Production of high-quality plasma discharges via real-time control of plasma current ramp-up using neutral gas injection in Aditya-U tokamak

Technical Summary: Real-Time Control of Plasma Current Ramp-Up via Neutral Gas Injection in ADITYA-U

Problem Statement
In conventional tokamaks utilizing Ohmic breakdown, the plasma current ramp-up phase is critical for achieving stable, high-pressure discharges. However, this phase is highly susceptible to failure due to unquantified dynamical variables, particularly the condition of the vessel wall and plasma-facing components from previous discharges. These conditions influence impurity influx and plasma resistivity, causing the actual plasma current rise rate ($dI_p/dt$) to deviate from pre-programmed profiles.

In the ADITYA-U tokamak, the primary vertical equilibrium magnetic field ($B_V$) coils possess a time constant ($\tau \approx 20$ ms) significantly larger than the characteristic time scale of plasma column motion ($\sim 1$ ms). While auxiliary fast feedback (FFB) coils mitigate this, they have limited control authority. If the current rise rate exceeds a critical threshold (approximately 6 kA/ms), the combined power supplies for the main and auxiliary vertical field coils become insufficient to maintain equilibrium. This leads to loss of horizontal position control, enhanced plasma-wall interactions, impurity influx, and the triggering of Magnetohydrodynamic (MHD) instabilities (such as tearing modes) due to unfavorable current density profiles. Consequently, discharges often terminate in disruptions.

Methodology
To address these limitations, the authors developed a novel active control strategy that supplements the existing $B_V$ feedback systems with real-time neutral gas injection. The core premise is that injecting a controlled quantity of fuel gas (hydrogen) increases plasma density and impurity content, thereby raising plasma resistivity. This increased resistivity shortens the resistive diffusion time, allowing deeper current penetration and increasing internal inductance, which collectively suppresses the current rise rate.

The implementation relies on a dedicated Digital Signal Processor (DSP)-based hardware controller (DCBH) with the following operational logic:

1. Monitoring: The DCBH continuously monitors the integrated signal from a Rogowski coil to calculate the plasma current ($I_p$) and its time derivative ($dI_p/dt$) over 5 ms intervals.
2. Threshold Detection: When the calculated current rise rate exceeds a preset threshold (experimentally set at 3.2 kA/ms for hardware testing and 4 kA/ms for control experiments), the DCBH generates a Transistor-Transistor Logic (TTL) pulse.
3. Actuation: This TTL pulse triggers a function generator, which activates a piezoelectric valve to inject a short pulse of hydrogen gas (approx. $10^{17}$ neutral particles per 1 ms pulse) into the vacuum vessel.
4. Feedback Loop: The system operates in real-time, intervening only when the ramp rate becomes excessive, aiming to keep $dI_p/dt$ within the manageable range of the magnetic equilibrium control systems.

Key Results
The study presents experimental validation using two consecutive discharges (#38970 and #38971) with identical pre-discharge settings, differing only in the activation of the gas injection (GI) system.

• Controlled Discharge (#38970): When the current ramp rate exceeded 4 kA/ms, the GI system triggered a 2.5 ms gas puff. This intervention successfully reduced the ramp rate to below 4 kA/ms. The discharge maintained a close agreement between the pre-programmed and required vertical equilibrium fields. Diagnostics showed a peaked Soft X-ray (SXR) profile with sawtooth oscillations, indicative of a high-pressure, stable discharge.
• Uncontrolled Discharge (#38971): Without gas injection, the current ramp rate continued to rise rapidly, leading to a significant deviation between the preset and required $B_V$. This resulted in outward plasma displacement, intense boundary interactions (evidenced by spikes in $C^{2+}$ line emission), and large MHD activity. The SXR profile exhibited multiple spikes characteristic of thermal quenches, and the discharge quality deteriorated significantly.

Hardware functionality tests confirmed that the DCBH accurately tracks the current slope and generates trigger signals at the precise moment the threshold is crossed, with a demonstrated delay of only a few milliseconds.

Significance and Claims
The paper claims that real-time neutral gas injection serves as an effective auxiliary control strategy for regulating plasma current ramp-up in tokamaks where magnetic feedback systems have intrinsic speed or authority limitations. The study demonstrates that:

1. Disruption Mitigation: Gas injection effectively prevents the current rise rate from exceeding the control capabilities of the vertical field power supplies, thereby avoiding the loss of equilibrium and subsequent disruptions.
2. MHD Stability: By moderating the ramp rate, the scheme prevents the formation of hollow current density profiles that trigger tearing modes and other MHD instabilities.
3. Complementary Role: This method does not replace high-performance feedback control but complements it by ensuring the ramp rate remains within the "manageable limits" of existing hardware.

The authors conclude that while optimization of gas quantity is necessary to avoid over-injection (which could trigger massive gas injection-like disruptions), the integration of this gas injection scheme with closed-loop magnetic control systems offers a robust pathway to achieving stable, prolonged plasma discharges in conventional tokamaks like ADITYA-U.