Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2018 / S19 (Particle accelerator science)

For geometrically similar FODO optics at fixed beam energy and damping partition, the natural horizontal emittance scales approximately as the cube of the bending angle per cell, ${\epsilon }_{x0}\propto {\gamma }^2{\theta }_b^3$. This follows from the radiation-integral expression ${\epsilon }_{x0}=C_q{\gamma }^2{I}_5/(J_x{I}_2)$: at fixed optical phase advance the dispersion-invariant scaling makes the ratio proportional to the cube of the cell bend.

Doubling the number of cells around a fixed circumference halves the bending angle per cell, givingThis estimate assumes the redesigned magnets actually preserve the relevant dimensionless optical structure, the same energy and circumference, and no dominant insertion-device emittance contribution. Merely counting twice as many quadrupoles without specifying the lattice would not guarantee the cubic improvement.

If the quoted coupling is $k={\epsilon }_y/{\epsilon }_x$, then ${\epsilon }_y=k{\epsilon }_x$. Combining the horizontal factor of eight with $k$ falling from $0.1$ to $0.02$ givesThis is the usual leading estimate treating the horizontal emittance in part a as unchanged by small coupling.

If one instead accounts for exact redistribution of a fixed natural-emittance sum, ${\epsilon }_x={\epsilon }_0/(1+k)$ and ${\epsilon }_y=k{\epsilon }_0/(1+k)$. Then the horizontal ratio is $(1/8)(1.1/1.02)\simeq 0.1348$ and the vertical ratio is $0.02696$, an improvement by about $37$. These are consistent refinements of the stated approximate factor forty, not a second independent reduction to apply afterward.

Far from the diffraction limit, and with negligible dispersion contribution, the source phase-space area scales as ${\epsilon }_x{\epsilon }_y$. At fixed photon flux, source-point optical functions and spectral conditions, brightness is inversely proportional to that product. The leading estimates therefore giveUsing the coupling-partition refinement in part b consistently gives about $275$ instead. Both estimates assume comparable current, photon flux and insertion devices. They would cease to apply once photon diffraction emittance or other source-size contributions become important; the question specifically excludes that regime.

The accelerating supply transfers $I_{e}V=50$ kW into the beam, but the electrons need not be discarded with that kinetic energy. An electrostatic recovery system decelerates the used beam before collection and returns most of its energy to the electric field or supply. The net required power then covers unrecovered beam energy and losses, rather than the whole circulating kinetic-energy flux.

For example, $99\%$ recovery leaves $0.01(50\mathrm{kW})\simeq 500$ W of unrecovered beam power, plus auxiliary loads. This explains the stated two-order-of-magnitude scale. It does not imply that gun heating, vacuum equipment, magnets and all electrical losses are automatically included in a $500$ W total plant budget.

Regenerative braking in an electric vehicle converts part of its mechanical kinetic energy back into electrical energy stored in a battery, rather than dissipating it all in friction brakes. An industrial recuperative heat exchanger transfers heat from a hot exhaust stream to incoming process fluid, reducing the required new heat input.

Both recover energy that would otherwise be rejected, while remaining subject to conversion losses and limits on the usefulness of the recovered form. Neither is a source of net energy over a complete cycle.

Express temperatures as energies $\Theta =k_{B}T$. A longitudinal cathode-energy spread of order ${\Theta }_c$ remains of that order during an ideal common electrostatic acceleration. At final mean speed $v_0$, a small energy spread produces $\delta v_{\parallel }\simeq \delta E/(m_e{v}_0)$. In the accompanying beam frame, the corresponding random kinetic-energy scale isThe last equality uses the nonrelativistic acceleration energy $E_e=m_e{v}_0^2/2$. This is the approximate temperature convention supplied in the hint; exact numerical factors depend on the emitted distribution and the definition of spread. At $50$ keV, relativistic corrections are modest but not strictly zero.

Electrostatic acceleration narrows longitudinal velocity spread relative to the fast mean beam even though it does not remove each electron's transverse cathode motion. In ideal magnetized cooling, slow proton motion couples to the electron guiding-centre dynamics, making this very small longitudinal spread relevant to the attainable ion temperature. Practical voltage ripple, space charge, alignment errors and ion heating can set much larger limits.

