Snake perturbations during pellet injection and LHCD in the HL-1M tokamak

Long-lived (1, 1) ‘snake’ modes were discovered nearly three decades ago, but basic questions regarding their formation, stability, and superb particle confinement—shown by surviving tens to hundreds of sawtooth cycles—have remained unanswered. High-resolution spectroscopic imaging diagnostics permit studies of heavy-impurity-ion snakes with unprecedented temporal and spatial resolution, making it possible to positively identify the SXR signals with specific ion charge states and to infer, for the first time, the perturbed impurity density, Zeff, and resistivity at the centre of these long-lived helical modes. The results show a new scenario for the formation of heavy-impurity-ion snakes, which can begin as a broad 1/1 kink asymmetry of the central impurity-ion density, that grows and undergoes a seamless transition to a large crescent-shaped helical island-like structure inside q < 1, with a regularly sawtoothing core. This type of formation departs strongly from the nonlinear island model based on a modified Rutherford equation proposed originally to describe the pellet-induced snakes and expanded further to account for the impurity effects (e.g. and ). These new high-resolution observations show details of their evolution and the accompanying sawtooth oscillations that suggest important differences between the density and temperature dynamics, ruling out a purely pressure-driven process. Instead, many features arise naturally from nonlinear interactions in a 3D MHD model that separately evolves the plasma density and temperature.

Section: Coexistence With Sawtooth Instabilitymentioning

confidence: 57%

Section: Introductionmentioning

confidence: 99%

On the formation and stability of long-lived impurity-ion snakes in Alcator C-Mod

Delgado‐Aparicio

Sugiyama²,

Granetz

et al. 2013

“…In the past years, progress was made in the HL-1M experiment in many aspects including confinement improvement, auxiliary heating and fuelling of plasma and wall conditionings [3,6]. In the off-axis ECRH, double sawteeth in soft x-ray (SXR) radiation were observed, which implies that reversed magnetic shear (RMS) could be formed during ECRH.…”

Section: Experimental Progress Of Hl-1m Tokamakmentioning

confidence: 99%

“…HL-1M started operation at the end of 1994 [1]. Since then, HL-1M plasma performances have been improved significantly, with wall conditioning and auxiliary heating and current drive [2,3]. Up to now, the maximum parameters of HL-1M are plasma current I P = 320 kA, line average density n e = 8 × 10 19 m −3 , toroidal magnetic field B t = 3 T and discharge duration of up to 4 s. To fulfil the objective of the device, four different systems of auxiliary heating and current drive were installed in HL-1M, which are an electron cyclotron resonance heating (ECRH) system of 0.5 MW/75 GHz, a lower hybrid current drive (LHCD) system of 1 MW/2.45 GHz, a neutral beam injection (NBI) beamline of 1 MW, and a ion cyclotron resonance heating (ICRH) system of 0.8 MW.…”

Section: Introductionmentioning

confidence: 99%

Recent experimental results from the HL-1M tokamak and progress in the HL-2A project

Liu

Yan²,

Zhou³

et al. 2004

Recent experimental results from the HL-1M tokamak and progress in the HL-2A project are presented in this paper. In HL-1M, strong fishbone instability was observed during off-axis electron cyclotron resonance heating (ECRH). This is the first observation of fishbone instability purely driven by energetic electrons produced by ECRH. The molecular beam injection (MBI) was first proposed and demonstrated in HL-1M. Recently, new results of the MBI experiment were obtained by increasing the pressure of the gas. A stair-shaped density increment was obtained with high-pressure multi-pulse MBI just like the density evolution behaviour during multi-pellet injection. It is shown that injected particles penetrated into the core region of the plasma. HL-2A is a divertor tokamak constructed at SWIP based on the original ASDEX main components. The mission of the HL-2A project is to explore the physics issues involved in an advanced tokamak. For the first phase, divertor (edge plasma) and confinement research will be emphasized. The major parameters of HL-2A are R = 1.65 m, a = 0.4 m, B t = 2.8 T, I P = 0.48 MA. The main parameters and characteristics of the subsystems such as power supply, pumping, diagnostics and auxiliary heating are presented in this paper. The first plasma of HL-2A was obtained at the end of 2002.

“…In JET device, lower hybrid heating and current drive (LHCD) was applied to generate a region with zero current density [11]. In the early 1990s, the LHCD experiments were carried out on the HL-1 M tokamak, and a slightly hollow current profile was achieved [12,13]. The results show that LHCD experiment successfully suppresses magnetohydrodynamic (MHD) instability [14,15].…”

mentioning

confidence: 99%

Stability nature of ohmic discharges with hollow current profiles on the HL-2A tokamak

Shen¹,

Dong²,

He³

et al. 2020

The equilibrium configurations with hollow current density profiles can effectively improve plasma confinement. The stability of the ohmic discharges with hollow current density profiles on the HL-2A tokamak was studied for various parameters, such as plasma poloidal beta β p , the axial and edge safety factors q 0 and q s , as well as the minimum safety factor q min . The results suggest that the restriction to the parameters of discharges with hollow current density profiles is stricter than that of conventional discharges with peaked current density profiles. In the case of q profile with reverse shear, the instability of the configuration is significantly improved with higher β p . As a result, the configuration with a hollow toroidal current profile is stable for high β p operations. Only if q 0 , q s and q min satisfy the specific limitations, i.e. on HL-2A, only if both q 0 ≳ 2.1 and q s ≳ 3 hold, together with q s − q 0 ≳ 0.7 and q 0 − q min ≳ 0.7, may the configurations with the reverse shears be stable for hollow-current-type Ohmic discharge. The results may give hints for the role of the discharges with hollow toroidal current densities in the coming advanced tokamak operations.