Controlling the rotation of drift tearing modes by biased electrode in ADITYA-U tokamak

Abstract

The influence of background plasma poloidal rotation on the rotation frequency of the m/n = 2/1 drift tearing mode (DTM) has been studied in ADITYA-U tokamak. The poloidal rotation velocity of the background plasma in the ion diamagnetic direction is increased or decreased by inducing an outward or inward radial electric field, respectively, through a biased-electrode placed in the edge region of the plasma. The rotation frequency of the preexisting drift tearing mode, rotating in the electron diamagnetic direction, concomitantly decreased or increased with the application of bias depending on its polarity. The positive-bias increases the background plasma rotation in the ion-diamagnetic direction from its pre-bias value, hence decreasing the DTM rotation frequency, whereas the negative bias reduces the plasma rotation velocity in the ion-diamagnetic direction, hence increasing the mode rotation. In addition to that, a short gas puff introduced during the positive and negative bias pulse further reduces the mode frequency, however, with different amplitudes in different bias-polarities. These observations suggest that the background plasma rotation contributes significantly toward the rotation of DTMs, and the rotation frequency of the magnetohydrodynamic modes can be modified by varying the poloidal rotation of the background plasma and/or the diamagnetic drift frequency.

Gyrokinetic simulations of electrostatic microturbulence in ADITYA-U tokamak

“…The experimentally recorded spectrum of electrostatic fluctuations shows a broadband of frequencies from ∼0 to 50 kHz (red) that matches well with the findings of the gyrokinetic simulations (blue) of ADITYA-U tokamak using GTC. The ion diffusivity near to the LCFS of tokamak predicted from the self-consistent simulations using GTC (see figure 8) gives a value ∼0.2 m 2 s −1 which roughly matches the experimental estimate derived from the density profile [62], and further cross-checked with UEDGE code simulations [63]. Due to the diagnostic limitations at the present time, a realistic experimental value of the electron heat conductivity is not available.…”

“…The reported experimental value of the electron heat conductivity has been estimated by assuming a diffusive transport and using an energy confinement time from power balance as χ e ∼ a 2 /4τ E , where a is the minor radius (0.25 m) and τ E is the energy confinement time [64]. Experimentally, τ E is obtained by the usual method of dividing the stored energy by the input power, that gives τ E ∼ 10 ms [62]. This estimate of the electron heat conductivity obtained from the experiment is χ e ∼ 1.5 m 2 s −1 , which is within 20% of the value χ e ∼ 1.2 m 2 s −1 obtained from the simulations (see figure 8).…”

Overview of recent experimental results from the ADITYA-U tokamak

“…These observations suggest that background plasma rotation contributes significantly to the rotation of DTMs, and these MHD modes can be stabilized by the modification in the background poloidal plasma rotation. A new electrode bias set-up as shown in figure 16 is developed for biasing a cylindrical-shaped tungsten electrode with a diameter of 8 mm and length of 15 mm [54]. In this experiment, the electrode position is fixed at r = 22.5 cm, which is 2.5 cm inside the LCFS.…”

Role of pinch in Argon impurity transport in Ohmic discharges of Aditya-U Tokamak

“…Strong local inward pinch of the impurities are commonly observed in several tokamaks with different discharge scenarios such as Ohmic, L-mode and Hmode 9,11,57 . With the finite loop-voltage (𝑉 = 𝐸 𝜙 × 2𝜋𝑅, 𝑤ℎ𝑒𝑟𝑒 𝑅 𝑖𝑠 𝑡ℎ𝑒 𝑚𝑎𝑗𝑜𝑟 𝑟𝑎𝑑𝑖𝑢𝑠) in the reported discharges, a radial pinch due to 𝐸 𝜙 × 𝐵 𝜃 , where 𝐵 𝜃 is the poloidal magnetic field, always exists 47 . As mentioned earlier and can be seen from figure 3a (inset), loop voltage increases by 50% after argon injection, hence the pinch also increases after argon injection.…”

“…Note here that for maintaining the density, multiple periodic injection of hydrogen gas-puffs are used, which are turned off prior to argon injection. The observation of periodic relaxation events in plasma parameters prior to argon injection is the result of the periodic hydrogen gas puffs 47 . As seen from the figure 3, the overall plasma parameters do not vary much after the argon injection except the soft-X-ray emission.…”