The stated conversion gives ${\Theta }_c\simeq 0.10$ eV. Hence the ideal longitudinal bath temperature isThis estimate equates the cooled proton thermal-energy scale with the effective electron bath scale, not the two particles' random speeds. The proton root-mean-square speed for a one-component convention $m_p{\sigma }_v^2=k_{B}T$ would be about $2.2\mathrm{m}{\mathrm{s}}^{-1}$.

The result is an ideal floor under the hinted cooling model. It is not a promise of a sub-millikelvin stored beam in a real ring, where noise, beam self-fields, intrabeam scattering and other heat sources enter the balance.

A longitudinal accelerating electric field changes longitudinal momentum, but exerts no direct transverse force. With a constant guiding magnetic field and adiabatic transport, the transverse gyro-energy is consequently retained from the cathode:More generally the magnetic moment of the orbit gives ${\Theta }_{\perp }/B\simeq \text{constant}$, so the equality assumes the same relevant field strength, not merely that a field is present all along the line.

In the strong-magnetization regime, many electron gyro-orbits occur during an important long-range electron-ion interaction and the gyroradius is small. The ion then interacts predominantly with a guiding centre; rapid transverse cyclotron motion does not act like an unmagnetized bath at the full transverse electron temperature. This allows a proton equilibrium scale far below $T_c$ despite the hot transverse gyro-motion.^[ref]

That conclusion is conditional on magnetization and collision scales. A finite gyroradius, nonadiabatic fields or transverse beam errors can restore heating and degrade cooling; it is not generally true that transverse temperature can never affect the cooled beam.

Let the bunch tail have the higher energy so that it catches the head. For an ultrarelativistic proton, $\delta v/v\simeq (\delta E/E)/({\beta }^2{\gamma }^2)$. A drift of length $S$ then changes the bunch length by approximately $S\delta v/v$. Requiring that to match the initial length givesTaking $\gamma \simeq 62.5/0.938\simeq 66.6$, $\beta \simeq 1$, and the given lengths yields $\Delta E/E\simeq 0.023$, about $2.3\%$. Treating $62.5$ GeV as kinetic rather than total energy changes that by only a few percent.

This is the linear ballistic compression requirement; a real finite-emittance bunch, nonlinear velocity-energy relation, uncorrelated energy spread and collective fields prevent exact zero length. The chirp is a correlated full head-to-tail spread, not automatically an rms energy spread.

The target collision energy requires $175$ GeV per lepton beam. Its initial energy after the half-linac is $62.5$ GeV, so each lepton must gain $112.5$ GeV. Define the transformer ratio as accelerating field experienced by the witness divided by decelerating field experienced by the driver. If the useful stage length exhausts the driver's $62.5$ GeV while delivering the required gain, thenThis matches depletion to acceleration and avoids leaving unused driver energy at the witness target. It is an ideal energy-transfer estimate, assuming suitable loading and longitudinal field profiles; a field ratio alone does not specify a practical wake or its efficiency.

For ideal transfer of the proton bunch's kinetic energy to the witness bunch,ThusThis is a minimum driver population for unit efficiency and full depletion under the stated energy budget. If only a fraction $\eta$ is usefully transferred, the required ratio becomes $1.8/\eta$. The equality to the transformer ratio uses the same ideal matching assumption as part b.

The mean witness gradient must be $112.5\mathrm{GeV}/1.1\mathrm{m}\simeq 102\mathrm{GV}{\mathrm{m}}^{-1}$. To turn this into a density estimate, assume an accelerating wake amplitude of order the cold plasma wave-breaking scale,Eliminating ${\omega }_p$ givesThe associated plasma wavelength is about $31$ micrometres, illustrating why a short driver structure is required. The given length and energy gain do not uniquely fix density without an assumption about wake amplitude relative to the plasma scale. Driver shape, beam loading, relativistic wave-breaking corrections and matching to a Positron witness are substantive additional issues; the estimate is a scale calculation, not a completed accelerator design.

If the driver can be fully depleted with transformer ratio six, the largest ideal witness gain from a $62.5$ GeV proton is $375$ GeV. Each lepton then hasFor equal opposing electron and Positron beam energies,The associated ideal population ratio would be $N_p/N_e=6$. This energy-budget ceiling ignores wake-generation and transfer losses, staging limits, emittance preservation and the difficulty of accelerating both charge signs with suitable focusing. It is the consequence of the stipulated transformer-ratio bound, not a prediction of an achievable upgrade.