Minutes of the SPL working group

meeting no. 87

date: 9. September 2008

present: G. Bellodi (AB/BI), S. Calatroni (TS/MME), C. Carli (AB/ABP), F. Caspers (AB/RF), E. Ciapala (AB/RF), C. De Almeida Martins (AB/PO), M. Eshraqi (AB/ABP), R. Folch (TS/MME), R. Garoby (AB/BI), F. Gerigk (AB/RF), S. Lanzone (AB/ABP), R. Losito (AB/ATB), M. Paoluzzi (AB/RF), M. Poehler (TS/CE), C. Rossi (AB/RF), J. Tuckmantel (AB/RF), M. Vretenar (AB/RF), U. Wagner (AT/CRG), R. Wegner (AB/RF)

agenda

1. General news, report from LHeC workshop (Roland Garoby)
2. Summary of HB'08 workshop (Frank Gerigk)
3. Klystron modulators for the LP/HP-SPL (Carlos De Almeida Martins)
4. Ideas to recover the stored energy from SC cavities (Fritz Caspers)
5. next meeting
6. linked documents in EDMS

1. General news, report from LHeC workshop (Roland Garoby)

• rate of SPL meetings: after a period, where the steering group worked on the frequency review it is now planned to raise again the frequency of the SPL study group meetings to approximately one per month,
• publications: The paper on the the layout of the new LHC injectors is ready and publication is imminent. The report on the SPL frequency review is expected for the end of September.
• collaborations: An agreement was signed with SNS, which should make it easier for people to visit the other institute. ESS-Sweden: for the beginning of 2009 2 people are expected at CERN, which are paid by ESS-S and who will be trained at CERN in topics related to high-power linacs. The two will start on beam dynamics and high-power RF. Two more will be working on target issues. A meeting with ESS-Bilbao will take place tomorrow (minutes). Cockroft institute: no news yet. Canada: expressed a will to contribute to the LHC upgrade, and they are interested in SC cavity technology.
• LHeC workshop: took place last week in Divonne. A follow-up workshop will take place in one years time. The idea is to continue the experimental work from DESY on electron/proton collisions, using protons/ions from LHC and electrons, which are supplied by a seperate accelerator. The favoured option seems to be to get the electrons from a linac, which might then be further accelerated by a ring machine (2nd stage). For this option one could use the high-energy part of the SPL as an electron re-circulator, providing a beam energy of ~20 GeV. The studies are still at the beginning, but could result in more interest and support for the SPL. It is clear that if this option has be foreseen, it will certainly increase the price tag of the initial SPL installations.
• Linac14: there is an effort to attract the Linac14 conference to Geneva (or surrounding area).

2. Summary of HB'08 workshop (Frank Gerigk)

F. Gerigk presented some highlights from the recent HB'08 workshop in Nashville. Some important news are:

• SNS reached 0.5 MW beam power and they are not yet loss limited in the linac! The limit for higher duty cycles is given by failing klystron modulators.
• SNS is advancing on laser H- stripping and they applied their formulas to the SPL: Above 3.5 GeV one use infrared light. Using 0.5 MW peak laser power on could get 98% stripping efficiency. In an idealised case one could strip LHC type beams with an average laser power of 0.01 W!
• SARAF has commissioned the first cryo-module.
• PEFP is now running a 20 MeV DTL at the KAERI site with very low duty cycle. The machine will be moved to a new site, and extended to 100 MeV. Future plans include a 100-200 MeV SC linac at 704 MHz, and an RCS.
• GSI is working on a code to understand the dynamic vacuum behaviour in SIS18 and SIS100.
• FNAL is pursuing the HINS project, a 60 MeV linac using first normal- and then SC spoke cavities together with SC solenoid focusing. They have moderate funding to continue construction and R&D.
• ISIS is settling on an upgrade plan involving i) a new 3 GeV RCS, and ii) a new 800 MeV injection linac replacing the present ISIS RCS. The ultimate goal is 2 - 5 MW primarily aimed at neutron production.
• ESS-Bilbao presented an updated linac design based on 352/704 MHz.
• ...

3. Klystron modulators for the LP/HP-SPL (Carlos De Almeida Martins)

C. De Almeida Martins presented the klystron modulator options for the LP and HP-SPL. The LP-SPL modulators will be based on the topology used in the 3 MeV test stand, which relies on a pulse transformer and a solid state high-voltage switch. This approach can probably be pushed to 10-15 Hz, but is not suitable for higher repetition rate (and higher duty cycle), because it is unlikely that one can find an HV switch or pulse transformer that can stand the strain. For this reason it is proposed to use a different topology for high-duty-cycle operation, based on the SNS approach. However, to make the system more reliable, one should use 4-5 independent modules in series, rather than a star connection with 3 modules as it is used for SNS. This should make it easier to ensure "soft switching" (switching when either the current or the voltage is close to zero). First simulation results look promising: even with a short circuit at the output, the system could run for hours without any damage, because it is self-limiting in current. The R&D effort is estimated at ~20 FTE's distributed over 5 years, together with a hardware budget of ~1.2 MCHF.

For the time being it is clear that the SPL study cannot invest these resources, so we will have follow closely the SNS development and see if a solution can be found to make their design approach more reliable.

4. Ideas to recover the stored energy from SC cavities (Fritz Caspers)

F. Caspers reported on the possibility to re-use the stored RF power from SC cavities in the SPL, which is otherwise reflected into an RF load. Recovering the reflected energy would require a four-port device after each klystron, instead of the standard circulator, which is foreseen in the present scenario. This would certainly add volume and cost to the RF distribution system. The most promising idea is based on the conversion of microwaves into a DC voltage, which could then be re-used to power the klystron modulators. First tests have achieved up to 80% efficiency and produced a voltage of 15-20 kV DC. The experiments were done for around 10 kW, and a frequency of 2.45 GHz, but they should be scalable to 704 MHz. A method that seems less promising is based on a "rectanna" (rectifier antenna), see Wikipedia.

Further suggestions:

• Autoresonance laser accelerator (Loeb & Friedland) (PR A)
• Theory of electron-cyclotron-resonance laser accelerator (Chen) (PR A)

5. next meeting: 15th October 2008, 9:00 in 864-2-C30 (Salle J.B. Adams)

6. Linked Documents in EDMS

• Summary of HB'08 (F. Gerigk)
• Klystron modulators for the LP/HP-SPL (C. De Almeida Martins)
• Ideas to recover the stored energy from SC cavities (F. Caspers)

-- FrankGerigk - 18 Sep 2008