Initial Studies of a Possible Option for the NLS based on X-band Technology R. Bartolini, C. Christou, J-H. Han, R.P. Walker Diamond Light Source, Oxfordshire, UK WORK IN PROGRESS !

The New Light Source (NLS) Project Ultra-fast Electron Dynamics and Attosecond Science, Imperial, May 2008 High Energy Density Science, Rutherford Lab., May 2008 Condensed Matter, Rutherford Lab., May 2008 Chemical Sciences, Daresbury Lab., May 2008 Advanced Photon Sources, Daresbury Lab., June 2008 Life Sciences, Diamond, June 2008 NLS Science Community Meeting, Royal Society, April 2009 Chemical Science Research at NLS, U. Manchester, June 2009 New Biological Imaging Possibilities at NLS, U. Cambridge, Sep Condensed Matter Research at NLS, Imperial Coll., Oct Materials under Extreme Conditions, U. Edinburgh, Oct Launched in April 2008 by STFC, supported by Diamond, to consider the scientific case and develop a conceptual design for a possible next generation light source based on a combination of advanced conventional laser and free-electron laser sources. Science Consultation Project Leader: Prof. J. Marangos, Imperial College, London

2.25 GeV cw superconducting linac for high repetition rate of regularly spaced pulses; initially 1 kHz increasing in later stages to kHz Three independently tuneable FELs in the 50eV – 1keV range, with harmonics up to 5 keV Short pulse duration ~20fs Synchronized to THz and laser sources Variable polarization Seeded FELs producing reproducible, transversely and longitudinally coherent pulses Based on the science case the following machine design emerged:

“The NLS project would have very high impact. It would have a major lead in both a national and international context. It would be a unique, world leading facility in the area of biological imaging and would open up exciting new research areas and develop new communities.” Report from STFC Science Board (Dec. 16 th 2009) and that “STFC re-assess the NLS project in 3-5 years time in order to ensure that STFC considers future user needs”. End of the official NLS Project: Conceptual Design Report, STFC, May 2010 available from: However, given the budget available, Science Board recommended: “no further funding for NLS development at this time”

A Normal-Conducting X-band NLS ? The cw superconducting linac technology chosen for NLS is expensive. A cheaper alternative might stand a better chance – in the future – of being funded. A good deal of the science that NLS was designed to address could be covered by the baseline 1 kHz performance. Operation of a normal conducting X-band linac appears to be possible at 1 kHz (or more). (S. Tantawi, ICFA Future Light Source Workshop, SLAC, March 2010) → Start by investigating the use of X-band technology to match the NLS baseline parameters

Design of an X-band Option for NLS S-band gun and accelerating sections S01 Gun X4H S02X01X02X03 X04X05 X07 undulators BC1 BC2 DL X08 X09 X06 X-band section for phase-space linearisation Main X-band linac Undulator(FEL) Bunch compressors

1 kHz S-band Injector Co-axial coupler (as DESY gun) to allow higher repetition rate (lower power dissipation in the coupler and better axi- symmetric water cooling) and optimum solenoid location Exchangeable cathode Peak field at the cathode 100 MV/m “S-band Photocathode Gun with a 1 KHz Repetition Rate”, J-H. Han et al., Proc. LINAC’10, THP104

“Design of 1 kHz Repetition Rate S-band Photoinjector”, J-H. Han, Proc. LINAC’10, THP105 Charge20 pC50 pC200 pC 100% projected emittance (mm mrad) central slice emittance (mm mrad) bunch length, FWHM (ps) Optimised beam properties after 4 S-band sections (137 MeV): Gun is followed by a number of 3 m S-band accelerating sections 12 MV/m gradient to allow 1 kHz operation

1 kHz X-band Linac Work by Z. Huang (presented by S. Tantawi, ICFA Future Light Source Workshop, SLAC, March 2010) suggests a soft-Xray FEL can be driven using an X-band linac using an a/ =0.13 structure such as T53VG3. Wakefields increase rapidly for smaller a/  1 kHz version of a XL-4(5) 50 MW klystron, and its modulator, don’t yet exist, but are believed feasible Base a design on 1 x 50 MW klystron per N x accelerating sections, no SLED, for simplicity and cost effectiveness

Assume an “effective” shunt impedance of 55 MOhm/m, similar to that of T53VG3 and in-line with independent calculations (C. Christou, WG1 presentation) Vary N, number of sections per klystron, with P in = 50 MW / N Linac cost = A*N section + B*N klystron + C*L A: sections: $100k waveguide etc.: $125k B: klystron: $226k modulator: $335k LLRF+TWT: $100k C: tunnel + RF gallery: £54k/m Choice of Gradient Linac cost estimates kindly provided by C. Adolphsen are “bare” purchase costs e.g. cost per RF station, based on 4 sections per single klystron = $1561k from NLS cost estimate

No. of sections per klystron: Linac Costs x 1.5 Current design based on 4 sections/klystron, ~ 35 MV/m average power dissipation (kW/m) at 1 kHz, 0.35  s

Accelerating Structure For more details see WG1 presentation, “X-band Linac Technology for a High Repetition Rate Light Source” by C. Christou SUPERFISH calculations of X-band ( GHz) structure shunt impedance vs. iris radius

Power loading on the iris averaged over the RF cycle Thermal analysis using QuickField 35 MV/m, 0.35  s What is an acceptable limit ? 10 kW/m

RF Parameters Accel. gradient36 MV/m L structure 0.53 m P in 12.5 MW P structure 6.9 MW Rep. rate1 kHz RF pulse length 0.35  s structure 4.6 kW/m P RF 50 MW RF 17.5 kW to allow 5-bunch operation with 50 ns spacing

Beam Dynamics and Parameter Optimisation For more details see WG3 presentation, “Beam Dynamics Studies for an X-band Linac Driven Soft X-ray FEL ” by R. Bartolini AstraElegantGENESIS S01 Gun S02X01X02X03 X04X05 undulators BC1 BC2 DL X06S03S04LHX4H Requirements at the exit of the linac for driving a FEL: - high peak current, low emittance and energy spread but also, to permit seeded FEL operation: - minimum variation of current, energy, energy spread, emittance, transverse position and angle along the bunch. Optimise location of bunch compressors (BCs) and X-band linearizer, BC strengths, accelerating voltages and phases …... taking into account space-charge in the injector, coherent synchrotron radiation in the bunch compressors and wakefields in the accelerating sections.

Start-to-end simulations Results similar to those obtained for NLS for the 50 pC SASE case: 3 GW pulses at 1 keV, 17 fs FWHM 200 pC seeded operation still under study, but looks feasible

Conclusion A first step has been made in considering a normal conducting X-band accelerator which meets the NLS baseline parameters So far it appears feasible, although modulator & klystron technology does not currently meet the requirements. An R&D phase would certainly be needed. First indications are that it could be 20-25% cheaper overall than the cw superconducting version (a £500m-class facility in end 2009 prices) … sufficiently interesting to pursue further. More work required on accelerating structure and beam dynamics optimisation, as well as more detailed cost comparison.

Thanks The organising committee for the opportunity to present this work DLS colleagues Jang-Hui Han (gun and injector), Chris Christou (linac) and Riccardo Bartolini (beam dynamics and start-to-end simulations) Sami Tantawi and Chris Adophsen (SLAC) for much useful advice

Thanks for your attention Comments welcome !