The RF system provides a synchronous accelerating voltage to make up for the energy individual particles lose due to synchrotron radiation and the energy a bunch can lose from higher order mode losses. The amplitude of the RF voltage and the magnitude of the losses determine an equilibrium synchronous phase angle. The particles will oscillate about this equilibrium phase angle according to initial conditions or perturbations to the system. The stability of these oscillations is a key concern.

In addition, the RF provides a longitudinal restoring force that keeps the particles bunched together inside the RF bucket. The bunch length will also oscillate about an equilibrium value depending on initial conditions and perturbations. At high intensities the stability of these oscillations is also a key concern.

The orbit circumference determines the revolution frequency of the bunch but since we keep the RF frequency fixed the equilibrium energy of the bunch adjusts itself to match this frequency. The bending magnet strength determines the bending radius for a given energy, so the configuration value of the dipoles will determine the equilibrium energy of the beam.

If the beam is injected away from this equilibrium energy it will oscillate at the synchrotron frequency (~100 kHz) around this energy. We can observe this as transverse position oscillations on a bpm in a dispersive region, or as accompanying phase oscillations when the bunch phase is compared to the 714 MHz drive frequency.

Similarly, the bunch will make synchrotron oscillations if it is injected away from the synchronous phase. Moving the phase of the injected bunch w.r.t. the 714 MHz determines this matching.

Phase and energy are analogous to position and angle in transverse phase space. Just as we need two correctors in a beam line to steer the orbit flat, it is necessary to optimize both launch energy and injection phase to remove an injection energy oscillation.

The bunch makes oscillations in both phase and energy. Since individual particles in the bunch have slightly different energy and phase they occupy a certain area in the energy-phase plane which we call emittance in the longitudinal phase space. The magnitude of the RF voltage determines the size of the RF potential well, or bucket, that should contain the bunch. Primarily, our concern is that the bucket be big enough to contain the bunch.

The orientation of the bunch within the bucket does determine whether there are bunch length oscillations (quadrupole mode at twice the synchrotron frequency). Typically, we are not concerned with this at injection, only later when we induce a bunch rotation just before extraction.

Instabilities manifest themselves as sudden beam loss in the ring or as jitter in the extracted beam. At high current the tolerance of downstream systems to jitter decreases.

The synchrotron oscillations of the bunch can grow or decay depending on the damping mechanisms present. One such mechanism is the tune of the main RF cavity. As the tuning angle is made more negative the damping increases. If the tuning angle is positive the oscillations are anti-damped.

Since we prefer to run the cavities at a zero tuning angle to maximize the gap voltage external damping is provided by a synchrotron feedback circuit..

When 2 bunches are present they can also make synchrotron oscillations in anti-phase with one another, in which case the centroid of the bunches does not oscillate and the above damping mechanisms are ineffective.

For this case special Robinson damping is supplied by a pi-mode cavity tuned tuned near to an odd revolution harmonic of the beam.

At high intensities not only the bunch position oscillates but also the bunch length. The bunch length oscillations can grow very rapidly but they cease once the bunch grows beyond a certain size. This gives rise to a characteristic sawtooth behavior as the bunch blows up and then damps down again.

This instability is avoided by ramping the gap voltage so that the bunch length never damps down below the instability threshold.

An effect of ramping the voltage down is an increase in the relative beam loading in the RF cavity. This can lead to a Robinson zero-mode instability where the beam begins to oscillate at an arbitrary frequency.

Beam loading compensation feedback (Flemming loop) reduces this effect.

In addition to the all-unit displays there are several summary displays:

Shows temperature and vacuum interlock status,

cavity gap voltages and forward and reflected power from each cavity.

klystron and power supply interlocks,

klystron voltages and power.

Status, timing and phases or gains of:

• S-Band loop

• Synchrotron loop

• Gap voltage amplitude loop

• Klystron phase COMPensation loop

• Flemming loop

Tuner loop status,

tuner positions and tuning angles,

status of bypass switch for beam presence signal.

Summarizes timing for all sample and holds

and gap voltage ramp.

For each ring there are two dedicated remote scopes with dedicated control system triggers for diagnostics of RF and beam derived signals

A remote switch allows a choice of signals to be displayed on either channel of each scope.

Shows the phase of the bunch compared to the 714 MHz reference frequency.

Shows the phase of the bunch compared to the 714 MHz cavity frequency.

Shows the phase of the bunch compared to the linac 2856 MHz reference frequency.

Monitor signal from the obsolete synchrotron feedback module which shows the peak sampled BPM sum signal

Output from an amplitude detector connected to the cavity pick-ups.

Monitors the drive voltage to the klystron.

HP PK POWER ANALZR panel <- DRNG RSCOPE INDEX

Video output from the HP Peak Power Analyzer.

<- DRNG RSCOPE INDEX

Video output from the remotely operable HP Spectrum Analyzer.

<- DRNG RSCOPE INDEX

Video output from the remotely operable Tektronix Spectrum Analyzer with real time digital signal processing.

The cavity tuners are controlled by a feedback loop, accessed from the COMP panel <- Feedback index <- RF index.

The cavity tune changes with beam loading and so must be sampled at the correct time in the store cycle:

The tuner control module has a beam presence input to prevent the tuner loop from responding when there is no beam in the ring.

Tuner position readback is calibrated in cm

The tuning angle is read at two times during the beam store:

Refer also to the RF Cavity Display

The cavity voltage is sampled twice during the store to be compatible with the gap voltage ramp: once at injection when the ramp has its maximum voltage (A) and once midway in the store when the ramp is at its minimum (B).

On the summary display only A is shown.

