Why bigger tokamaks with larger plasma current are desired

As mentioned in Sec. 2.8, the beta limit study on JET tokamak shows that the maximal $\beta_t$ obtained is proportional to $I_p / a B_{t 0}$. This means the maximal plasmas pressure obtained is proportional to $I_p / a$, i.e.,

$$\langle p \rangle \propto \frac{I_p}{a}$$ (93)

The total plasma energy is given by $E \approx \langle p \rangle 2 \pi R a^2$, where $R$ is the major radius of the device. Using Eq. (93), we obtain

$$E \propto I_p a R.$$ (94)

Since fusion power is proportional to the plasma energy, the above equation is written as

$$P_{\ensuremath{\operatorname{fusion}}} \propto I_p a R,$$ (95)

which indicates, to obtain larger fusion power, we desire bigger tokamaks with larger plasma current.