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

2. 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

3. 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

4. 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

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

6. Tests of the bare cavity
at FREIA

7. 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).

8. Unlocked Self-Excited Loop: an Ideal Resonator

9. Generator driven configuration
SC Resonator
Generator driven configuration
SEL

10. 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.

11. Self-Excited Loop for the spoke

12. Test plan

13. Day 1, zoomed in view

14. 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

15. Cavity re-conditioning: X-ray emission
1 hour

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

17. Q-slope measurements

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

19. 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

21. Microphonics

22. Microphonics measurements

23. Microphonics spectrum: dissipated power ~ 50 W

24. Microphonics in the time domain

25. Microphonics at low and high power
dissipated power

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

27. 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.

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

29. Dynamic Lorentz Force detuning

30. Amplitude modulation of the self-excited loop

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

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

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

34. 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

35. Comparison with the Argonne double spoke cavity