Poloidal beta and internal inductance measurement on HT-7 superconducting tokamak

Diamagnetic measurements based on the compensation of TF current diffusion in J-TEXT

Due to the existence both of toroidal ripples and toroidal field (TF) current diffusion, the toroidal flux changes with time when the TF current is at the flat-top. A diamagnetic measurement based on the compensation of TF current diffusion has been built in J-TEXT to solve this problem. The measurement system includes a double-loop installed in the vacuum vessel and an array of small printed circuit board (PCB) magnetic probes placed on the mid-plane of one TF coil. A model was proposed to analyze and compensate the effect of TF current diffusion. Experiment results show that the residual flux is about 1 × 10^-4 Wb after the compensation and it can meet the need of diamagnetic measurement in J-TEXT.

Diamagnetic measurements by concentric loops in the HL-2A tokamak

The diamagnetic concentric loop method in the HL-2A tokamak is described in this article. The system consists of two concentric poloidal loops with different areas enclosing the plasma column and a short time constant differential integrator, RC < 1 ms. The diamagnetic flux in HL-2A ranges from 1 mWb to 2 mWb for typical discharges with plasma current Ip = 100-400 kA. The integrator output ranges from 0.1 V to 0.2 V with time constant RC = 0.5 ms, and differential area ΔS∕Sout ≈ 7%. Using hybrid analog-digital compensation, the integration drift can be well compensated within 5 mV∕10 s, which can meet the requirement of the concentric loop system. In this method, the measurement of differential area ΔS is not required. The vacuum toroidal flux can be compensated by adjusting the resistance in the integration circuit for several discharges with toroidal field only, which minimizes the additional error produced by a measurement of differential area. The diamagnetic concentric loop system improved the signal to noise ratio by using the short time constant integration. The system with a resolution of ±0.2 kJ can be used to study rapid changes in plasma stored energy, such as the additional power absorbed by the plasma, and the energy loss caused by edge localized modes.

Diamagnetic flux measurement in Aditya tokamak

Measurements of diamagnetic flux in Aditya tokamak for different discharge conditions are reported for the first time. The measured diamagnetic flux in a typical discharge is less than 0.6 mWb and therefore it has required careful compensation for various kinds of pick-ups. The hardware and software compensations employed in this measurement are described. We introduce compensation of a pick-up due to plasma current of less than 20 kA in short duration discharges, in which plasma pressure gradient is supposed to be negligible. The flux measurement during radio frequency heating is also presented in order to validate compensation.