Spherical vs Conventional Tokamak
Low aspect ratio versus the classic thin donut: how the spherical tokamak trades a crowded center column for better stability and pressure efficiency.
- Conventional
- Aspect ratio 3–4, roomy center column
- Spherical (Kronos)
- Aspect ratio 2.0, crowded high-field center stack
- Benefit
- Higher β, better stability, compact for its output
- Cost
- Center-stack shielding & neutron budget are tight
The difference between a conventional and a spherical tokamak is aspect ratio — major radius over minor radius.
A conventional tokamak (aspect ratio 3–4) is a thin donut with a roomy central column, giving plenty of space for the inboard magnet and shield. The trade is lower plasma pressure efficiency for a given field.
A spherical tokamak (Kronos MetroVolt: aspect ratio 2.0) squeezes toward a cored apple. This raises the plasma pressure the machine can hold for a given magnetic field, improves stability, and makes the machine compact for its output. The price is a crowded center stack: the inboard magnet leg and shield must do their jobs in very little radial space — which is why the design carries an explicit neutron-fluence budget for the shielded center stack and leans on a high-field REBCO conductor with a graded winding pack.
The low-neutron D–³He fuel choice eases exactly this pressure: fewer neutrons means a smaller shield fits in the tight center-stack geometry.