Test of the dressed spoke cavity Vitaliy Goryashko on behalf of the FREIA Group October 2014

Cold RF Tests spoke cavity DEDICATED RF TESTS CW, HIGH GRADIENT cavity Q_0 vs Eacc microphonics transfer function of the Lorentz force detuning parameters of mechanical modes BASIC RF TESTS VERY LOW POWER calibration loaded Q cavity spectrum cavity tuning spoke cavity STANDARD RF TESTS PULSED, HIGH PEAK POWER accelerating gradient X-ray emission multipacting barriers cavity tuners ACCELERATOR MODE PULSED, HIGH PEAK POWER nominal voltage profile fast RF feedback feedforward with piezotuners

Equipment and Power Levels DEDICATED RF TESTS CW, up to 100 kW phase-locked LLRF (self-excited loop) high-power amplifier cavity resonance monitor (built in into LLRF) BASIC RF TESTS some mW VNA spectrum analyzer scope spoke cavity STANDARD RF TESTS PULSED, up to 400 kW signal generator high-power amplifier data acquisition system scope with I/Q mode spectrum analyzer ACCELERATOR MODE PULSED, up to 400 kW signal generator high-power amplifier LLRF with feedback and feedforward

Q-slope: can we do it at FREIA? Calorimetrical measurement of Q0: JLab procedure close inlet and outlet valves of the cryomodule record the pressure as a function of time apply a known amount of resistive heat to the helium again record the pressure a function of time build the calibration curve: the rate of pressure rise vs. heat load apply RF to the cavity, record the pressure rise calculate the dynamic RF load using the calibration curve

Lorentz Force Transfer Function for a 325 MHz Spoke; ANL Dangerous frequencies that must be avoided in the cavity environment

Tests of the bare cavity at FREIA

Self-Excited Loop (SEL) no need for frequency tracking during turn-on or amplitude ramping quickly bring up cavity gradient without running the tuners free of electromechanical instability J.R. Delayen, “Phase and Amplitude Stabilization of Super-conducting resonators”, Ph.D. Thesis, Caltech (1978).

Unlocked Self-Excited Loop: an Ideal Resonator

Generator driven configuration SC Resonator Generator driven configuration SEL

Unlocked Self-Excited Loop: FREIA test bench A perfect sinusoidal signal amplifier attenuator A signal from the cavity is sent back to the amplifier through the attenuator. cavity The next step is to close the phase loop.

Self-Excited Loop for the spoke

Test plan

Day 1, zoomed in view

Day 2, zoomed in view Q-circle method: Makes use of the NI system Time-consuming (~10 hours per test) but fully automated by Krish

Cavity re-conditioning: X-ray emission 1 hour

Transmitted signal upon re-conditioning re-conditioning started 15 minutes later

Q-slope measurements

VNA Standard reflective type measurement forward cavity reflected Reflection coefficient S11 Frequency

cavity Self-excited loop cavity reflected signal limiter forward drive signal 10 KHz span phase shifter amplifier The ratio of the reflected signals to the forward one gives S11

Microphonics

Microphonics measurements    

Microphonics spectrum: dissipated power ~ 50 W

Microphonics in the time domain

Microphonics at low and high power dissipated power

Quasi-static Lorentz Force Detuning The loop amplitude is modulated with a 10-sec period.

Quasi-static Lorentz Force Detuning cont’d The loop amplitude is modulated with a 20-sec period. The 7.7 Hz resonance is suppressed and the overall microphonics level is reduced.

Quasi-static Lorentz force detuning: 20-sec Hz/(MV/m)^2  

Dynamic Lorentz Force detuning

Amplitude modulation of the self-excited loop

Cavity voltage and frequency modulation 3 Hz 3 Hz 100 Hz 100 Hz

Cavity voltage and frequency modulation cont’d 318 Hz 318 Hz 600 Hz 600 Hz

Transfer function of the dynamic Lorentz force detuning noise goes up because the cavity voltage modulation goes down

Guillaume’s simulations of the cavity mechanical modes Frequency Mode 1 & 2 212Hz Beam tube on CTS side 3 & 4 265Hz & 275Hz Spoke bar/Helium vessel 5 & 6 285Hz Coupled mode Cavity/Helium vessel 7 313Hz Helium vessel 8 to 11 315Hz to 365Hz 12 396Hz beam tubes Mode 1: 212 Hz Mode 3: 265 Hz Mode 5: 285Hz Mode 12: 396Hz

Comparison with the Argonne double spoke cavity