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PS Landau RF System Study Group
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Europe/Zurich
864/2-B14 - SALLE J.B.ADAMS (CERN)
Present: R. Calaga, H. Damerau, N. Esfahani, G. Favia, A. Lasheen, M. Morvillo, M. Paoluzzi
Follow-up from last meeting and updates
- Heiko reported on the final beam tests with the existing 40 MHz cavities as Landau RF systems during acceleration. Both cavities perform differently with beam. As expected from earlier RF measurements, C40-77 can only produce ~60% of the voltage of C40-78 when operated far below the resonance. Nonetheless both cavities together can provide ~28 KV in total starting from 175 ms after transition. This corresponds to 17% of the main RF voltage.
- Due to the limited bandwidth it is not possible though to operate the cavities closer to transition, hence they remain a Landau system which can only be used at beam energies above ~13.5 GeV (total).
- Final beam tests with protons have been performed by operating all feedback systems together and adding stability margin with the existing 40 MHz system for Landau damping during the second part of acceleration and at the flat-top. The LIU intensity (2.6 1011 p/b) can be accelerated not only with nominal longitudinal emittance (0.35 eVs, Tomoscope foot tangent fit), but also its 10-15% smaller emittance. Actually even reducing the emittance further would still not be an issue for longitudinal stability, but for the beam loading in the 40 MHz and 80 MHz cavities. Heiko stressed that a wealth of data at the LIU intensity has been recored during the 2018 proton run.
- The successful operation of the existing 40 MHz cavities as Landau RF system, imposes the question if a dedicated Landau system is really justified. Following the review meeting in September, R. Garoby also recommended that the 'Dedicated Landau RF system development is valuable but not necessary'.
- A checkpoint meeting with the LIU management will therefore be held on 24/01/2019 with the objective to (i) summarise the current status of dedicated Landau RF system (three cavity options below) and to (ii) review the need of a dedicated system based the beam measurements from the 2018 run. Heiko showed first ideas for the agenda.
Updated parameters for cavity options (Giorgia, Mauro, Nasrin)
- Nasrin presented the updated parameters for the Garnet tuned cavity options (single gap), the first variant with a mushroom capacitor and minimum amount of garnet material, the second variant with no capacitive shortening (other than the gap capacity) and more garnet material. The RF power required for both cases is with 32.5 kW and 35 kW rather similar (for a phase offset between RF and beam of 30 deg). This power requirement is dominated by the over-coupled amplifier. The design choices remain unchanged for an RF voltage of 2x20 kV instead of 2x25 kV, hence the amplifier tube is a RS1084CJ.
- Georgia showed the parameter table for the ferrite options (double gap), operating at 30 MHz or 40 MHz. For the latter case, the required RF power, again for phase offset between RF and beam of 30 deg, is almost 37 kW. This is comparable to the garnet option, although the quality factor is smaller. Again the RS1084CJ tube is chosen for the amplifier. The beam impedance of both garnet and Finemet cavity options is defined by the direct feedback, which is mandatory. The final amplifier must thus be close to the cavity.
- Heiko showed the parameter table for the Finemet option, based on the measurements with the 4-ring shroud presented by Mauro during the last meeting. He suggests two options: a double-gap resonator with twice 8 rings (1.25 kV/ring) and a shorter variant with twice 5 rings (2 kV/ring). Due to the inherent shunt impedance and large bandwidth of the cavity, the amplifier can be located outside the tunnel, hence choosing solid state technology. The RF power for the latter option to reach a total voltage of 20 kV per cavity is about 50 kW and about 70 kW for a phase offset of 30 deg. In contrast to the other options, only passive techniques (e.g., mismatched feeder line) could be applied to reduce the beam impedance.
- Mauro commented that the shroud set-up with the PS Booster rings (smaller diameter) was realistic model scaled in radial direction. He also suggested to keep the variant with 2x5 rings as baseline and confirmed that the higher shunt impedance measured (presented during the last meeting) was due to magnetic and electric coupling (coupling factor n2 = 1.86).
- Heiko asked Alexandre to check again the influence of the inherent beam impedance of the Finemet option on longitudinal beam stability.
Follow-up
- Preparation of checkpoint meeting with LIU management (main topic of next working group meeting).
- Measurements with Finemet cavity shroud.
- Tests of small garnet and ferrite samples.
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