Superfast BPM Processor Scheme and First Results Stephen Molloy, QMUL 2 nd Mini-Workshop for Nano-Project 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 from hybrid and mix with 714MHz from ATF control system. Low-pass filter with 200MHz cutoff –5-pole Chebyshev chosen due to low latency, and strong out of band fall-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 better than ~50ps. –This is possible, and has been achieved. 714MHz LO input to the mixer should be very phase stable with respect to the beam. 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

Simulated Output - 100μm

Comments Positive points –Simulated output is linear with beam position. –Hybrid common-mode residual shifts BPM centre in a predictable way. –Imperfect cable lengths (within achievable limits) merely shift BPM centre. –Mixer leakage easily reduced by low-pass filter. Potential problems –Relatively large amount of power at beam bunching frequency remains after low-pass filter. –Predicted latency of entire system is ~6ns. Remember latency should be <5ns for feedback experiment.

Alternative Filtering 3 or 4-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. –Total latency should be equal to or less than the previous scheme as the band-pass poles have a larger bandwidth than the low-pass poles

Simulated Response of 3-pole Bessel BPF

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 –Longer by ~0.5ns when 4-pole band-pass filter is used.

Recent Beam Tests Single bunch tests –Verified cable lengths were correct. –Stepped through each component in turn and verified signal. –Found correct LO phase. –Verified output was correlated with beam position. –Measured latency.

Recent Beam Tests Multi-bunch tests –Calibrated each of three processors using corrector magnets. –Recorded many extraction pulses to measure resolution.

Reminder!

Single Bunch - Raw Signals

Single Bunch - After hybrid

Single Bunch - After Band-Pass Filter

Single Bunch – After Mixer

Single Bunch – Final Difference Output

Single Bunch - Corrector Sweep

Latency Measurement Triggered the scope with a sum signal. Recorded raw stripline signal, and final output of processor. Results –3-pole band pass scheme – ns –4-pole band pass scheme – ns –No band pass, 5-pole low pass – ns

Multibunch – 3-pole band-pass filter

Multibunch – 4-pole band-pass filter

Multibunch – No bandpass, 5-pole lowpass

Calibration Run – BPM13

Bunch charge during Dec 9 th shift