1. 10 MHz amplifier status G. Favia
H. Damerau, V. Desquiens, S. Energico, M. Migliorati, M. Morvillo, M. Paoluzzi, C. Rossi
LIU-PS MEETING
02/05/2017

2. Outline 10 MHz amplifier after YETS 2015-16
Validation of the upgrade strategy of the 10 MHz cavities amplifier
Evaluation the total impedance of the 10 MHz RF system
Development of a model simulating the 10 MHz system
10 MHz amplifier after EYETS
Ongoing studies and potential improvements

3. Outline 10 MHz amplifier after YETS 2015-16
Validation of the upgrade strategy of the 10 MHz cavities amplifier
Evaluation the total impedance of the 10 MHz RF system
Development of a model simulating the 10 MHz system
10 MHz amplifier after EYETS
Ongoing studies and potential improvements

4. 10 MHz amplifier upgrade 𝑍 𝑐𝑎𝑣 = 𝑍 1+𝐴𝛽
10 MHz RF upgrade:
A new prototype amplifier has been built and installed in the PS ring during the winter technical stop in 2015
𝑍 𝑐𝑎𝑣 = 𝑍 1+𝐴𝛽
Loop gain
Higher loop gain ~6 dB more at 10 MHz
Improved stability
Vacuum tube configuration kept for radiation hardness reasons

5. Reduction of the impedance
Validation of the upgrade
To evaluate the impedance reduction measurements have been performed on the PS cavity 11, when driven by a standard amplifier and by the upgraded one:
Reduction of the impedance
by a factor two

6. Impedance measurements
n-TOF single bunch beam to neglect the multi bunch effect
Measurements of the beam induced voltage (no RF on the gap )
Single gap impedance
The impedance is computed from the beam induced voltage:
N.B: The total impedance is twice the impedance of one gap.

7. Evaluation of the PS 10 MHz cavities impedance
A measurement campaign has been performed for the evaluation of the contribution of the eleven 10 MHz cavities to the longitudinal impedance in the PS at h=8, 16 and 21
Upgraded
amplifier
𝑅(𝑍)[Ω]
Measurements of the impedance of all PS cavities provide an important input for the present campaign which aims at an impedance reduction and provide cures to instabilities, such as coupled-bunch instabilities.

8. Measurements at high RF voltage
Beam instabilities are observed during accelerations (~10 kVp /gap)
Additional measurements took place to analyse the performance of the PS cavities when the accelerating RF voltage is generated across the gap
Induced voltage spectrum
Upgraded amplifier
Beam spectrum
Standard amplifier

9. Benchmark of simulation model (Pspice)
Time domain simulations were carried out, and were found in good agreement with the measurements.
measurements
simulations

10. Benchmark of simulation model (Pspice-CST)
The two descriptions have been combined into an innovative model running in the CST environment
effectiveness of Pspice in describing the amplifier chain behaviour
accuracy of CST in modelling the cavity resonator
amplifier non-linearities
ferrite dispersive properties
feedback reduction
The model includes:

11. Outline 10 MHz amplifier after YETS 2015-16
Validation of the upgrade strategy of the 10 MHz cavities amplifier
Evaluation the total impedance of the 10 MHz RF system
Development of a model simulating the 10 MHz system
10 MHz amplifier after EYETS
Ongoing studies and potential improvements

12. Miller effect issue cavity CM final ~6 dB Local loop ~30 dB
Predriver +
Driver
~6 dB
Local loop
~30 dB
Close to the anode (bare cavity) resonance the amplifier stage operates with positive feedback when the source frequency lies below the nominal frequency, and with negative feedback when it is above.
MILLER EFFECT
The peak of the anode resonance is thus shifted both in amplitude and, since the amplifier output impedance is complex, in frequency.

13. Neutralization 3 MHz TOTAL LOOP ~26 dB CM z 10 MHz -CM LOCAL LOOP
Predriver +
Driver
10 MHz
-CM
LOCAL LOOP
~22.5 dB

14. Alternative loop gain measurements
Measurements of the loop gain with final off/on can provide an estimation of the loop gain
~25.5 dB
~28 dB
Measurement method validated by simulations
3 dB improvement
Further investigations needed
~21 dB
~24 dB

15. Outline 10 MHz amplifier after YETS 2015-16
Validation of the upgrade strategy of the 10 MHz cavities amplifier
Evaluation the total impedance of the 10 MHz RF system
Development of a model simulating the 10 MHz system
10 MHz amplifier after EYETS
Ongoing studies and potential improvements

16. Solid state upgrade strategy
2015 upgrade main improvements:
Change of tubes working point→higher forward gain
Compensating network & new grid resonator→improved stability
Maximum performance achieved with the present vacuum tube configuration
𝑍 𝑐𝑎𝑣 = 𝑍 1+𝐴𝛽
Higher forward gain needed → a solid state upgrade solution is under development

17. Preliminary simulation results
Solid state upgrade strategy
Predriver + driver replaced by a single transistor-based stage + driver stage→ HIGHER FORWARD GAIN
Two blocks using two ARF479 (~420W) power mosfets in push-pull configuration (class AB operation)
MAIN STAGE
Preliminary simulation results
~30 dB
DRIVER
Up to 300V on the final grid achievable
Power
Combiner
75 Ω
Grid resonator
Two VRF151 power mosfets in push-pull configuration
(class A operation)

18. 2017 plan
Measurements of the impedance of the new amplifier prototype installed in cavity 11
Engineering of the vacuum tube upgrade solution
Finalization of the studies and development of a solid state upgraded prototype by September 2017

19. Conclusions
An upgrade strategy of the 10 MHz amplifier has been validated: an impedance reduction by a factor can be achieved
A measurement technique of the cavities impedance has been developed
An innovative combined Pspice-CST model has been developed and validated; it will be used for the beam-cavity interaction studies for the remaining PS cavities
There are still some uncertainties on the effective loop gain of the amplifiers
A solid-state upgrade strategy is under development: it aims at achieving an higher impedance reduction at 10 MHz

20. Thank you
for you attention