Superfast BPM Processor Scheme Stephen Molloy, QMUL Nano-Project Mini-Workshop at ATF

FONT at ATF Micron-level stabilisation of the ATF extraction line beam. Measure position of start of train, and correct the end. –To be accomplished within 56ns train. –Latency must be kept low. BPM processor should work in <5ns.

Processor Design Obtain difference between top and bottom striplines with a hybrid. Mix with 714MHz from ATF control system. Filter to remove unwanted mixer products and beam bunching power.

Filters 3-pole Bessel band pass filter before mixer. –Centred at 714MHz, bandwidth ~400MHz. –Reduces 357MHz entering mixer. –Reduces out of band power entering mixer, thus increasing dynamic range of BPM. Less 357MHz means low pass filter requirements are relaxed. –Less poles results in faster filter. –3-pole Chebyshev with ~170MHz cut-off.

Issues Non-zero hybrid isolation causes zero-offset. –BPM centre can be moved with variable attenuation if necessary. Stripline -> hybrid cables must be matched in time to a few ps. –This is possible, and has been achieved. 714MHz LO input to the mixer should be very phase stable with respect to the beam. –Should be tuned to the correct phase. A lot of power at the beam bunching frequency. –Low-pass filter must limit this to a very small value before signal reaches feedback amplifier.

Phase Stability of 714MHz

Tuning of LO phase ● Split difference signal to two hybrids using zero degree splitter. ● Split LO using 90 degree splitter. ● Adjust phase of incoming LO so the phase measurement output has no dependence on beam position.

Simulated Output - 100μm

Simulated Output - 1μm

Comments 357MHz beam bunching is not observed. Power at 714MHz –Due to mixer leakage. –Level equivalent to DC output when beam has 1μm displacement. Predicted latency ~4.5ns.

Reminder!

Single Bunch - Raw Signals

Single Bunch - After hybrid

Single Bunch - After Band-Pass Filter

Single Bunch – After Mixer

Single Bunch – Final Difference Output

Multibunch – 3-pole band-pass filter

Calibration

Warning!!! The following slides show very preliminary data!!! They show the first results from closing the feedback loop with no optimisation of the system at all!!! In the following week work will be done to optimise the main loop gain, the delay loop gain, the timing of the amplifier turn on, etc., and we will then be in a much better position to demonstrate the effectiveness of this system.

Feedback – Difference and Sum

Normalised Feedback

Further Work ● Optimisation of the system, including ● Amplifier gain ● Delay loop gain ● Delay loop timing ● Beamline layout (to minimise latency) ● Automation of the LO phase adjustment ● Christine Clarke (QMUL) has developed a Matlab algorithm to optimise the LO phase automatically. ● Adjusts voltage controlled phase shifters via GPIB power supplies.