Cavity Processing for  <1 SRF Cavities TESLA Technology Collaboration Meeting April 19-22, 2010 Fermilab Speaker: Mike Kelly April 20, 2010

Spring 2001 Q High-pressure rinsing first used with a low-  SRF cavity 10 years ago Found dramatic performance increase from HPR consistent and repeatable if cavity kept clean

 Large SRF performance gains for low-  still possible with already established techniques  However, low-  SRF slow to move with the advancing state-of-the art –No linear collider to drive up performance, drive down cost  Applications for high-gradient low-  –Medical Isotopes (accelerators as solution to Mo99 crisis) –National Security (non-destructive interrogation methods) –Of course…cost savings for basic science accelerators  Path to higher performance for low-  –Cavity design –Make good use of existing demonstrated techniques –Cavity diagnostics Background: Low-beta SRF Cavity Processing

Niobium-to-stainless braze Die formed and EB- welded 3 mm (also 2 and 4 mm) RRR>250 niobium sheet (AES) Cavity inside stainless He jacket; alternatively titanium or niobium Designed in 3D, using MAFIA Microwave Studio ProE/ANSYS Background: Low-  Mechanical Design and Fabrication

3 mm full penetration (50 kV x 64 mA x 60)/6 inch/min 32 kJ/inch 3 mm key hole(50 kV x 62 mA x 60)/27 inch/min 6.9 kJ/inch 2 mm full penetration (50 kV x 50 mA x 60)/9 inch/min 16.7 kJ/inch 2 mm key hole(50 kV x 48 mA x 60)/27 inch/min 5.3 kJ/inch Weld type parameters heat content Key hole Full penetration  We have a combination of full penetration and key hole welds in high magnetic field region  Key hole welds have substantially lower heat deposited –Are these welds as good, better or worse than through welds? –Must be measured with detailed cold test diagnostics (2 nd sound, thermal mapping) Background: Electron beam welding for low-  QWR

Background: Clean room assembly of cavity string for  =0.14, Jan. 2009

 Highest gradients for operational cavities in this range of beta (separate vacuum system)  Beam steering correction works  Vacc = 14.5 MV in 4.6 m module length (Eacc= 8-9 MV/m for all cavities) –4 cavities limited by VCX tuner –2 cavities limited by field emission (10 R/h max allowed at cryostat wall) –1 cavity limited by quench An ATLAS Energy and Intensity Upgrade: Phase I Commissioned in June 2009

 Lower beta cryomodule with 7 cavities/4 solenoids  Design goal Vacc = 20 MV in 5 m module length –No VCX tuner limitations (overcoupling plus fast piezo-tuner) –Tapered to reduce Bpeak; no cost in real estate ATLAS Energy and Intensity Upgrade: Phase II Commissioning in July 2012 (for scale)

Pushing performance for low-beta SRF cavities  Benefits for ATLAS at Argonne –Replace aging split-ring cryomodules –Higher energies (30-40% beam energy increase with Phase I) –Much higher beam currents (steering corrected drift-tube face)  …But also real possibilities for high-gradient low-beta for applications –National security (non-destructive interrogation methods) –Nuclear medicine (accelerators as solution to Mo99 crisis)  To push for better performance in next upgrade… 1.Presently our VCX fast tuner limits new cavity performance to ~8 MV/m (though average quench limit ~12 MV/m); replace with a piezoelectric transducer + 4 kW coupler 2.Better performance through the use of techniques learned in e-cell collaborations; particularly horizontal electropolishing on completed jacketed niobium cavity 30 cm  =0.077 f=72.5 MHz B p /E acc = 4.8 mT/MV/m E p /E acc =3.25

Processing: EP has been adapted to various shapes Co-axial half-wave Double spoke Quarter-wave Split-ring QWR EP in Joint Facility

Cathode for spoke housing Cathode for end plate  Hand wound cathodes from ½” 3003 Al tubing  Typically polished using Siemens intermitant EP Processing: Simple electropolishing fixturing for cavity sub-assemblies

Processing: Light 10  m (flash) BCP after final welding to remove weld residues

Processing: Upgraded EP fixturing for recent QWR’s EP in joint facility Exploded view of apparatus Average surface removal Substantial top/bottom differential polishing – still not fully satisfying

 Not just a case of “me too”, but this is in principle a better way of processing these cavities  Requirements –Suitable cavity access ports for cathode, acid flow, gas flow –Some engineering for an electropolishing tool –Availability by Dec for new prototype QWR  Benefits –All rf surfaces receive the full bulk EP –Easy direct water cooling using helium jacket –Helium jacket/final machining before delicate EP’d rf surface –No final buffered chemical polishing –Low acid flow rate needed due to external cooling; <1 lpm –Major reduction in overall EP effort and number of procedures –Large fraction of required components are common to e-cell polishing Processing: Horizontal electropolishing on a completed quarter-wave niobium cavity

Processing: Existing elliptical cell cavity electropolishing system

Processing: QWR electropolishing based on existing design for mechanical and electrical hardware sliding Bosch rail rotating carbon brush assembly

Processing: Four cathodes - one loaded through each coupling port cavity interior cathode (hollow Al tube) cavity port helium jacket HDPE guide bushing and flange No cathode bag (nearly pointless anyway)

water acid Processing: Schematic of acid/water flow through cavity  Direct water cooling and low acid flow rate (<4 lpm) makes this scheme easier than would be the case with lpm as for 9-cell EP

An aside…electropolishing for 650 MHz 5-cell cavity  Scaled cavity geometry shown with the existing EP hardware –Cavity with twice radial dimension of the 1.3 GHz 9-cell fits into the existing system with modest modification (no cavity frame shown, may need to shim under blue stands) –55 gallon acid handling limit OK –2 ½ times surface area, EP supply OK, 50% larger chiller –Cavity handling similar to 9-cell (crane in hi-bay, hoist in chemistry room) –No major difficulties in adapting EP to this geometry

Processing: HPR for QWR’s in the joint ANL/FNAL facility  Cavity fixed, wand rotates and translates  Rinsing/assembly sequence similar to that for e-cell –An initial HPR –Partial assembly of rf pickup, blanks, inter-cavity spool –Final HPR –Dry, pump out, leak check

Summary  Many of the performance gains achieved for e-cell cavities based on (fairly) well established techniques have not been fully realized for low beta cavities.  Electropolishing and high-pressure rinsing are two of these techniques –These are being addressed with the present ATLAS intensity upgrade QWR’s  Performance limitations due to low-  cavity fabrication have received little detailed study –Requires a similar effort with diagnostics and inspection as for e-cell development  Moderate (mediocre) performance will be the rule until most/all of these techniques are integrated into fabrication and processing

A few words on e-cell processing at ANL… Cavities Processed at the ANL/FNAL SCSPF in 2010 DateCavity NameCavity TypeEP Type Target Removal (μm)Process Run Time (min) 1/27/2010TB9RI0269-CellBulk /28/2010TB9ACC0079-CellLight2070 2/15/2010TB9RI0269-CellBulk /18/2010TE1ACC0031-CellLight /22/2010TE1CAT0021-CellBulk /26/2010TB9RI0249-CellLight2070 3/30/2010TB9RI0269-CellLight2070 4/2/2010TB9AES0031-CellLight2070 4/7/2010TE1CAT0011-CellLight2070 4/8/2010NR-61-CellLight2070 4/16/2010TE1CAT0011-CellLight /20/2010NR-61-CellLight30100

Typical 9-cell cavity electropolishing parameters