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

Abstract: Robust control of plasma current ramp-up is an absolute necessity, as an efficient and uncontaminated plasma current ramp-up is essential for achieving prolonged, high-pressure tokamak plasma discharges. In conventional tokamaks with Ohmic breakdown, the plasma current ramp-up is achieved primarily with pre-fixed temporal profiles of the applied toroidal electric field and the equilibrium magnetic field (Bv). The pre-fixed temporal profiles of these fields are often insufficient to maintain a successful plasma current ramp-up, as several unquantified dynamical variables, such as the condition of the vessel wall and plasma-facing components, influence the plasma current rise. Fuel gas injection in an appropriate quantity at a suitable time during the current ramp-up is therefore used to control the plasma current rise rate, ensuring successful plasma current start-up in Aditya-U. The gas injection time and gas quantity are controlled based on real-time measurement of plasma current rise rate using a digital signal processor (DSP) controller. This special control scheme is capable of achieving the plasma current to rise nearly at the desired rate, resulting in a successful start-up and a stable plasma discharge.

• The paper demonstrates a real-time gas injection feedback mechanism that effectively moderates plasma current ramp-up in the Aditya-U tokamak.
• It implements a DSP-based controller triggering gas puffs at critical dIₚ/dt thresholds, aligning the actual plasma state with programmed equilibrium requirements.
• Experimental results reveal reduced MHD instability, mitigated plasma-wall interactions, and improved operational reliability.

Real-Time Control of Plasma Current Ramp-Up via Neutral Gas Injection in the Aditya-U Tokamak

Introduction

This work addresses a central challenge in tokamak operation: precise and robust control of plasma current ($I_p$) ramp-up, especially given the stochastic influences from vessel wall conditions and plasma-facing components. Traditional ramp-up strategies, which rely on pre-programmed toroidal electric and equilibrium magnetic fields, often fail to accommodate the real-time variability caused by unquantified impurities and evolving device parameters. This study proposes and experimentally validates a real-time, feedback-driven strategy for moderating $I_p$ ramp rates through dynamically triggered neutral gas injection. The method is demonstrated on the Aditya-U tokamak, offering a new operational degree of freedom that stabilizes ramp-up trajectories and mitigates MHD instability and disruptions.

Experimental Implementation

Plasma Operation and Diagnostics

The Aditya-U tokamak (major radius $R_0 = 0.75$ m, minor radius $a = 0.25$ m) was employed across a window of $I_p$ ($80$–$200$ kA) and line-averaged density ($1$–$4\times 10^{19}\,\mathrm{m}^{-3}$) with typical core $T_e$ of $I_p$0–$I_p$1 eV. Hydrogen gas puffs of $I_p$2 atoms per $I_p$3 ms were administered using a fast piezoelectric valve triggered by feedback electronics. Real-time line-averaged density was measured with a 100 GHz heterodyne interferometer; impurity influx and boundary interactions were tracked via time-resolved visible spectroscopy and soft X-ray (SXR) tomography. Rogowski coils and cosine coils, with cross-checks from fast-camera imaging, enabled high-speed measurement of $I_p$4 and plasma position.

DSP-Based Feedback Controller

Central to the control architecture is a digital signal processor (DSP)-based controller that receives integrated Rogowski coil signals, computes $I_p$5 at $I_p$6 ms intervals, and generates a TTL output upon crossing a tunable ramp-rate threshold (demonstrated at $I_p$7–$I_p$8 kA/ms for critical events). The triggered pulse actuates the piezoelectric valve for neutral gas injection, whose timing and quantity are directly controlled by the pulse width and amplitude from the downstream logic. The system's performance was validated across diverse discharges, confirming accurate, low-latency triggering of gas puffs in response to real-time plasma current conditions.

Experimental Results

Verification of Closed-Loop Hardware

The real-time hardware was verified to reliably monitor and trigger intervention at specified $I_p$9 thresholds. Discharges with different ramp-up characteristics led to hardware-triggered pulses precisely at the points when the preset ramp-rate thresholds were crossed.

Impact on Plasma Ramp-Up and Stability

Parallel discharges with and without real-time gas injection under otherwise identical pre-discharge conditions were analyzed in detail. Key findings include:

• Threshold-Limited Ramp Rate: Without feedback, $R_0 = 0.75$0 exceeded $R_0 = 0.75$1 kA/ms, resulting in unmanageable departures from the equilibrium magnetic field requirement, pronounced outward plasma displacement, and intense plasma-wall interactions. Feedback-triggered gas injection promptly reduced $R_0 = 0.75$2 below $R_0 = 0.75$3 kA/ms, bringing the actual vertical equilibrium field in close agreement with programmed values and stabilizing plasma position.
• Stability and Disruption Suppression: Uncontrolled discharges exhibited heightened impurity line emission (e.g., C$R_0 = 0.75$4), pronounced SXR spikes (thermal quench signatures), and increased MHD activity. Actively controlled discharges suppressed these features, instead exhibiting peaked SXR profiles with sawtoothing—indicative of stable, high-pressure operation.
• Plasma-Wall Interaction Mitigation: The gas injection scheme directly limited deleterious plasma-boundary interactions by decreasing the ramp rate and thermalizing wall-induced perturbations.

Control Mechanism Insight

The gas puff-induced density rise, impurity radiation, and increased H$R_0 = 0.75$5 output lower the plasma temperature locally, increasing the plasma resistivity and accelerating current diffusion. This effect increases the internal inductance and flattens the current profile, collectively reducing the ramp-up rate. The system thus ensures that $R_0 = 0.75$6 remains within the operational envelope of the ADITYA-U vertical field feedback systems and avoids MHD-induced disruptions.

Implications and Future Outlook

This feedback-triggered gas injection technique is a pragmatic auxiliary to closed-loop electromagnetic feedback, extending the operational robustness of ramp-up control in devices where the time constant of power supply systems limits their response to rapidly evolving plasma conditions. The method provides a pathway for merging actuator-diverse control (magnetic and particle) in intermediate-scale devices, and it is particularly relevant where large-scale power supplies cannot respond on the millisecond timescale.

Optimization of injected gas is critical: excessive flux may risk premature disruptions analogous to those observed in massive gas injections. Future work will focus on developing adaptive algorithms to tailor gas pulse profiles to varying plasma-wall conditions and integrating the scheme with more advanced model-based feedback controllers. There is substantial potential for its application in larger, next-generation machines with similarly constrained vertical field coil time constants and highly nonlinear wall interactions.

Conclusion

The implementation of real-time, neutral gas injection as a feedback actuator for plasma current ramp-up control in the Aditya-U tokamak demonstrates reliable moderation of $R_0 = 0.75$7, effective stabilization of equilibrium and position, and significant mitigation of disruption-prone events. By exploiting the dynamic interplay between density, impurity radiation, and current diffusion, this method enhances both operational reliability and performance envelope of Ohmically heated tokamaks. As advanced tokamaks push toward longer-pulse, higher-pressure operation, such actuator-diverse real-time control strategies will be essential components of disruption avoidance toolkits.