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Notes From SwissFEL

Notes From SwissFEL. A. AKSOY, A. LATINA. Layout. There are many things which can not be compressed Bunch compressors Beam transport Photon lines S-band section Do we gain enough with high gradient?. Stability. Amplitude and phase stability of X-band RF modules is critical;

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Notes From SwissFEL

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  1. Notes From SwissFEL A. AKSOY, A. LATINA

  2. Layout • There are many things which can not be compressed • Bunch compressors • Beam transport • Photon lines • S-band section • Do we gain enough with high gradient?

  3. Stability • Amplitude and phase stability of X-band RF modules is critical; • SwissFEL X-band linearizer requires • phase jitter <0.07 deg • amplitude jitter < 0.1%. • The jitter is dominated by the driver amplifier and the HV stability of the modulator. • the modulators pulse to pulse HV needs to be stabilized to better than 1e-10. • We should make calculation as soon as possible not only for RF but also alignment..

  4. Breakdown • SwissFEL operating @100Hz has breakdown-rate of 4x10^-8 • which means ~1 BD/h over the entire facility. • CLIC operating @500Hz has breakdown-rate of 10^-7 which means 50 BD/h. • Breakdown-rate tolerances are still very tight • In case of a breakdown, thermal variation can be such that the system gets detuned, and we will loose a lot of time for recovering • We should study for fast tuning (additional module or controlling the detuning..)

  5. RF issues • X-band will have high power density at pulse compressor (500MW for X-band) • can cause high breakdown-rate. • Even at C band the breakdown occurring in cavity is less than breakdown occurring in waveguide • We should be careful with the length of compressed RF pulse. • The filling time of the cavity is already about 100ns. • For a pulse length of 150 ns might be challenging to accelerate several bunches. • We should take into account stucture length

  6. Issues about 500 Hz repetition • Pulse to pulse RF phase and amplitude stability problems with the modulators The linear voltage level needs time for being corrected. • 100 Hz already gives problems but probably manageable.. • Cooling down, lot of heat dissipation which make the cavity engineering challenging. • Needs 4 times more cooling than 100 Hz. More operation cost (SwissFEL would be 25 MCHF/year) • Operation S-Band @500Hz would be challenging • Beam loss @500Hz due to dark current would increase cost of infrasturcure • SwissFEL building 160 MCHF for 100 Hz repetition • Photon detectors @400 Hz can work.

  7. Undulators • In general high “k” values are better: • they allow you change the undulator gaps to tune the wave length. • Photon flux is proportional to K. • Large K values require big undulator period length and higher beam energies • If one wants to tune undulator strength “K” • Planar standart is prefereble • Constant wake field from beam bipe • Easy operation and machining • .However there exist limitation on period length. • The wake due to small aperture varies wiht K for in-vacuum undulators • One needs to make fine tunning for each K • in this sense going to higher energy and small laser wavelength, standart undulators has advantage

  8. User Requirements; Hard X-ray • Running at 15 GeV would give 5 times more photons and relax the micro-bunch instability by a factor 2 • Undulators would be similar, max 60 meters long each but longer period • means easiest fabrication • Relax a bit the emittance requirements • Photon lines would be longer to avoid damages on the screen • ~150 m instead of ~100 m lines • Single molecule/crystal imaging will need about 20 GeV machine in order to get 10^34 photon /pulse. • But at higher energy the incoherent synchrotron radiation starts deteriorating the electron beam along the undulator line. • Very high photon density per pulse would make the machine unique. • This allows decreasing the size of the samples (easy crystallization) increasing the chance of getting defect free samples • Example Interesting physics at 14 KeV photon energy • “Mössbauer effect”. Very sharp edge of Fe.

  9. Soft x-ray • Energy would be around 2 GeV • User requires high average laser power • Users work with the idea that they don’t need to re-use the samples) • Superconductive RF is the solution / MHz rep rate (take LCLS-II) • However full CLIC time structure, 150 bunch /pulse and 500Hz rap-rate would be useful for soft X-rays

  10. Recommendation from SwissFEL: • Choose one good application for proposing FEL. High photon power at one photon pulse. E.g. Single-Crystal Imaging, which is not covered by any FEL.

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