1. XFEL Project (accelerator) Overview and recent developments
R. Brinkmann, DESY
TESLA Collaboration Meeting, Frascati May 2003

2. TESLA TDR, March 2001: integrated XFEL

3. Considerations leading to XFEL TDR-update, autumn 2002
Avoid strong “coupling” of XFEL and LC parts of TESLA project during construction, commissioning and operation stages (and: approval)
Gain flexibility in operation parameter space
 XFEL driver linac in separate tunnel
Limitation of additional cost:
Reduce linac length & energy to ~1.5km, 20 GeV
Difference in accelerator cost:
(sep. linac – TDR2001) = 196 Mio. €
Reduce # of photon beam lines 10  5, # of experiments
30  10

4. Comparison of parameters (1Å, fixed-gap undulator)

5. Injector: = TTF-II

6. Simulations (60MV/m cathode field) indicate  < 1 mm*mrad possible
Including BCs (CSR!), phase space structure non-trivial  start-to-end simulations
Linac wakefields unlikely a serious problem
Exploration of bunch parameter space (e.g. charge vs. emittance & bunch length) ?

7. 3.9km

9. Proposed modifications in recent discussions:
Shortening of the accelerator (Injector + linac + collimation/diagnostic etc.) tunnel to ~2km
XFEL site layout not necessarily linked to LC site (except: don’t exclude LC construction at foreseen Hamburg/S.H. site)
TTF-like modules with 8 cavities per module; # of modules per Klystron ?
Flexibility in duty cycle ? (see below)

10. Duty cycle limitations I: Cryogenics
TDR layout for module/He distribution (GRP  , pressure drop) allows for upgrade to 800 GeV
 At ~23 MV/m, 1.5km linac could be operated up to about 20 Hz rep rate (~2% duty cycle)
Required cryogenic plant would have approx. the size of one of the six TESLA-500 LC plants
From cryogenics point of view, could scale duty cycle as 1/energy2

11. How about CW operation??
At 23 MV/m, Q0 = 1010, rf losses at 2K are 55W/m CW  20 GeV linac would require 3 times total 2K capacity for TESLA-500 LC (forget it…)
At half gradient (linac length  3km!), Q0 = 21010, rf losses at 2K are 7W/m CW  required 2K capacity still 4…5 times one 5km unit of TESLA-500 LC ( considerable additional investment and operating costs, modification of He distribution?)
A big issue for all (near-) CW considerations: at present, no suitable beam source available

12. Duty cycle limitations II: RF system
present design of modulator/klystron station can operate at max. 10 Hz, 10MW, 1.4ms pulse length, 65% efficiency (average power into klystron ~220kW)
Higher duty cycle at lower peak power possible as long as average power from modulator/into klystron gun is kept  above limit (careful: DCRF efficiency drops at lower power!) concerns: IGCTs at high rep rate, RF drive power
Scale beam pulse current with acc. Gradient (=beam energy)  loaded Qext constant, or: optimise for ~constant beam current, variable Qext
Assume 33% RF power overhead: Prf = 1.33 Pbeam

13. Max rep rate and beam power, four vs. six modules/klystron
Remark: av. Beam power is maximum possible - because of beam dump (solid absorber option) we wanted to limit Pav 600kW

14. What can we gain from variable Qext? Example (6 modules):
Keep Ib = 5 mA const., scale Qext  Eacc
Attractive option: shorter pulses/higher frep (RF gun!)

15. XFEL – Linear Collider Synergies
Working towards getting ready for start of construction of 20 GeV s.c. linac in ~2 years from now is a big step forward for making TESLA technology available for large-scale projects
The issues in common for developing the GeV LC s.c. linac and getting ready for constructing the XFEL linac (by far) outweigh those issues which may be different and may require potential priority conflict discussions
There is also overlap between LC and XFEL for a number of other design issues and sub-systems (e.g. failure handling/operational reliability, beam size and profile monitoring, fast orbit feedback, LLRF…)

16. Example 1: Tunnel Layout
E.g.:
Electronics in tunnel/radiation environment ( test in DESY-LINAC-II)
Handling of RF and cavity failures
Stray fields?
Supports and alignment

17. Example 2: fast kicker systems
Damping ring: re-distribute the train of bunches in time (compress/de-compress at injection/ejection)
XFEL user beam lines: distribute bunches within a train to different beam lines (possibly extraction points at different energies, etc…)
Technology for both applications may also be similar to fast orbit feedback requirements

18. As for the LC, DESY will work out a site proposal for the XFEL
Not yet decided: green field vs. near-DESY

19. HERA tunnel Ellerhoop (barn)
Expected orbit vibration from linac FODO lattice: ~0.1 (rms) from “noisy” (workday) HERA data – not a problem (?)
HERA tunnel
Ellerhoop (barn)

20. Concluding remarks
Sequence of projects on scales of time & realisation probability: TTF-II (scTech.&VUV-FEL) – XFEL – LC
XFEL is a great project for DESY+Partners – and a great opportunity for DESY to host the project
It is also a great opportunity for all of us