A proposal for updating the X-band test program (plus some 30 GHz)

1. A proposal for updating the X-band test program (plus some 30 GHz) W. Wuensch 31-7-2007

2. Motivation for restating and updating the X-band test program: • With the consequences of the big parameter change settling in and an increasing understanding of breakdown, incorporate our current understanding of what should be covered by the testing program. • Absorb the feedback from the structure workshop and ACE • Plan a program that addresses most major issues at X-band, with a complimentary 30 GHz program. • Include among our objectives reasonable determinations of effect on gradient of: geometry, different types of damping, different technologies. We want to show not only the gradient but also why we have the gradient. • Include more structures to make sure we have statistics both for the final results and for guidance along the way. • Plan (roughly for the moment) to use KEK and the multiple NLCTA slots in parallel over the next 1.5 to 2 years – which is the typical lead/prediction time compromise.

3. Main categories for test: • Geometrical scaling of iris – diameter but also thickness, ellipticity, phase advance. • The effect of damping - direct effect on gradient, indirect effect through fundamental mode parameters. • The effect of tapering – can we go to/beyond constant P/C? How do single geometry results translate into tapered structures. • The effect of couplers – length efficiency, relevance of 1st cell effect in low breakdown rate regime. • Pulse length and pulse shape – both for efficiency and for understanding • Breakdown rate and distributions – continue to disentangle effects occurring during conditioning from those occurring at low BDR. • Physics of breakdown – light etc. • Manufacturing technologies – Understand options since the compromise between breakdown, pulsed surface heating, material states, damping topology, wakefield measurement resolution and mass production is unlikely to be sorted out by 2010. • Preparation, conditioning techniques • Full structures – we need to operate reasonably efficient, fully equipped structures at 100 MV/m. • Materials – both for potential increased performance but also ‘lever arm’ for understanding. For the moment we will only pursue this at dc spark and 30 GHz.

4. Pulse shape with T53 disk, undamped, ‘light’ taper Retest CERN built vg1.1 disk, undamped, no taper SLAC built T26 disk, undamped, constant gradient CERN built CLIC_vg1 disk, undamped, P/C taper Current active pipeline CERN built CLIC_vg1 quadrant, damped, P/C taper KEK built CLIC_vg1 disk, undamped, P/C taper KEK built CLIC_vg1 quadrant, damped, P/C taper CLIC_vg1 (in rf design) disk, damped, P/C taper

5. vary iris thickness, ellipticity add waveguide damping add damping slots in different technologies vary phase advance reference iris vary iris diameter add tapering Testing program – the Big Picture full structures

6. Circular irises, diameter and thickness 30 GHz values are scaled to 12 GHz reference iris

7. diamond turned disks, brazed • Sami’s mode launchers • 10 cells, constant geometry • two structures of each type (6+1 pipeline) Iris diameter and thickness scaling test geometrical variation of gradient

8. Iris scaling • Why 10 cells? To give at least some kind of length for breakdown rate, although not perfect because of constant impedance. Obviously shorter is cheaper (6). • The bigger the range the better for iris scaling. What limits our range? • Available power on the high side. This is why bigger irises are better tested at 30 GHz • Hitting the surface field limit on the low side – we want to know if and where this holds, but we want to avoid it in a scaling test. Some new ideas which are brewing are expected to shed some light on what a power flow to surface field limit transition experiment might look like. • For iris ellipticity and phase advance we will follow the same system of 10 cell constant impedance brazed structures. The new ideas new ideas referred to above are relevant here so a detailed proposal will follow.

9. Add tapering • Standard reference: T53 exists, change to steeper tapering with T26 already very interesting, (in pipeline) • CLIC_vg1’s in disks have reference in iris scaling test (but not standard reference) (in pipeline). This test tries to avoid pushing the limit from power flow to surface field. • CLIC_vg1’s in quadrants with damping also relevant but this test mixes tapering and damping (in pipeline) • Further tapering requires confidence on tolerances at low group velocities and ideally the single geometry test referred to in the previous slide.

