Ge Zhuang

Weak dissipation can control wave propagation, mode competition, and instability thresholds in plasmas, yet the physical origin of large branch-to-branch differences in collisional damping is often obscured by dielectric-tensor calculations. We show that weak collisional damping in cold plasmas is governed by wave-energy partition. In the one-rate cold plasma model with independent-relaxation collisions, the damping rate of a positive-energy collisionless eigenmode equals, to leading order in weak collisions, the collision frequency multiplied by the fraction of the total wave energy stored in plasma motion. This result recasts the standard perturbative damping formula into a compact and physically transparent law, immediately explaining why field-dominated branches such as whistlers can be much less damped than the collision frequency, whereas quasi-electrostatic modes can exhibit damping of comparable magnitude. Analytic examples for Langmuir, transverse electromagnetic, whistler, and extraordinary waves show that the energy-partition form classifies weak collisional damping across distinct branches and provides a simple diagnostic for mode competition in multibranch plasma-wave systems.

I-mode combines high energy confinement and stable temperature profiles similar to L-mode. Simultaneous detachment of the targets was observed during an I-mode discharge in EAST. To investigate the detachment process, SOLPS-ITER simulations were performed before and after neon seeding. The outer midplane transport coefficients were adjusted to reproduce the experimental plasma profiles, with an absolute percentage deviation of less than 28% for the electron density in the region of (\rho < 1.05). With neon seeding, the target electron temperature and ion saturation current density decreased, accompanied by enhanced momentum loss and radiation in the divertor region. The radiation region extended from the targets toward the X-point, while neon remained mainly confined to the divertor region, with a core concentration below 3% of the deuterium plasma density. These results suggest that enhanced divertor momentum loss and radiation are associated with the reduction of target plasma parameters during neon-seeded I-mode detachment.

The enhanced confinement of tokamak plasmas (H-mode) makes it a preferred regime for achieving fusion power production goals in future devices such as ITER. Nevertheless, low confinement mode (L-mode) remains worthy to investigate in reactor relevant conditions, primarily due to no/reduced requirements for ELM and divertor heat load control. In this regard, this study aims at exploring a new potential approach to maximise the achievable fusion gain Q _fus . This approach attempts to increase the core density with enhanced pellet fuelling and then investigates the feasibility of high Q _fus L-mode operation in ITER. The JINTRAC integrated modelling suite has been employed for core-edge transport and source modelling, using the HPI2 module for pellet fuelling. In some of the scenarios considered, the core density reaches up to ∼185% of the Greenwald density, n _GW , with edge densities approaching n _GW , motivated by recent re-evaluations of the density limit that suggest a power-dependent threshold. We compare core transport modelling results obtained by applying the semi-empirical Bohm–gyro-Bohm (BgB) or the quasi-linear gyrokinetic TGLF-SAT2 anomalous transport models, with interpretive vs predictive impurity transport modelling, and pellet fuelling describing continuous vs discrete particle sources. The core plasma confinement of high-density L-mode operation in ITER predicted by the TGLF-SAT2 model is significantly better than that predicted by the BgB model, resulting in a significantly improved Q _fus . Fusion performance metrics, including P _fus and Q _fus , exhibit only minor changes when switching from the interpretive impurity model to the predictive SANCO model, and/or from the continuous ad-hoc pellet model to the discrete HPI2 pellet model. The highest Q _fus value predicted in the ITER high-density L-mode simulations is ∼4, with indications that further improvement may be limited by increased transport associated with electro-magnetic turbulence at elevated plasma beta. This integrated modelling prediction demonstrates the potential of improved Q _fus L-mode operation in ITER and future fusion devices, while exploring its boundary.

Millimeter-wave interferometry is a standard diagnostic for line-integrated electron density, but conventional implementations typically rely on discrete high-frequency sources, mixers, and separately aligned reference paths. Here, we report an integrated frequency-modulated continuous-wave interferometer for plasma diagnostics using a commercial 60–64 GHz radar system-on-chip and, critically, an in situ common-path reference architecture that enables phase-stable operation with industrial hardware. The key design feature is the intentional use of aperture-clipping reflection from the diagnostic-window flange as a stationary reference echo, while the transmitted beam core traverses the plasma and is reflected by a mirror. Because the reference and measurement echoes are generated by the same chirp and share the same transceiver, launch optics, and diagnostic port, differential phase extraction suppresses common-mode oscillator drift, thermal drift, and mechanical vibration without a separate reference arm. A high-duty-cycle chirp configuration and chirp-by-chirp FFT phase retrieval provide a temporal resolution of 120 μs. Experiments on the Keda Linear Magnetized Plasma device show that the common-path-referenced scheme reduces the phase standard deviation from 0.34° to 0.17° and suppresses the dominant 22 Hz vibration component by more than 12 dB, yielding a minimum detectable line-integrated density of 2 × 1014 m−2. These results demonstrate a compact and scalable interferometer architecture for plasma diagnostics based on commercially integrated millimeter-wave hardware.

The implosion dynamics of a magnetically driven capsule-shaped theta pinch plasma is experimentally investigated in this work, focusing on the coupling between the external drive coil and the imploding plasma column. The electron densities were measured by a five-channel optical-fiber interferometer. The local density (or plasma mass) evolution during implosion is obtained by layer-stripping inversion technique. It is observed that plasma mass increases during implosion, which implies that the fixed mass assumption in the classical Lee model is not valid. Here we employ the modified snowplow model with variable mass with momentum conservation constrain to study the dynamic process of implosion. The temporal evolution of magnetic field distribution and plasma current were measured by radial magnetic probe arrays. Plasma current also reveals clear discrepancies from predictions of the conventional Lee model with fixed plasma-to-coil magnetic coupling assumption. An improved variable-coupling model is developed by incorporating time-dependent mutual inductance to explain the experimental phenomena.

Doppler reflectometry (DR) can measure the turbulence in magnetic confinement fusion plasma by detecting backscattered signals, and has been widely used in fusion devices. On the experimental advanced superconducting tokamak (EAST), a Backscattering and Forward-scattering integrated DR system has been deployed during the 2024 experimental campaign, which can measure the turbulence across multiple scales at the same location, simultaneously. In the scattering process, the wave vector values satisfy the Bragg condition: m k ⊥ = k s − k i . For Doppler backscattering (DBS), it detects the m =−1 scattered signal, whereas for the forward scattering, it detects the scattered signal with m > 0 .   To investigate the scattering process around the cutoff layer for DR, a two-dimensional (2D) plane-shaped O-mode full-wave simulation based on the finite-difference time-domain (FDTD) method has been done. It was found that the signal received by the DBS is mainly the m = −1 scattered signal, while the turbulence measured by the Doppler forward-scattering (DFS) corresponds mainly to low-k fluctuations ( k ⊥ < 3c m − 1 ). Besides, the results indicate that for both backscattering and forward scattering, the intensity of the scattered signals increases with turbulence level, while the Doppler shift frequency remains unchanged. Therefore, based on these simulation results, the integrated Backscattering and Forward-scattering DR system can simultaneously measure the large scale (low- k ) and small scale (high- k ) turbulence at the same position in the plasma.