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As the plasma beta (β) increases in high-performance tokamaks, electromagnetic turbulence becomes more significant, potentially constraining their operational range. To investigate this turbulence, a cross-polarization scattering (CPS) diagnostic system is being developed on the HL-3 tokamak for simultaneous measurements of density and magnetic fluctuations. In this work, a quasi-optical system has been designed and analyzed for the Q-band CPS diagnostic. The system includes a lens group for beam waist size optimization, a rotatable wire-grid polarizer for polarization adjustment, and a reflector group for measurement range regulation and system response enhancement. Laboratory tests demonstrated a beam radius of order 4 cm at the target measurement location (near the plasma pedestal), cross-polarization isolation exceeding 30 dB, and poloidal and toroidal angle adjustment ranges of ±40° and ±15°, respectively. These results verify the system's feasibility through laboratory evaluations. The quasi-optical system has been installed on the HL-3 tokamak during the 2023 experimental campaign to support the development of CPS diagnostics. [ABSTRACT FROM AUTHOR]

Avoiding the major disruption is of paramount importance in future reactor-level devices, for which understanding the disruption mechanism is essential. In this work, MHD instabilities and related dynamic interactions during the major disruption have been investigated in the ohmically heated plasmas in the HL-2A tokamak. It is reported that the interaction between a kind of edge-oscillating-mode (EOM) perturbation and a rotating m/n = 2/1 tearing mode (TM) inside the plasma plays an important role in inducing the mode locking and the subsequent disruption. The EOM perturbation is oscillating in the laboratory frame, which is proposed to be originally generated by the penetrated error field at the plasma edge and is modulated by the rotating 2/1 TM mode. Before mode locking, the 2D electron cyclotron emission imaging shows that the momentary coupling of the EOM and the 2/1 mode can be decoupled each other and the mode structure does not alter significantly. After the mode locking, the EOM and the 2/1 mode expand and couple each other and induce the heat transfer from the core to the edge. The influence of the TM instability and the EOM perturbation on surrounding plasmas prior to the mode locking has also been presented. The results deepen our understanding of the disruption dynamics related to the external field perturbations, especially in the presence of TMs inside the plasma. [ABSTRACT FROM AUTHOR]

Plasma Position Reflectometry (PPR) is planned to provide plasma position and shape information for plasma operation in future fusion reactors. Its primary function is to calibrate the drift of the magnetic signals due to the integral nature of magnetic measurement. Here, we attempt to measure plasma position using ordinary mode (O-mode) and extraordinary mode (X-mode) reflectometry systems on two tokamaks. A new physical model based on the phase shift is proposed to deduce the relative movement of the cut-off layer without density inversion. We demonstrate the plasma position measurements by absolute measurement from density profile inversion and relative measurement from phase shift. The combination of X-mode and O-mode reflectometers can minimize the limitations of single polarization reflectometry and further increase the accuracy of plasma position measurement. These results could provide an important technical basis for the further development of a real-time control system based on PPR. [ABSTRACT FROM AUTHOR]

The suppression of runaway electrons (REs) is influenced by the size of pre-existing 2/1 magnetic islands. Simulations of argon (Ar) massive gas injection into J-TEXT plasmas with pre-existing 2/1 magnetic islands of different width are performed with the 3D magnetohydrodynamic code called NIMROD (Sovinec et al 2004 J. Comput. Phys. 195 355). Results show that the ratio of remaining REs is not monotonically dependent on the width of pre-existing 2/1 magnetic islands. Large enough pre-existing 2/1 magnetic islands can lead to strong magnetic perturbation and strong magnetic surface stochasticity to expel the REs. The critical width for the suppression of REs is to be 0.31 as the minor radius. Under the condition of suitable small pre-existing 2/1 magnetic islands width, with island width ranging from 0.07 to 0.11 times the minor radius, the mitigation of REs is achieved. The duration of magnetic perturbations δB/B(n = 1) (exceeding (4 to 6) × 10−3) is a key for RE loss, and high amplitude of high n modes plays an important role in RE loss. [ABSTRACT FROM AUTHOR]