The desired gap voltage is set by the gap voltage ramp controls. The gapv max and min are presently set only in units of DAC volts. When these devices are trimmed only the output of the controller is checked for the trim function. The gap voltages readback on the summary display are the actual cell voltages, but are No Control readbacks.

The gapv max is the default gap control voltage. Only when the gap voltage triggers are activated does the gap voltage ramp down to gapv min during the store cycle.

The klystron is capable of 60 kW peak in its present configuration with the ramp. The dual sample and hold reads power levels at the two times at the klystron and at the cavity waveguides.

see also memo by J.Judkins in DR Operating Manual.

Sets the phase of the damping ring RF w.r.t. S1

Reference phase when the S-band loop closes

Reference phase at the end of the phase ramp prior to extraction, relative to the linac.

This phase is pulsed to allow the two time slots to be tuned independently.

Phase of RTL compressor w.r.t. to phase ramp

In principle moves all phases together in the correct relationship

The stability criterion is the jitter tolerance in the linac and the occurrence of flier pulses. The RF setup must control the bunch length and beam loading

The gap voltage is set to maximum at injection for best capture of the beam. It is then ramped downward to prevent the onset of sawtooth instability. Just before extraction the voltage is ramped back up to reduce the bunch length, but done fast enough that the sawtooth does not have time to reoccur.

Beam loading compensation is provided by the Flemming loop. The loop stability is controlled by the feedback phase and the feedback gain.

This module provides an intermediate voltage to the cavities for the period of time when there is no voltage ramp during rate limiting. This is to keep the average RF power in the cavity the same and hence its temperature constant.

Bunch rotation minimizes the bunch length just prior to extraction in order to reduce chromatic effects in the RTL compressor region.

Immediate consequence of correct bunch rotation is a reduction in RTL PLIC

The bunch rotation is performed by making two, short, downward pulses in the RF gap voltage (munches).

The klystron can deliver around 60 kW peak power, beyond which it saturates so that the output power and hence the gap voltage is no longer linear w.r.t. the drive power.

There are a variety of conditions which can cause the klystron to saturate:

• the requested gap voltage is too high, GAPV MAX

• the ramp is incorrectly timed so that the GAPV MAX is held for too long a period in the cycle.

• the munch amplitude is too large

Several different feedback loops interact with varying response times within the RF control electronics. Refer to the schematic diagram. Stable operation require that they all function correctly.

• The slowest loop with a response time of the order of seconds relying on mechanical plungers.

• Ensures that the cavity is tuned to the correct frequency in the presence of beam. A bypass switch allows the beam presence to be over ridden for low current operation.

• The loop gain is set from the front panel of the Tuner Controller.

• Its control signal is derived from a phase comparison of the 714 MHz waveguide pickup signal and the cavity pickup signal (loading angle).

• Fastest loop operating at MHz around the klystron and cavity.

• Feeds the cavity pickup signal back to the klystron drive to compensate for the change in loading on the klystron as the bunch goes through the cavity.

• Stability is dependent on the remotely adjusted phase and gain of the fedback signal.

• Operates with a response of around 10 kHz.

• Ensures that the gap voltage follows the desired voltage function during the store cycle, but is not fast enough to respond to beam loading transients as the Flemming loop does.

• The amplitude detected signal from the cavity pickup is fed into the Gap Voltage Controller module in the DR Alcove racks.

• The gain of the loop is set on the front panel of the controller module

• The control signal from the module can be remotely monitored. If the signal exceeds 10 V at any time during the store cycle this is an indication that something within the loop is saturating (the klystron).

• Necessary to launch the beam into the linac.

• Phase locks the damping ring 714 MHz RF to the 2856 MHz linac S-band RF so that the beam can be put into the linac at a predefined phase.

• The phase lock loop only closes 1.7 ms after injection at which point the phase ramp can then move the bunch over to the desired linac phase.

• Adjusts the phase of the 714 MHz reference to compensate for phase changes that may occur across attenuators in the loop that are used for amplitude control.

• When the loop is closed the KLY_COMP_MON phase (PHAS 24) should be zero indicating the loop is locked.

• In the rare event that the loop loses lock because of radical tweaking, opening and closing the COMP Loop is usually sufficient to restore lock. PHAS 23 may need to be moved to bring the loop in range, but this is unlikely unless hardware has failed.

• Provides external damping of the synchrotron 0-mode bunch oscillations

• The beam phase measured from a BPM electrode is compared to the 714 MHz reference and fedback to a fast 714 MHz phase shifter.

• The feedback electronics can distinguish between two bunches in the ring and hence the 0 and the pi-modes of oscillation. Only the 0-mode is damped in this loop.

The pi mode cavity is an additional passive cavity added to the rings to provide Robinson-like damping to the pi-mode (coupled) oscillation mode of the two bunches.

Pi mode is when the two bunches make synchrotron oscillations in the ring in anti phase with one another. Without the pi-mode cavities this oscillation can grow unchecked.

When two equal bunches are in the ring only even revolution harmonics are generated in the bunch spectrum. If pi-mode oscillations occur the symmetry is broken and odd revolution harmonics also appear.

The pi-mode cavity is designed to absorb energy from this unwanted mode by tuning the cavity to the lower synchrotron sideband of an odd revolution harmonic (tuning it to an upper sideband would make the oscillations grow).

The cavity is tuned in the same way as the main RF cavities with mechanical tuners, but there is no tuner loop for the pi-mode cavities.

The cavities must be tuned to the correct frequency:

It is usually sufficient to restore the tuners to their configuration values, but the tune of the cavities can be measured as follows:

For single bunch operation the tune of the cavity may be too close to the revolution harmonic of the beam so that it absorbs a lot of RF power, outgasses and trips on vacuum.

The solution is to detune the cavity to a lower frequency: