ELM Mitigation Through Magnetic Perturbation generated by Divertor Biasing
Current on the HL-2A Tokamak
T. F. Sun1, B. T. Cui2, X. Q. Ji1, G. Z. Hao1, Yi Liu1, Y. Q. Liu3, S. Y. Liang1
1 Southwestern Institute of Physics, Chengdu 610041 China,
2 Department of Engineering Physics, Tsinghua University, Beijing 100084 China,
3 General Atomics, San Diego, California 92186-5608, USA
Controlling the large heat flux on thedivertor target is a critical issue for fusion reactor
design. It is however challenging to install andutilize these coils in fusion reactor environment,
due to space limitations and high neutron fluxes[1]. Development of a simple, flexible and robust
syetm for heat flux control is therefore muchdesirable and also urgent.
Recently, a divertor biased target system,with 2 × 2 array of plates, has been designed,
constructed and installed on the HL-2A tokamak[2]. When the applied voltage UBT increases from∼ 0 V to ∼ 60 V，ELM mitigation appears after
20ms. Before that, splitting of the divertor strike
points has been observed, in the presence ofcurrents driven by the divertor biased target
system and flowing in the scrape-off layer (SOL)
region. The ELM induced ion saturation current(Jsat) extends from Z ≃ −81 cm to Z ≃ −86 cm.
When the SOL currents exceed a critical value,
the secondary strike point at Z ≃ −85 cm appears.
Meanwhile, the peak position of Jsat induced byELMs moves up.
Power spectrum of electron density
fluctuations in r=0.34m near the plasma edgemeasured by BES indicates that the bias-driven
magnetic perturbation significantly enhances thepower spectrum in the high-frequency (30–50kHz) range whereas the peaked power spectrum
around 40 kHz. The magnetic perturbation
generated by the divertor biasing current
mitigated the edge localized mode (ELM) in
high-confinementmode
plasmasthroughaffecting the heat flux pattern on the divertor
target plate. Toroidal modeling with themagnetohydrodynamic code shows that themagnitude of the dominant resonant magnetic
field perturbation including the plasma response,generated by an 80 A biasing current in SOL, iscomparable to that produced by the ELM control
coils in HL-2A. The magnetic field topology
mapping and the thermal particle orbit tracing,both inside the plasma and in the SOL region,
confirm the divertor strike point splitting effect.
In coclusion, these results clearly indicates thatELMs can be mitigated by the biasing technique
for the case studied here.

Figure 1. Effects of the biased targets on the ELM
behavior and the divertor heat flux in HL-2A
H-mode discharge 39043. Shown on left panels
are time traces for (a) the electron density ne (inblue) and the toroidal plasma current Ip (in red),(b) intensity of the Dα emissions (in blue) in theouter divertor and the plasma stored energy Wdia
(in red), (c) the biasing voltage UBT and thebias-driven SOL current I#1 − I#3, and (d) thespatiotemporal distribution of the ion saturation
current density on the outer divertor target.

Figure 2. Power spectrum of electron density
fluctuations in r = 0.34m from 1610 ms to 1660 ms
near the plasma edge measured by BES
References[1] S. J. Zinkle, Phys. Plasmas 12, 058101 (2005).
[2] B. Cui et al., Fusion Eng. and Des. 173, 112963(2021).