Plasma Shaping and Elongation

Elongation stretches the plasma vertically to raise current and pressure limits; on a spherical tokamak it is naturally large but must be actively stabilized.

Shape as a control variable

Beyond triangularity, the other major shaping knob is elongation — how tall the plasma cross-section is relative to its width. Spherical tokamaks are naturally elongated, which raises the plasma current a machine can carry at fixed field and improves the pressure limit. Shaping is not decoration; it directly sets how much fusion-relevant plasma the magnets can hold.

Hyperion combines strong elongation with negative triangularity delta -0.30. The pair is chosen together: elongation for current and pressure capacity, negative triangularity for edge behavior and heat exhaust. The shaping is produced and held by poloidal-field coils.

The stability cost of elongation

A tall plasma is vertically unstable — it tends to drift up or down and must be actively held in place by a fast feedback control system. Lose that control and the plasma moves into the wall. Elongation therefore buys performance at the price of a hard real-time control requirement, which is one reason the machine is paired with an active plasma-control stack. The nearby conducting structures slow the instability enough for feedback to catch it, but the growth rate still sets a firm floor on how fast the position sensing and coil response must act, and that requirement flows directly from the shaping the design chooses.

• Elongation raises current and pressure capacity
• Combined with delta -0.30 for edge and exhaust benefits
• Cost: vertical instability requiring active feedback control

This page describes a design-and-simulation study, not a built machine. Construction begins Q2 2027; first-of-a-kind first tritium is targeted near 2030. No hardware net-gain is claimed before FOAK.