and monoenergetic neutron tests. These results extend the validation of our spectrum-based SEE prediction methodology, previously demonstrated in a D–D fusion neutron environment in 2021, to a D–T fusion neutron field. The novelty of this work is therefore the first experimental validation of this complete prediction approach in a D–T fusion environment, using real-time neutron spectrometry and independent device sensitivity data, rather than a new SEE model. The experiments also show that a local B 4 C shield reduces the bit-flip rate by absorbing thermal neutrons, and we discuss the conditions under which such shielding can improve electronics reliability in tokamak and accelerator neutron environments.
Abstract Reliable disruption prediction across tokamaks is needed for next generation devices such as ITER, SPARC, and BEST, where disruptive target-machine data will be scarce by design. For a transferred predictor, target performance is limited by both the source-domain error and the mismatch between the source and target feature distributions. We use this distinction to revisit our previous J-TEXT to EAST study and to add three components for few-shot and zero-shot operation. An upgraded physics-guided feature extraction (PGFE-U) reduces geometry-dependent differences in Mirnov, soft x-ray and absolute extreme ultraviolet array features. A floating labelling strategy replaces a fixed pre-disruption window with shot-dependent precursor onset labels. An estimation of the feature distribution (EFD) supplies target-machine z-score statistics from outside the classifier training set. With the supervised CORAL backbone, the area under the receiver operating characteristic curve (AUC) on the same EAST test set reaches 0.957 in a few-shot setting using only 10 disruptive EAST discharges, whereas the previous work required 110 EAST discharges to reach 0.890, and the partial AUC over false positive rates up to 10% (pAUC) rises from 0.428 to 0.858. The zero-shot AUC reaches 0.892 with no EAST discharge used for classifier training, against 0.592 for the same model normalised with J-TEXT statistics, and the zero-shot pAUC is 0.619 against 0.084. The EFD statistics are estimated from real EAST shots; a per-feature Monte-Carlo scan shows that the sensitivity is concentrated in a few global discharge parameters, the quantities most reliably estimated at the design stage. Shapley additive explanation based attribution shows that both transferred models preserve most of the qualitative feature-to-disruption trends of the abundant-data baseline. These results indicate that improving feature quality, labels and normalisation statistics can recover a large fraction of abundant-data performance when target-machine shots are limited.
