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

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 2016-17 Ongoing studies and potential improvements

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 2016-17 Ongoing studies and potential improvements

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

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

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.

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.

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

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

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:

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 2016-17 Ongoing studies and potential improvements

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.

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

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

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 2016-17 Ongoing studies and potential improvements

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

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)

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

Conclusions An upgrade strategy of the 10 MHz amplifier has been validated: an impedance reduction by a factor 1.5-2 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

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