10. Add damping • T-type reference iris with waveguide damping. Turned and milled cells, brazed, 10 cells, constant geometry, Sami’s mode launchers (2) • T-type reference iris with slots. Milled quadrants, 10 cells, constant geometry, Sami’s mode launchers (2) • CLIC_vg1’s in quadrants with damping also in pipeline but this test mixes tapering and damping (in pipeline) • Lots of combinations at 30 GHz.

11. Manufacturing technology • CLIC_vg1 will be made in undamped disks, damped disks and damped quadrants. Mixes damping with technology in a tapered structure (in pipeline) • Reference iris in quadrants, undamped, 10 cells, constant geometry, Sami’s mode launchers (2) • Reference iris with slots is a cross check how contact without gap compares with slot. (in pipeline) • At 30 GHz we will have slot/contact and contact/brazed disk comparisons.

12. What about HDS? • CLIC HDS (2). I will return to how this structure fits in the Big Picture.

13. Summary 8 structures/tests in pipeline +14 proposed new structures = 22 slots needed at SLAC and KEK to the end of 2008. More testing slots than structures through end of year. More structures than testing time next year. SLAC can make disk based structures. KEK can make disk based and will be able to make quadrant based structures. Finding a person to spend significant time at SLAC would be a huge added value.

14. Outlook • What I believe will happen: • 30 GHz HDSs work as predicted. First results by end of August. • T26 is built by November and works as predicted by the end of January. • CERN-built quadrant CLIC_vg1 is built by November and works as predicted by the end of January. KEK-built structure confirms result. • What we would do after: • Go for whatever the prototype quadrant based CLIC structure looks like at that point. Built by CERN and KEK. Going to very low vg, which implies low power, high surface field and tight tolerances becomes crucial region to investigate. • Push X-band HDS hard. • Scaling tests in disks because scaling is necessary information for CLIC design process. SLAC and CERN build these. • Lower priority on specialized damping or technology experiments on a because we can sort out optimum solutions later.

15. Outlook • What might go wrong: • Poor results from the 30 GHz HDSs. • T26 is built by November and works as predicted by the end of January • CERN-built quadrant CLIC_vg1 is built by November and works as predicted by the end of January. KEK-built structure confirms result. • Analysis: Problem in slots but not quadrants. • What we would do after: • Go for whatever the prototype quadrant based CLIC structure looks like at that point. Built by CERN and KEK. Going to very low vg, which implies low power and high surface field, crucial region to investigate. • Scaling tests in disks because scaling is necessary information for CLIC design process. SLAC and CERN build these. • Pursue specialized damping and technology experiments to sort out what is going on.

16. Outlook • What might go wrong: • 30 GHz HDSs work as predicted. First results by end of August. • T26 is built by November and works as predicted by the end of January • Poor results from CERN-built quadrant CLIC_vg1. KEK-built structure confirms result. • Analysis: Problem in contact (virtual leak?) but not slots nor quadrants. • What we would do after: • Go for whatever the prototype disk based CLIC structure looks like at that point. Built by CERN and KEK. Going to very low vg, which implies low power and high surface field, crucial region to investigate. • Push X-band HDS hard. • Scaling tests in disks because scaling is necessary information for CLIC design process. SLAC and CERN build these. • Pursue specialized damping and technology experiments to sort out what is going on (improved contact, braze quadrants?).

17. Outlook • What might go wrong: • None of the 30 GHz HDSs work as predicted by end of year. • T26 is built by November and works as predicted by the end of January • Poor results from CERN-built quadrant CLIC_vg1. KEK-built structure confirms result. • Analysis: Quadrants seem to give problems. • What we would do after: • Top priority is introducing waveguide damping in brazed disks • Scaling tests in disks because scaling is necessary information for CLIC design process. SLAC and CERN build these. • Pursue specialized damping and technology experiments to sort out what is going on.

18. Outlook – parallel track in pipeline T26 and CLIC_vg1 brazed comparison will give us a good information about the effect of varying tapering. Ricardo will speak about the 30 GHz program and in particular about new ideas. Flow charts which include all these tests too are complicated to make (and useless in my opinion). It is mainly important to know that we have structures in the pipeline to adapt to most outcomes.

19. Actions SLAC testing: pulse shape and old vg1.1 structure Start to move on iris-scaling structures. Contact SLAC to see if they can produce them. Start disk based waveguide damping reference cell design. Find a person to spend significant time at SLAC.