1. Bejing 2004: the Choice of Cold Technology Giorgio Bellettini International Workshop on Thin films Applied to Superconducting RF: Pushing the Limits of RF Superconductivity Legnaro INFN National Laboratory, October 9, 2006

2. Content 1)Charge of the International Technology Recommendation Panel 2)Linear Collider options 3)ITRP methods of work 4)Scientific, technical, social issues 5)Recommendation

3. Charge: Tesla versus JLC/NLC “The International Technology Recommendation Panel (the Panel) should recommend a Linear Collider (LC) technology to the International Linear Collider Steering Committee (ILCSC). “ “On the assumption that a linear collider construction commences before 2010 and given the assessment by the ITRC that both TESLA and JLC- X/NLC have rather mature conceptual designs, the choice should be between these two designs. If necessary, a solution incorporating C-band technology should be evaluated.” The ITRP interpreted this charge as being to recommend a technology, not a design. However, for comparison purposes the parameters of the existing designs were used. The 5.7 GHz RF warm machine design (C-band) was not found superior to the JLC/NLC design (at 11.4 GHz) and was not influential in the global technology comparison. A description of CLIC was heard but that option was not considered.

4. ITRP method of work A large amount of written material was studied ITRP visited DESY, SLAC, KEK Each Panel Member interacted personally with members of the community A matrix of evaluation parameters was built on which each Panel Member expressed his view.

5. Evaluation Matrix Matrix parameters which turned out to be more important: Scientific issues Technical issues Physics operation issues Schedule issues Social impact of the LC It was agreed that cost of either machine could not be reliably assessed. Cost was removed from the matrix.

6. Scientific issues: machine luminosity CHARGE: “Luminosity and reliability of the machine should allow the collection of approximately L = 500 fb**(-1) in the first 5 years of running” The design luminosity of both machines L = 3,4.10**34 (cm-2.s-1) in TESLA, L = 2,5.10**34 (cm-2.s-1) in NLC was found to be adequate.

7. Scientific issues: machine energy CHARGE: “The machine should allow for an energy range between 200 and 500 GeV and allow for energy scans in this range with operation as dictated by physics”. “Both technologies were found to offer this flexibility” CHARGE: “The maximum c.m.s. energy should be 500 GeV”. “The machine will be designed to begin operation at 500 GeV, with a capability for an upgrade to about 1 TeV, as the physics requires. This capability is an essential feature of the design. ”

8. Significance of the proposed energy step If the LC ranges up to  s  1 TeV: From the SLC to the LC, factor ~ 10 From LEP2 to the LC, factor ~ 5 Compare with past experience: From PETRA to LEP2, factor ~ 5 From the ISR to the SpS Collider, factor ~ 10 From the SpS Collider to the Tevatron Collider, factor ~ 3 From the Tevatron collider to the LHC, factor ~ 7 We would be fully consistent with past experience.

9. If SUSY is there beyond the SM At least one SUSY Higgs, gauginos, sleptons… A Linear Collider can measure detailed properties of several supersymmetric particles: masses quantum numbers lifetimes decays  AN ENORMOUS PROGRAM even below 500 GeV

10. Energy reach versus operation reliability CHARGE: “The Panel will make its recommendation based on its judgment of the potential capabilities of each conceptual design for achieving the energies and the peak and integrated luminosities needed to carry out the currently understood scientific program”. “The warm technology allows a greater energy reach for a fixed length, and the damping rings and positron source are simpler. The Panel acknowledged that these are strong arguments in favor of the warm technology. The superconducting technology has features, some of which follow from the low RF frequency, that the Panel considered attractive and that will facilitate the future design.”

11. Some TESLA/ NLC Parameters (500 GeV) TESLANLC Design Luminosity (·10 34 cm -2 sec -1 )3.42.5 Linac Repetition Rate (Hz)5120 No. of Bunches per Pulse2820192 Bunch Separation (nsec)3371.4 Bunch Train Length (  sec) 9500.267 Loaded gradient (MeV/m)23.850.0 Two-Linac-Length (km)3013.8 Total Site AC Power (MW)140195 Plug to Beam Efficiency (%)(*) (*) Includes estimated loss in couplers and HMO absorbers 23.38.8

12. RF Parameters (500 GeV) TESLAJLC/NLC RF Frequency (GHz)1.311.4 Loaded Gradient (MV/m)23.850 Klystron Peak Power (MW)9.775 RF pulse length (  s) 13700.4 Filling Time (  s) 4200.120 Bunch Train Length (  sec) 9500.27

