High Gradient and High Q R&D Topics 1- Explore Q > for TESLA parameter flexibility –higher luminosity through higher rep rate, longer rf pulse… 2- Improve geometry of TESLA cavity to increase accelerating field –reduce peak magnetic field by 10% –allow peak electric field to rise 20% 3- Understand gradient limitations related to dark current and voltage breakdown in copper for NLC

Gradient Related R&D con’t 4 - Study gradient vs rf pulse length –Can we get 45 MV/m for 100  sec instead of 30 MV/m for 1000  sec? 6- Streamline TESLA cavity production and preparation –Heat treat stack of half-cells,electropolish half-cell (instead of 9-cell structure) –Spin entire 9-cell structure out of one tube (done for copper) 21,000, 9-cell cavities

Item 1: Potential Q improvements can lead to Q from to Achieved in single cells already, Need to achieve in 9-cells

Proposed New Geometry 10% higher Eacc for same Hpk Simulations show that new shape will not be prone to multipacting Explore Improved TESLA Cavity Shape

100  m More surface studies needed to show how to get to 40 + MV/m? Better Surface Treatments Lead to High Gradients in TESLA Cavities

J. Wang 1995 Eacc = MV/m Dark Current and Voltage Breakdown Studies Important for Warm NLC

SLAC DS2 1.8 m long structure - Cell 8, Downstream Side AFTER RF R. Kirby/SLAC But Sustained Sparks Cause Individual Sparks Craters not harmful Reduce dark current (condition ing) Too many craters=> surface erosion=> cavity detuning

Previous Studies Show Many Common Features between RF and DC Breakdown DC single breakdown events are very fast (few ns) like in cavities Cause few isolated craters (not harmful) emission halted! (processing) But multiple breakdown events can be long and cause severe surface damage (erosion)

Digital Video of Spark 1 mm Copper Single Breakdown Event

20 um ? CERN Diamond Machined Copper cathode, HPR Craters from single breakdown event Ion bombardment Starburst region

Sustained Spark at 170 MV/m

50 microns 20 microns SEM

1) Diamond Machined Cu, as for cavities 2) Mn Particles found After furnace brazing 3) Sparked 4) Mn particles Were here - Now craters Mn particles Were here - Now empty craters Mn particles outside starburst survived the spark Contaminants are one source of field emission and breakdown

20 microns 70 MV/m Spark Starbursts present on copper are carbon depleted areas due to ion bombardment Auger Detection can find residue of original emitter Vanadium from Auger SEM 10 microns Intentional V particle: Strong emitter On copper surface

Simulation Results Modelling of breakdown has started But could benefit from more effort

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

ElectronsIons 50 microns

Resources Needed for First Year Existing RAs and SRA Graduate Students K 2 Technician support 150 K Equipment 150 K Materials and Supplies 100 K Outside Contracts 100 K Total 630 K