Gyro-Bohm Transport
The favorable turbulence scaling in which diffusivity shrinks with the normalized gyroradius.
The scaling
Gyro-Bohm transport is the expected scaling when turbulence is driven at the ion-gyroradius scale (k_perp rho_i ~ 1). The diffusivity is the Bohm value multiplied by the normalized gyroradius rho-star = rho_i / a:
D_gyroBohm = rho_star * D_Bohm = (rho_i / a) * (T / eB)Because rho-star shrinks as the machine gets larger or the field gets stronger, gyro-Bohm transport improves with size and field, the opposite of the pessimistic Bohm limit. This favorable scaling is a central reason larger and higher-field tokamaks confine better.
Local, gradient-driven
Gyro-Bohm transport is inherently local: the flux at each radius depends on the local gradient, temperature, and field, not on the global machine size directly. This is why dimensionless-parameter confinement studies vary rho-star to test whether a plasma is truly gyro-Bohm.
Departures
- Turbulence spreading and large eddies can push transport toward Bohm scaling
- Zonal flows and profile shearing suppress turbulence, improving on gyro-Bohm
- Electron-scale turbulence adds a separate, smaller-scale channel
How it is quantified
Gyrokinetic simulations output heat flux as a function of gradient; the gyro-Bohm-normalized flux collapses across machine sizes when the scaling holds. Reduced models fit this to give fast transport coefficients for whole-device modeling.
For a compact high-field spherical tokamak such as the Hyperion breeder (16.84 T peak field, 8 T on-axis), the strong field and the gyro-Bohm improvement are what make good confinement plausible at small size, which is verified by turbulence simulation rather than assumed.