13. Tesla linac layout 1500  s, 5Hz, 130 kV, 150 A TESLA: one 10 MW klystron driving 36 one meter long cavities (= 3 cryo- modules a 12 cavities) with 230 kW/m. 20 m TESLA 2820 bunches/train, 337 ns apart, 5 Hz, head-on collisions

14. NLC linac layout 1.6  s, 120Hz, 500 kV, 2.12kA NLC: 8 x 75 MW klystrons, supplying (after pulse compression) a peak power of ~100 MW/m. 192 bunches/train, 1.4 ns apart, 120 Hz, requires X-angle

15. Accelerating power TESLA has 572 Klystrons, ~ 1 Klystron/GeV The NLC has 4064 klystrons, ~ 8 Klystrons/GeV TESLA transfers the power to the beam in 1 millisecond, and the JLC/NLC in 0.3 microsecond. Over 3 orders of magnitude higher peak power in JLC/NLC. Space density of power on beam : TESLA 0.226 MW/meter, NLC ~100 MW/meter after bunch compression. About 500 times larger density in JLC/NLC

16. Time structure of beams LC pulses are trains of bunches. 1 ms long trains are separated by 200 ms in Tesla (5 Hz) 0.3 microsecond trains are separated by 8.3 ms in NLC (120 Hz) Within trains, bunches are separated by 337 ns in Tesla Within trains, bunches are separated by 1.4 ns in NLC Adequate intra-bunch time helps to preserve luminosity. If some bunches miss the collision one has time to react within a Tesla train and ridirect later bunches. If malfunctioning is signaled by front of train one can abort the rest.

17. ITRP comment on beam time structure “The long bunch interval of the cold machine permits inter-bunch feedback and may enable increased beam current”.

18. TESLA NLC CLIC Iris diameter In Tesla, iris diameter is a = 70 mm. Transverse wake potentials are proportional to a**-3, must align cavity to within 0.5 mm. Aligment required to within 10-100 micron in NLC (Note: even 5 mm are hard to get at the Tevatron)

19. Installation tolerances in TESLA and NLC* (  s = 500 GeV) Quad to survey offset: TESLA ~300 , NLC ~50  ** Structure to structure offset: TESLA ~300 , NLC ~25  Structure tilt : TESLA ~240  rad, NLC ~ 33  rad  Installation alignment simpler in TESLA. Final emittance preservation based on BPM`s easier in TESLA.  Dynamical realignment of elements based on BPM`s is planned hourly in NLC. ILCTRC Second Report (2003), megatable 7.19 ** At Fermilab present accuracy in magnet alignment at installation is ~250 

20. Alignment issue Luminosity stabilization (jitter in beam size and axis, final focus vibration) very challanging for all LC. Re-alignment every some months required in any LC to correct for slow ground motion. Continous re-alignment required in NLC to correct for frequent minor motion.

21. ITRP comment on beam alignment “The large cavity aperture of the cold machine reduces the sensitivity to ground motion”

22. Importance of a system test DESY will build XFEL and pave the way to a cold ILC by clearing a number of issues, including components reliability and linac cost. This will provide for free a testbed for the ILC. “The construction of the superconducting XFEL free electron laser will provide prototypes and test many aspects of the linac”

23. Social issues The large electrical bill of a multi-MW research facility will raise running budget questions and might face popular criticism. Total site AC power at 500 GeV is: Tesla 140 MW, NLC 195 MW luminosity/power is 1.9 times larger in Tesla. “The use of superconducting cavities significantly reduces power consumption”

24. Rational of ITRP reccomandation The superconducting technology has features, some of which follow from the low rf frequency, that the Panel considered attractive and that will facilitate the future design: The large cavity aperture and long bunch interval simplify operations, reduce the sensitivity to ground motion, permit inter- bunch feedback, and may enable increased beam current. The main linac and rf systems, the single largest technical cost elements, are of comparatively lower risk. The construction of the superconducting XFEL free electron laser will provide prototypes and test many aspects of the linac. The industrialization of most major components of the linac is underway. The use of superconducting cavities significantly reduces power consumption.

25. Oversimplified rational and recommendation “The main linac and rf systems, the single largest technical cost elements, are of comparatively lower risk.” “We recommend that the linear collider be based on superconducting rf technology. This recommendation is made with the understanding that we are recommending a technology, not a design.”

26. CONCLUSIONS It was a hard decision, with heavy and far reaching consequences The detailed design of a cold ILC being addressed is drifting appreciably from the TESLA design and indicating how difficult the real job will be We judged that it would be less difficult. However, the cold ILC will be by no means an easy machine. The future of HEP depends to a large extent on its success.