Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2016 / S19 / 1 / b / Solution

The beam rigidity is $B\rho \simeq p/(0.2998\mathrm{GeV}/(c\mathrm{T}\mathrm{m}))\simeq 1.67\times 10^5$ T m. Equal opposing strengths have $f=B\rho /(G{L}_q)$, and weak separated-doublet net power is $1/F\simeq L/f^2$. Taking $F\sim L^{\ast }=50$ m gives the design familyThere is not a unique pair $(L_q,L)$ without a spacing/layout choice. A compact estimate $L\sim L_q$ gives about 18 m for each, showing immediately that neither is extremely small compared with 50 m and the advertised asymptotic approximation is only rough. For example $L=30$ m gives $L_q\sim 14$ m in the same estimate.

An equal-strength doublet also has exact parallel-input focal distances $f^2/L-f$ and $f^2/L+f$: it is astigmatic, so a single exact common 50 m focus cannot be imposed without specifying incoming optical functions or allowing unequal strengths. The family above is a scale estimate of net focusing, not a complete collider final-focus design.