Density Limit Experiments and Core-localized Kinetic MHD Activities in HL-2A Ohmic Heating Plasmas

TL;DR

This study investigates the density limit in HL-2A Ohmic plasmas by combining edge fueling to exceed $n_e_G$, core diagnostic observations of core-localized MHD (clMHD) activity, and gyrokinetic simulations. It shows that the Greenwald threshold can be surpassed (up to $n_e/n_e_G\approx1.5$ for about $500$ ms) without immediate radiative collapse, yet core KBM/AITG-like modes emerge and correlate with disruptions. Using the GENE code with experimental profiles, the clMHD activity is identified as KBM/AITG, with frequencies and toroidal mode numbers that evolve with density and broaden radially—enhancing core transport and constraining density. These results offer a coherent mechanism linking core MHD instabilities to the density limit and provide disruption-precursor indicators for high-density operation in tokamaks.

Abstract

The density limit is a mysterious barrier to magnetic confinement nuclear fusion, and is still an unresolved issue. In this paper, we will present the experimental results of the density limit and core-localized kinetic MHD instabilities on HL-2A. Firstly, the high density shots with $ne/ne_G>1$ have been achieved by the conventional gas-puff fuelling method in Ohmic heating plasmas, and the corresponding duration time is close to $t\sim500$ ms ($\sim$ $30τ_E$), where $τ_E$ is the global energy confinement time. Secondly, it is found for the first time that there are kinetic MHD instabilities in the core plasmas while $ne/ne_G\sim1$. The analysis suggests that the core-localized MHD activities belong to Alfv{é}nic ion temperature gradient (AITG) modes or kinetic ballooning modes (KBM), and firstly it is found on experiment that they trigger the minor or major disruption of bulk plasmas while the density profile is peaked. These new findings are of great importance to figure out and understand the origin of the density limit.

Figures (13)

• Figure 1: (a1) Time traces of plasma current $I_p$ and total plasma radiation power $P_{rad}$for shot #38261. The red and green solid lines correspond to $I_p$ and $P_{rad}$, respectively. (a2) Time traces of $I_p$ and $P_{rad}$ for shot #38262. (b1) Time traces of line-averaged plasma density $ne$ and oxygen impurity spectral line $I_{OVI}$ for shot #38261. The blue and magenta solid lines show $ne$ and $I_{OVI}$, respectively. The black arrow marks the relative density level at the time of peak density. (b2) Time traces of $ne$ and $I_{OVI}$ for shot #38262. (c1) Spectrum of microwave interferometry signal for shot #38261, where the microwave interferometry channel passes through the center of the plasma. The white arrow indicates the trend of spectral broadening. (c2) Spectrum of the central microwave interferometry signal for shot #38262.(d1) Magnified view of spectrum as indicated by black rectangular in subfigure (c1). The black arrows indicate the onset time of clMHD modes. (d2) Magnified view of spectrum as indicated by black rectangular in subfigure (c2). The dashed elliptical line denotes signature of the clMHD modes.
• Figure 2: (a) Time evolution of the density profile for shot #38261, obtained from Abel inversion of laser interferometry data. The black arrow marks the core density before the density-limit disruption, with $ne\sim 6.03\times 10^{19} \mathrm{m^{-3}}$.(b) Time evolution of the density profile for shot #38262. The core density prior to disruption is $ne\sim 5.87\times 10^{19} \mathrm{m^{-3}}$. Here $\rho=r/a$ is the normalized radius. The observed density ramp-up and peaking in both shots result from plasma self-organization under gas puffing.
• Figure 3: (a) Time traces of plasma parameters for shot #38522. (b) Time traces of plasma parameters for shot #38581.The following quantities are shown: plasma current $Ip$ (red line); total radiation power of plasma $P_{rad}$ (green line); line-averaged electron density $ne$ (blue line); oxygen impurity level $I_{OVI}$ (magenta line); carbon impurity level $I_{CIII}$ (magenta line); core electron temperature $Te$ (red line); plasma stored energy $W_E$ (blue line); soft X-ray (SXR) signal $I_{SX}$ (red line); time-differential poloidal magnetic perturbation $dB_{\theta}/dt$ (blue line); gas-puffing signal $GP$ (blue line) and NBI power $P_{NBI}$ (red line). Black arrows mark the times when the ratio $ne/neG$ reaches 1.0 and its maximum value.
• Figure 4: (a) Time evolution of the density profile for shot #38522, obtained from Abel inversion of laser interferometry data. The black arrow indicates the core density before the density-limit disruption, $ne\sim 8.2 \times 10^{19} \mathrm{m^{-3}}$. (b) Time evolution of the density profile for shot #38581. The core density prior to disruption is $ne\sim 7.6\times 10^{19} \mathrm{m^{-3}}$. Here $\rho=r/a$ denotes the normalized radius. The overall drop in plasma density after 1500 ms corresponds to a minor disruption occurring around 1660 ms.
• Figure 5: (a) Microwave interferometry spectrum before the density-limit disruption in shot #38522, showing instabilities in the 0–130 kHz range.(b1) Microwave interferometry spectrum before the minor disruption in shot #38581, with instabilities emerging in the 0–100 kHz range.(b2) Microwave interferometry spectrum before the major density-limit disruption in shot #38581.