Increasing the intensity at 450 GeV: RF issues

1. Increasing the intensity at 450 GeV: RF issues A. Butterworth AB/RF

2. Possible RF issues • RF High Power RF system • total beam intensity • Low Level RF system & beam control • what feedback loops do we need? • half-detuning • coupled-bunch instability threshold • RF Synchronization system • multi-batch injection in 2 rings

3. RF High Power system • Amplification chain, klystrons, waveguides, circulators, loads, etc. • Machine protection: Beam instability due to loss of control of a cavity becomes an issue at total intensity above ~1/2 nominal (~250mA) • would normally generate a beam dump interlock • RF equipment protection becomes an issue at total intensity above ~0.5mA total beam current • beam interlocks foreseen to protect cavities and loads from power coupled out of beam • with 43 bunches of 4x1010 we are at about 0.6% of nominal, or 3mA • i.e. still far away from any beam stability problems • however we may already need to connect beam interlocks for RF equipment protection

4. Low Level RF system

5. Cavity control feedback loops • Tuner Loop: Adjusts resonant frequency of the cavity to minimize klystron current. Commissioned and available from Day 1. • “Half detuning” technique used to minimize klystron power transients – requires knowledge of incoming beam intensity before first injection (timing/telegram) • RF Feedback Loop: Reduces the cavity impedance at the fundamental (by factor of 20 for Q=20000, by 180 at Q = 180000). Transient beam loading + longitudinal stability. Commissioned and available from Day 1. • 1-Turn Delay Feedback: Adds another factor 10 reduction on the revolution frequency side-bands. (Transient beam loading + longitudinal stability). Will not be commissioned on Day 1.

6. Half detuning • Abort gap ~ 3 ms • Half detuning principle: • If cavity tuned for beam current Ib, Ig minimal when beam present, but large power surge during abort gap • If cavity tuned for zero beam current, Ig minimal during abort gap, but large power surge during beam segment • Half detuning = detuned for Ib/2: • Modulus of Ig constant • Imaginary part (w.r.to Vacc) changes sign with/without beam Without half-detuning: beam current present It Vacc Ig -Ib Without half-detuning: beam current absent It=Ig Vacc Ig

7. Cavity control feedback loops • Tuner Loop: Adjusts resonant frequency of the cavity to minimize klystron current. Commissioned and available from Day 1. • “Half detuning” technique used to minimize klystron power transients – requires knowledge of incoming beam intensity before first injection (timing/telegram) • RF Feedback Loop: Reduces the cavity impedance at the fundamental (by factor of 20 for Q=20000, by 180 at Q = 180000). Transient beam loading + longitudinal stability. Commissioned and available from Day 1. • 1-Turn Delay Feedback: Adds another factor 10 reduction on the revolution frequency side-bands. (Transient beam loading + longitudinal stability). May not be commissioned on Day 1.

8. Will we need the 1-T feedback? • With RF feedback only, total cavity impedance ~ 0.4MΩ at the 400MHz fundamental • cf. instability threshold Rsh ~1MΩ at 400MHz for nominal beam • so even with nominal beam we would be just on the limit of being able to run with RF feedback only • With 43 bunches of 4x1010 ~ 0.6% of nominal intensity, we are a factor of 150 below the coupled bunch instability threshold at injection • (since the cavity step response ~10µs is large compared with the bunch spacing of ~2µs, the threshold scales approximately linearly with beam current) • Conclusion: it is not obligatory to commission the 1-Turn feedback at this stage

9. Beam control loops • Phase loop: Locks phase of RF onto beam (fast) to minimize emittance blowup. Can damp injection oscillations of a single batch. • Synchro loop: Locks phase of RF and beam onto frequency program (adiabatically, time constant >> phase loop). • Radial loop: Adjusts frequency to maintain radial position constant at the radial pickup. • The phase, synchro and radial loops must be commissioned with a single pilot once RF capture is achieved. • For a single batch, the phase loop will jump onto the injected beam and absorb the injection phase error. However for multi-batch injection the incoming batch will have a different phase from the circulating beam, and without the longitudinal feedback system we will have longitudinal blowup on subsequent batches. • Longitudinal feedback: Damps longitudinal oscillations due to injection errors and coupled bunch modes, using the main 400MHz RF system. Should ideally be commissioned before injecting more than one batch.

10. RF synchronization system • Synchronizes bunch into bucket transfer between SPS and LHC, generates pre-pulses for SPS extraction and LHC injection kickers. Commissioned and available from Day 1. • For multi-batch injection requires injection bucket number and ring identifier to be supplied on-the-fly via the timing system.

11. Summary • RF Power system may need connection to beam interlocks for equipment protection • RF feedback sufficient to avoid coupled bunch instabilities – no need for 1-Turn feedback • Longitudinal feedback should ideally be commissioned before injecting multiple batches if we want to control longitudinal blowup • Beam intensity, injection bucket number and ring identifier must be available via timing system