Experiments of disruption mitigation with massive gas injection have been conducted in J-TEXT tokamak with various impurities (He, Ne, Ar and He&Ar (90%:10%) mixtures) injection. A 17-channels polarimeter-interferometer (POLARIS) has been used to analyze the evolution of the electron density profile during pre-TQ and TQ phases. Experiments show that both the impurity species and applied high voltage of the valve have an obvious influence on the impurities deposition location and electron density profile. Moreover, with the high spatial resolution of POLARIS, a rather accurate impurity mixing efficiency has been obtained. The results show that Ar, the high atomic number (high-Z) impurity, has high cooling efficiency and evident mitigation effects, but the mixing efficiency for Ar is only of order 5%–8%. As for the low-Z impurity, He has fast propagation velocity and can reach high mixing efficiency, which is found to be of order 20%–40%. The use of the mixtures of high-Z and low-Z impurities (He&Ar (90%:10%)) can combine the fast penetration of He and high cooling efficiency of Ar, and the mixing efficiency is up to the order 20%–40%, improving the disruption mitigation efficiency and time response of disruption mitigation system. [ABSTRACT FROM AUTHOR]

The generation of runaway electrons (REs) during disruptions is a key issue for the safe operation of large tokamaks. For better design, a reliable scenario to suppress RE generation and for the investigation of RE generation during disruptions is highly essential. On J-TEXT, RE generation is strongly dependent on the pre-disruption electron density, toroidal magnetic fields (BT) and magnetic perturbations. RE generation can be avoided in discharges with a low BT or a high electron density. For discharges with a high BT, a high electron density threshold is required to suppress RE generation. However, this threshold decreases with the application of resonant magnetic perturbations (RMP) which is applied before the thermal quench. The enhancement of magnetic perturbation increases the RE loss during disruptions, leading to robust runaway suppression in the discharges with a relatively low electron density. The electron density threshold required for RE suppression reduces with the increase of RMP strength and the m/n = 2/1 mode RMP ismore efficient than the m/n = 3/1 mode RMP for the reduction of density threshold, where m and n are the poloidal and toroidal mode numbers, respectively. The NIMROD simulation is applied to investigate the transport of REs during disruptions, which indicates that the 2/1 mode RMP can create stronger magnetic perturbations during a disruption, resulting in a high loss ratio of RE seeds. All results provide evidence of the significant effect of RMP mode and amplitude on the electron density threshold for RE generation, which might give an insight into future large reactor tokamak operation with high electron densities. [ABSTRACT FROM AUTHOR]

Plasma disruption is one of the major challenges for ITER. A large fraction of runaway current may be formed due to the avalanche generation of runaway electrons (REs) in disruptions. Current researches find that the generation of REs may be hard to totally suppress during the disruptions, and lead to the formation of large runaway current in future fusion devices. Runaway currents carry huge magnetic and kinetic energies that need to be dissipated safely. Runaway current dissipation by high-Z impurities has been performed on J-TEXT. Runaway currents are formed by an injection of ~1019 argon atoms, and then large quantities of argon or krypton impurities are injected by the massive gas injection valve to dissipate the runaway current during the runaway current plateau phase. The dissipation efficiency increases with the increase of injected impurity quantity. When the injected impurity quantity exceeds 2 × 1021, the dissipation efficiency becomes saturated. Up to a 28MA s-1 runaway current dissipation rate and a 15% energy dissipation rate can be achieved. Analysis shows that the saturation of dissipation efficiency is caused by the decrease of impurity assimilation rate with the increase of total injected impurity quantity. The decrease of impurity assimilation rate may be caused by the saturation of impurity density growth rate during the short dissipation phase. A simple estimate also shows that the increase of internal inductance leads to the slowing down of the growth of runaway current dissipation rate during the runaway current decay phase. The results may have important implication for ITER disruption mitigation. [ABSTRACT FROM AUTHOR]