1. Electropolishing of Dressed ILC 9-cell Cavity
Joe Ozelis
Fermilab
on behalf of C. Baker, D. Bice, T. Khabiboulline, A. Rowe, B. Stone – FNAL
&
M. Kelly, R. Murphy, T. Reid – ANL
TTC Meeting, 28 February - 2 March, 2011, Milan

2. TTC Meeting, 28 February - 2 March, 2011, Milan
Introduction
Re-processing of cavities (HPR and/or EP/BCP) has often been used to recover or improve bare cavity performance.
HPR used primarily if field–emission is the limitation
otherwise additional chemical processing (EP/BCP) is attempted (stubborn FE, low Q, or quench).
New techniques are being developed to target suspected defects or “features” identified via optical inspection or thermometry
local grinding
laser or EB re-melting
These techniques, while (eventually) adaptable to dressed cavities, do not address widespread stubborn FE, low Q, or unidentified quench sources.
There is a desire to utilize “global” repair (e.g., EP) on dressed cavities, so that costly, time-consuming, and risky removal of He vessel could be avoided. This was attempted at FNAL/ANL with cavity TB9AES002.
TTC Meeting, 28 February - 2 March, 2011, Milan

3. TB9AES002 Performance History
Cavity TB9AES002 had been processed and tested at JLab
reached 33MV/m in vertical test
subsequently chosen for dressing
one of the 2 dressed cavities for the S1/Global CM
A He vessel was welded on, then the cavity received US cleaning, HPR, and assembly for vertical test (no time for horizontal testing).
Performance of dressed cavity quite poor
quenching at 19.8 MV/m (no FE)
cavity tested 3 times
with HOM probes connected
with HOM probes disconnected, and
with HOM probes removed
Additional HPR not likely to improve performance (no FE). Try additional EP.
TTC Meeting, 28 February - 2 March, 2011, Milan

4. TB9AES002 Performance History
Cavity repeatedly quenched at 19.8 MV/m.
Additional HPR not likely to improve performance (no FE). Try additional EP.
 Tested 1/4/2010, HOM’s removed.
Tested 11/18/09, 12/20/09, with and without HOM’s connected
TTC Meeting, 28 February - 2 March, 2011, Milan

5. Dressed Cavity EP - Setup
Used the same hardware and operating parameters as other ILC cavity EP processes. No real modifications to tooling.
Without access to the cells how do we make the anode connection?
Anode connection made at the beam tubes.
Cavity TB9AES002 mounted in the EP tool at ANL (tuner and 2-phase helium pipe removed).
The beam tubes were cleaned with scotch-brite and ethanol to make a good electrical connection
TTC Meeting, 28 February - 2 March, 2011, Milan

6. Dressed Cavity EP – Process Monitoring
thermocouples
thermocouples
The advantage of not having the helium 2-phase pipe?
Presents the opportunity to obtain important cavity surface temperatures that will help steer the EP process.
Attached thermocouples on the equator and iris of cell #3
Two thermocouples placed on each beam tube 180° apart, a 3rd thermocouple on each beam tube is placed under the anode connection.
This allows us to observe any localized heating from the anode connection (contact resistance), help ensure current uniformity (process uniformity).
TTC Meeting, 28 February - 2 March, 2011, Milan

7. Dressed Cavity EP – Process Parameters
Process conditions for the light EP of a dressed 9-cell were essentially identical to those of a bare 9-cell cavity. Material removal was 20mm, based on integrated current msmt.
The power had to be turned off twice to keep the equator of cell #3 below 30°C.
EP Process Conditions
Bare 9-cell Light EP
Dressed 9-cell Light EP
Voltage
18 V
Avg Current*
~160A
~170A
Cavity Equator Temp
25-30°C
* The current is affected by the niobium concentration of the acid. For comparison purposes the avg current of a typical 9-cell EP with similar acid quality was used.
TTC Meeting, 28 February - 2 March, 2011, Milan

8. TB9AES002 Performance After EP-1
Initial FE onset at ~9MV/m, eliminated by performing low-power (~ W) pulse-processing.
FE re-appeared at 18MV/m (and not appreciably improved by additional pulse-processing).
Cavity reached ~27MV/m, limited by FE-induced quench. Low- and medium-field Q somewhat lower.
Q-drop at ~23MV/m due to FE-loading and/or no 120°C bake.
Cavity performance significantly improved from 19.8 MV/m quench limit previously observed, but performance appears to be dominated by FE. Need to do additional HPR.
TTC Meeting, 28 February - 2 March, 2011, Milan

9. TB9AES002 Performance After EP-2
Field emission eliminated by additional (4) HPR cycles.
Cavity performance is identical to within measurement error.
Quench limit (non-FE) at 28.4 MV/m, Q-drop beginning at ~24MV/m (not due to FE-loading).
Quench could be due to global heating (low Q, Ploss ~175W) and not a localized defect. Diagnostics (therm, SS) not available for dressed cavity.
Field emission has been eliminated by additional HPR, but high field Q-drop remains, due to inability to bake dressed cavity. Nevertheless, performance improvement (19.8  28.4 MV/m) substantial.
TTC Meeting, 28 February - 2 March, 2011, Milan

10. TB9AES002 Performance After EP-2
How did EP of dressed cavity affect field-flatness?
Field Flatness before dressed EP = 98.3 %
Field Flatness after dressed EP = 96.2%
Field flatness has degraded somewhat, especially near the coupler end, but is still close to the ILC spec of 98%.
TTC Meeting, 28 February - 2 March, 2011, Milan

11. TTC Meeting, 28 February - 2 March, 2011, Milan
Summary
A dressed 9-cell ILC cavity has been successfully electropolished at the FNAL/ANL cavity processing facility.
Cavity performance has been improved substantially :
elimination of non-FE quench at 19.8 MV/m
gradient limit now 28+ MV/m
FE not a problem with sufficient rinsing
field-flatness still reasonable
Future plans
120°C bakeout to eliminate high-field “Q-drop”
This procedure shows promise for improving/regaining performance of dressed cavities w/o removal of He vessel.
TTC Meeting, 28 February - 2 March, 2011, Milan