R&D For Accelerating Structures

H. Padamsee

TESLA

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

2002

9-cell

0 5 10 15 20 25 30109

1010

Denis Kostin, DESY, MHF/SL

Module 5 cavities. Vertical test results. C45 AC60 AC61 AC65 AC66 AC79 AC78 AC62

Q0

Eacc [MV/m]

Q

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

30 MV/m

100 m

Single Cells

Need to UnderstandSC 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 1010

• Decrease number of klystrons from 680 to 330 by doubling rf pulse length, increase Q to 5 x1010 => 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 x1010

Lower mean free path, Q = 3 x1010

Lower Temperature

T = 1.7 K, Q = 1.2 x10 11

T= 1.5 K , Q = 4.3 x1011

TESLA Q = 1010

Need to Shield Earth’s magnetic field to 0.5 mOerstedHow to achieve this in an accelerator cryostat?

T 1.8 K, Q 5 x 1010

Saclay CAVITY C117 F = 1.3 GHz, T = 1.6 K

1E+08

1E+09

1E+10

1E+11

1E+12

0 5 10 15 20 25 30

Eacc (MV/m)

Q0

CEA Saclay, GECS, Jan. 1999

Q0 > 2. 1011 !!

Surface Resistance

Extremely low residual resistance experimentally obtained

Lower CostOld 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 = 65-75 MV/m

SLAC DS2 1.8 m long structure - Cell 8, Downstream Side AFTER RF

R. Kirby/SLAC

Sustainedspark

Individualsparks

Pits are harmful to small irisG. Loew and J. Wang

1.8 meter 11.4 GHz 1 meter TESLAStanding Wave

20 cm Standing Wave CavityVg ≈ 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 Spark1 mm

20 um

?

CERN Diamond Machined Copper cathode, HPR No intentional contaminants

Augerlater

QuickTime™ and aDV - NTSC decompressor

are needed to see this picture.

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

Electrons Ions

Ions

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 - YBCOLow Field & High Field Surface

Resistance

MgB2

A Comparison of Superconductors @ 1 GHz

XMgB2 - 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 Qext = 106 to Qext = 2107

NLC Couplers100 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