Breakdown and Start-Up Control

Breakdown is the moment the gas becomes a plasma; control must reliably initiate and grow the discharge from nothing.

The beginning of a discharge

Before there is a plasma there is neutral gas in the vessel. Breakdown is the process of ionizing that gas and forming an initial plasma current channel. It is a delicate, condition-sensitive step: if breakdown fails or is patchy, the discharge never starts or starts badly, so control must make it reliable.

Conditions for breakdown

Breakdown needs a loop voltage from the solenoid to accelerate seed electrons, a gas pressure in a suitable range so electrons ionize before being lost, and a field configuration with a low stray field so the electrons stay confined long enough to build an avalanche. Control sets these conditions and times the solenoid swing to initiate the avalanche.

Null formation

The poloidal field coils are configured to create a region of near-zero field, a magnetic null, where the plasma first forms. A clean, well-positioned null gives reliable breakdown; a poor one leaves stray fields that carry electrons into the wall before a plasma forms. Real-time control positions the null and holds it during the initial current rise.

Burn-through

Just after breakdown the young plasma must heat enough to fully ionize its impurities, a phase called burn-through. If radiation from partially ionized impurities exceeds the available heating, the plasma cools and the discharge fails. Control manages the loop voltage and fueling through this radiative barrier into a growing, stable current channel.

In the Kronos program

The Hyperion breeder, as a spherical tokamak with limited solenoid flux, must break down efficiently to conserve volt-seconds for the main discharge. Start-up control positions the null, times the solenoid swing, and manages burn-through before handing off to ramp-up control. The scheme is validated in simulation ahead of first plasma.