R&D For Accelerating Structures H. Padamsee

TESLA Niobium, one meter length, rf = 1.3 GHz Copper, 53 cm, rf = 11.4 GHz

cell

Q Why do RF Losses increase above Eacc = 20 MV/m?

30 MV/m 100  m Single Cells

Need to Understand SC materials and surface science Why do rf losses increase with surface roughness? Why do rf losses decrease with simple baking of 140 C and 48 hours Why does sc quench below rf critical field ≈ 50 MV/m?

Possible Benefits from higher Q Double luminosity with 1.6 x AC power (eg double rep rate, increase Q to 5 x Decrease number of klystrons from 680 to 330 by doubling rf pulse length, increase Q to 5 x10 10 => increase reliability Reduce AC power of 800 GeV TESLA at 35 MV/m

BCS Contribution still important 2 K

T = 2 K, Rs = 14 n , Q = 2 x10 10 Lower mean free path, Q = 3 x10 10 Lower Temperature T = 1.7 K, Q = 1.2 x10 11 T= 1.5 K, Q = 4.3 x10 11 TESLA Q = Need to Shield Earth’s magnetic field to 0.5 mOersted How to achieve this in an accelerator cryostat? T 1.8 K, Q 5 x 10 10

Lower Cost Old vs. New Fabrication Methods Hydroforming Spinning Half-Cells Ebeam Welding

Better Material Control Methods Needed for 20,000 cavities

1.8 meter 11.4 GHz

J. Wang 1995 Eacc = MV/m

SLAC DS2 1.8 m long structure - Cell 8, Downstream Side AFTER RF R. Kirby/SLAC Sustained spark Individual sparks

Pits are harmful to small iris G. Loew and J. Wang

1.8 meter 11.4 GHz 1 meter TESLA Standing Wave 20 cm Standing Wave Cavity Vg ≈ 0% 3% Vg = 3 - 5%

Juicy R&D topics in physics of RF breakdown What is the trigger? field emission from microparticles? gas evolution from ion bomardment? How to keep the overall emission current low at operating gradient? How to get low spark rate ?

Digital Video of Spark 1 mm

20 um ? CERN Diamond Machined Copper cathode, HPR No intentional contaminants Auger later

Sustained Spark

50 microns 20 microns 170 MV/m SEM

Electrons Gas Distribution - starts by surface desorption -or melting of emitter tips - builds up by ion bombardment - may be ultimately dominated by cathode material

ElectronsIons 50 microns

2.25 RF Periods Many Questions Remain How does the plasma ball move around when fed with stored energy?

Compare Nb/Cu with Bulk Nb

Nb-3-Sn - Best Performance

HTS - YBCO Low Field & High Field Surface Resistance

MgB2

A Comparison of 1 GHz X MgB2 - estimate R = 4 X 10 -8

Linac Cavity Input Coupler: TTF3 The TTF3 coupler tested to 1.8 MW peak, 1.3 ms pulse length, 4.68 kW average power (TW); coupling is adjustable from Q ext = 10 6 to Q ext = 2  10 7

NLC Couplers 100 MW Lots of sparking near coupler cell Is it the same?

The TTF4 coupler is supposed to be multipactor-free Linac Cavity Input Coupler: TTF4