Recent LFD Control Results from FNAL Yuriy Pischalnikov Warren Schappert TTF/FLASH 9mA Meeting on Cavity Gradient Flatness June 01, 2010

Resonance Control at FNAL Recently tested first blade tuner equipped cavity in HTS at FNAL Two piezos to control LFD

Measuring LFD Complex envelope of the probe signal described by 1 st order ODE Rearrange terms to extract the width and the dynamic detuning from the probe and forward IF signals

Correcting the IF Signals Accurate detuning calculation requires accurate LLRF signals Change the piezo bias and examine IF signals for – Saturation Decay of probe and reflected should be exponential – Cross-contamination of forward and reflected signals Amplitude and phase variations of the forward power during the fill and flattop Tails on forward power during the decay

Uncorrected and Corrected Signals Applied corrections based on assumption that forward tail is due exclusively to cross- contamination by reflected signal – Assumption is likely only partially true Correction reduces the amplitude and phase variation and forward signal from 5-15% to a few percent

Delay Scan Developed and tested in CCII Excite piezo with a series of short pulses at various delays Measure the dynamic detuning for each pulse

Response Matrix Subtract the mean detuning (LFD) to get response matrix Invert the response matrix and determine combination of pulses needed to cancel out the mean using LS

Measuring the Cavity Width Plot of P*(P- 2*F) vs P*dP/dt should give straight line through origin Provides a consistency check on correction and detuning calculations

Dynamic Detuning Measure the dynamic detuning at 35 MV/m during the pulse – Using long fill and short flattop because of HTS klystron limitations – Each curve is detuning at different piezo bias Maximum detuning ~1 kHz during fill and flattop

Standard Detuning Compensation Manual adjustment of pulse parameters – Flat phase during flattop at 33 MV/m – Demonstrates that cavity can be tuned at high gradients

LS LFD Compensation Implemented an adaptive version of the LS procedure that worked successfully in CCII Able to maintain flat phase during both fill and flattop Able to track the resonance as cavity was ramped down from 15 MV/m to 35 MV/m and back up again Flattop square and phase flat to few degrees at 35 MV/m LFD reduced to level of microphonics

Residual Detuning Detuning at 35 MV/m after compensation is comparable to microphonics – 50 Hz Peak – 15 Hz RMS Probably limited by low pass filter in FF loop – May be possible to reduce residual detuning further by increasing the LPF cutoff frequency

One Possible Approach Cavity response described well by exponential decay For a given beam current and Q, can determine the forward current required for a flat field Piezo could be used to move cavity off resonance for a short period early in the fill – Provides individual control of the effective fill time of each cavity Optimum detuning profile during fill TBD

Summary Excellent results with LFD so far, more progress to come – Reduce LFD from 1kHz (fill+flattop) to <50Hz peak at 35 MV/M May be possible to control field flatness of individual cavities by detuning them during the fill – Optimum detuning profile during fill still to be determined Piezo is still provides very crude control when compared to LLRF – Response time limited by mechanical resonances of cavity and inertial of mechanical tuner Response time may limit to feedforward – Passive only Piezo can divert power from cavity to reflected and vice versa but cannot inject any power into the system