Schematic diagram of the diamagnetic concentric loop system. The system consists of two poloidal loops with different areas enclosing the plasma column and the differential integrator. R2 is an adjustable resistance.
Experimental arrangement of the concentric loop system on HL-2A. (a) The schematic view of poloidal cross section and concentric loops. (b) Photograph of concentric loops and the block. (c) Schematic diagram of the block.
Schematic diagram of the low drift integrator for the concentric loop system. (a) Analog drift compensation. The diamagnetic flux is integrated by integrator A as shown in Figure 1 , and the drift is measured by the other one, integrator B. (b) Block diagram of digital drift compensation. The residual drift of analog integrator is measured by analog to digital converter (ADC) before every discharge, and digitally compensated during plasma pulse.
Digital drift compensation results. (a) The residual drift of analog compensation appears to be stable within several minutes. (b) Typical residual drift before and during a plasma discharge. (c) Digital drift compensation result during the plasma discharge phase, <5 mV/10 s.
Vacuum toroidal field flux compensated by adjusting the value of R2. #18223 (R2/R1 = 7.51%), #18224 (R2/R1 = 7.47%), and #18225 (R2/R1 = 7.52%). (a) Time evolution of toroidal field. (b) Output signals in discharges with vacuum toroidal field only.
Time evolutions of a vacuum poloidal flux compensation discharge (red line) in contrast with those in a plasma discharge (black line). The poloidal field coils currents are compensated by individual powering of each of the coils. (a)–(d) refer to the plasma current, toroidal field, ohmic field coil current, vertical field coil current, respectively. (e) Signal measured by the concentric loop system. (f) Diamagnetic flux.
Typical time evolutions of δΦdia and Wdia in an ohmic discharge (black line) and a discharge heating by ECRH and NBI (red line). The increase of Wdia, from 16 kJ to 30 kJ, is observed in the plasma heated by ECRH and NBI, PECRH = 0.75 MW, PNBI = 0.48 MW.
Time traces of the divertor Dα and stored energy Wdia in an ELMy H mode discharge on HL-2A. In the zoom windows, the energy loss induced by ELMs can be calculated with the time derivative of Wdia.
Estimation of the concentric loop system in HL-2A. The integrator output in Figure 1 , Φ meas , ranges from 0.1 V to 0.2 V with diamagnetic flux δΦdia ∼ 1–2 mWb, differential area ΔS/Sout ∼ 7%, time constant R1C = 0.5 ms.
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