Microphonics Discussion For LLRF Design Review Tom Powers 13 June 2016 Not for release outside of JLAB There are several MSWord documents located at: M:\asd\asddata\C100Microphonics2016

LLRF Implications due to Microphonics Background microphonics. Requires more RF power and makes the cryomodule more susceptible to trips due to outside excitation. Phase/RF Power signals in excess of about 60° / 12 kW will cause the cavity to loose regulation and trip off. Microphonic transients driven by outside sources, such as trucks and construction cause cavities to trip. When in SEL mode, with no gradient regulation, microphonics causes perturbations in cavity gradients which can lead to ponderomotive instabilities or trips/quenches if they are operated near the prompt quench field or other gradient driven limits. “Excessive” energy jitter due to microphonics that is correlated from cavity to cavity as well as, occasionally, from cryomodule to cryomodule.

Microphonic Mitigation Activities Completed and documented. – Log and review extensive data for. Background tuner motion during down. 0L04 trips during operations Background microphonics for C100/R100/F100 Energy stability on cavity by cavity basis (synchronous in each cryomodule) for the south linac as well as for the machine in Arc 1 and Hall A line. – Preliminary tests on tuner stack damper – Waveguide mechanical mode transfer functions SL22 – Waveguide/beam pipe/ion pump to cavity transfer function F100 Near Term Future – Design install, test and optimize 2 or 3 waveguide constraint/damper designs. – Choose the “best” solution and install on all C100/R100/F100 cryomodules. – Test and optimize the prototype tuner damper on the F100 and repeat on a C100 that has waveguide constraints installed. – Investigate the 100 Hz noise and try to mitigate. – Investigate and optimize the setup of the extra CM feet for reduced microphonics. – Monitor a cryomodule during a thunderstorm using accelerometers and low power cavity resonance monitor.

Example of Microphonics Driven Trip Normal 20 Hz dominated microphonics for extended period. 40 Hz Burst on cavity 8 followed by trip on cavity 5 Similar events recorded for trips initiated by cavity 1 40 Hz is a typical waveguide mode 0L04 Trip recorded in Feb

Energy Jitter in Hall A Line The energy jitter between 8 and 40 Hz is from the C100 cryomodules. It represents about 30% of the energy jitter in the CEBAF. While it may be OK for most physics experiments it does lead to moving beam spots in the arcs and can not help the beam aperture. This could be reduced if more gain could be applied to the cavity feedback loops at low frequency. Also there is a pulsed energy instability leading to two beam spots during 5-pass tune up operations.

Waveguide Transfer Function Examples Upper Strike WG 8 perpendicular to beam line lower transition flange. Measure acceleration at Waveguide transition lower flange (EW is in direction of Beam line) Lower Strike WG 8 perpendicular to beam line lower transition flange. Measure WG just before it enters the vacuum vessel

Example of Tuner Damper Improvements (There is Hope) In both cases the excitation was an impulse strike just above the beam line entrance to the cryomodule up stream end. Red is microphonic frequency transfer function. White is acceleration transfer function of the top of the tuner motor along the beam line direction. Upper is without a tuner damper Lower is without tuner damper struts. 12 Hz is the string mode for the F100 9 Hz is the tuner stack vibrational mode.

Known and Suspected* Modes LCW Tuner Stack “10 Hz” String mode “20 Hz” Half String Modes *Waveguide 5 *Waveguides 5, 8 *Down Stream Ion Pump *Waveguides 2, 6 *Waveguides 7 Single Cavity modes plus *Various Waveguide Modes NL22 Data Taken in GDR mode on 2 Feb. 2016, no sandbags

Microphonics and controls Spectrums

Transient Effects as Compared to Steady State Microphonics NL22 with polybead-bags on Tuner Stacks 4 and 5 Full Bandwidth Shown Area of Interest Note: According to all published data +/- 20 Hz should be OK

Transient Effects as Compared to Steady State Microphonics NL22 with polybead-bags on Tuner Stacks 4 and 5, Full Bandwidth Shown Step is probably cavity tuner operating

Transient Effects as Compared to Steady State Microphonics NL22 with polybead-bags on Tuner Stacks 4 and 5, 70 Hz to 500 Hz content Shown Same Scale as previous plot. It is not clear if this is when the tuner is running or just when it is starting to run.

Transient Effects as Compared to Steady State Microphonics NL22 with polybead-bags on Tuner Stacks 4 and 5 70 Hz to 500 Hz content Shown Same as last slide with decreased vertical full scale, also slowing all cavities Note that 5 Hz is not considered an excessive amount of microphonics.

Output RF Phase Signal Full Bandwidth Same data set as the previous slides only it is the RF phase for the klystron output rather than the cavity frequency shift. Note: A 60° phase shift is about all that the FCC can do without going unstable.

Output RF Phase Signal 70 Hz to 500 Hz content Shown Content between 70 and 500 Hz. Note the peak excursions due to the just 100 Hz component are almost big enough to cause a fault. Also remember that the 100 Hz microphonics excursion was less than +/- 5Hz.

Concept for Frequency Dependent Gain

Needs/Issues For Dealing With Microphonics Work to understand the source of the 100 Hz pulsed driving terms. If it is the tuner motor investigate the parameter space for ramp speed, velocity and micro stepping rates to minimize it. Work to understand and eliminate the microphonics that seem to be driven (i.e. 30 Hz, 40 Hz, etc.) Continue the effort reduce the susceptibility of the CMs to external vibrations. If possible use the waveguides and tuner ports as a means to damp microphonics that are introduced into the system. Flexible frequency parameters in the gain loops. Expend efforts to reduce the gain at higher frequencies with a goal of reducing excessive 100 Hz driven phase excursions while improving regulation at microphonics frequencies. This implies more than just turning up the integral gain term. Separate out phase and magnitude loops, either though an I/Q phase rotation or by using phase and amplitude control loops so that different gain / frequency domain parameters can be applied to each. Provide amplitude and phase error signals to both the DAC ports and EPICS scope tool so that the errors are out of the bit noise on both. Provide an appropriate scaling on both. Provide triggering synchronous to tune up beam so that the gains can be set to optimize both CW and pulsed operation. Determine if feed forward is necessary/desired for pulsed operation.

Backup Slides

Microphonics Spectrum Examples 2L24 one C100 3 Note: Modes at 40 Hz, and 30 Hz are not modes of cavity string.

Examples of Pulsed Detuning with 90 to 150 Hz Content RF Phase Shown, Data filtered BPF 70 Hz to 500 Hz C100 < 4 C100 >3