Dazhang Huang MUTAC Review LBNL, April 2008 Fermilab MTA 805 MHz RF Program

April 9th, 2008D Huang, MUTAC08 LBNL2 Collaborators IIT: Dazhang Huang, Yagmur Torun, Dan KaplanIIT: Dazhang Huang, Yagmur Torun, Dan Kaplan Fermilab: Alan Bross, Al Moretti, Zubao QianFermilab: Alan Bross, Al Moretti, Zubao Qian ANL: Jim NoremANL: Jim Norem LBNL: Mike Zisman, Derun LiLBNL: Mike Zisman, Derun Li JLab: Bob RimmerJLab: Bob Rimmer Imperial College, UK: Ajit Kurup (working on 805 cavity automation RF control system)Imperial College, UK: Ajit Kurup (working on 805 cavity automation RF control system)

April 9th, 2008D Huang, MUTAC08 LBNL3 Outline IntroductionIntroduction –Goals & approaches Button material testButton material test –Motivation –Experiment setup –Procedures –Measurements and data analysis E  B studyE  B study –Concept –Experiment schematic SummarySummary

April 9th, 2008D Huang, MUTAC08 LBNL4 Introduction Cavity material test:Cavity material test: –Goal: find materials and coatings that can withstand high peak surface field in strong magnetic field –Approach: use 805 MHz cavity to test buttons made of various materials E  B test:E  B test: –Goal: study RF breakdown limit when accelerating field E  magnetic field B Will be extended to study arbitrary anglesWill be extended to study arbitrary angles –Approach: rotate 805 MHz cavity 90  in solenoid field

April 9th, 2008D Huang, MUTAC08 LBNL5 Cavity material ( “ Button ” ) test “Button” system in pillbox cavity designed for easy replacement of test materials“Button” system in pillbox cavity designed for easy replacement of test materials Tested so far: TiN-coated Cu & Mo, bare Mo and WTested so far: TiN-coated Cu & Mo, bare Mo and W To be tested: Cu (electro- polished & unpolished), BeTo be tested: Cu (electro- polished & unpolished), Be –More to come button 0.75  radius Molybdenum buttons Be window button holder (1.7x field enhancement factor on button surface)

April 9th, 2008D Huang, MUTAC08 LBNL6 Button test: Experiment setup I SC solenoid 201 MHz cavity 805 MHz cavity RD46 “chipmunk” detector

April 9th, 2008D Huang, MUTAC08 LBNL7 Button test: Experiment setup II Vacuum, radiation levels, LHe level, solenoid current & voltage monitoredVacuum, radiation levels, LHe level, solenoid current & voltage monitored Accelerating gradient measured with pickup probe inside cavityAccelerating gradient measured with pickup probe inside cavity Data recorded in computer for later analysisData recorded in computer for later analysis

April 9th, 2008D Huang, MUTAC08 LBNL8 Button test: x-ray detectors 10 x-ray detectors in MTA hall10 x-ray detectors in MTA hall –9 fast scintillation counters, counting rate limit: ~ 10 MHz –1 NaI-xtal energy measurement, counting rate limit: ~ 1 MHz Detectors frequently used in button tests:Detectors frequently used in button tests: –#8 (small scint. paddle) –#16 (NaI crystal) –RD46 “chipmunk” (measuring integrated x-ray radiation dose in 20 sec.) Det. 8 Det. 16 Solenoid

April 9th, 2008D Huang, MUTAC08 LBNL9 Button test: Procedures x-ray measurements:x-ray measurements: –RF pulse length  20 μs –Use electronic gate covering RF pulse –Record x-ray events for 1000 RF pulses Cavity conditioning procedure:Cavity conditioning procedure: –Raise RF amplitude slowly –RF power automatically tripped by: bad vacuum (>1e -8 Torr)bad vacuum (>1e -8 Torr) high radiation level (>200mrem/hr)high radiation level (>200mrem/hr) modulator errormodulator error –On trip, reduce RF amplitude until stably below trip levels –After 5  10 min, gradually increase RF amplitude –Iterate to find maximal gradient without button surface damage We measured the maximal accelerating gradient at B fields up to 3.5 T in 0.25 T increments.We measured the maximal accelerating gradient at B fields up to 3.5 T in 0.25 T increments. gate Det 8 signal RF

April 9th, 2008D Huang, MUTAC08 LBNL10 Button test: coating issue After 1st (Fermilab-coated) TiN_Cu button test, observed  80% of TiN coating lostAfter 1st (Fermilab-coated) TiN_Cu button test, observed  80% of TiN coating lost LBNL then coated 2 new TiN_Cu buttons via 2 different techniquesLBNL then coated 2 new TiN_Cu buttons via 2 different techniques –LBNL coating gold, unlike Fermilab’s (color determined by thickness) After test of LBNL TiN_Cu button #2, observed smooth surface w/ no coating lossAfter test of LBNL TiN_Cu button #2, observed smooth surface w/ no coating loss Cu TiN coating Old (FNAL-coated) button New LBNL button #2

April 9th, 2008D Huang, MUTAC08 LBNL11 TiN_Cu data:TiN_Cu data: –less stable than rest, maybe due to loss of TiN coating Mo data:Mo data: –generally above W data –Mo appears to withstand higher surface field than W Button test results: tests:2008 tests: –more systematic conditioning to avoid coating damage New LBNL coated TiN_Cu button:New LBNL coated TiN_Cu button: –data appear more stable than FNAL-coated TiN_Cu –better performance at high magnetic field & 2008 LBNL TiN_Cu2 – LBNL TiN_Cu2

April 9th, 2008D Huang, MUTAC08 LBNL12 Where x-rays come from RF fields in the cavity may induceRF fields in the cavity may induce –multipactoring –field emission –sparking –… As a result:As a result: –electrons, ions, …, stripped from cavity walls, hit surfaces inside cavity  x-rays

April 9th, 2008D Huang, MUTAC08 LBNL13 Accelerating field on axis (MV/m) Button test: x-ray radiation I LOG-LOG plotLOG-LOG plot All curves display power-law growth, ~E 13, consistent w/ Fowler-Nordheim field- emission law which can be approximated by:All curves display power-law growth, ~E 13, consistent w/ Fowler-Nordheim field- emission law which can be approximated by: I ~ E n, where I is field- emission current, n depends on work function and local field LBNL-coated TiN_Cu button #2, chipmonk RD

April 9th, 2008D Huang, MUTAC08 LBNL14 Button test: x-ray radiation II LOG-LOG plotLOG-LOG plot Det. 16 (NaI crystal) saturated at higher accelerating fieldsDet. 16 (NaI crystal) saturated at higher accelerating fields Again, very sensitive to accelerating gradient,Again, very sensitive to accelerating gradient, display power-law growth LBNL-coated TiN_Cu button #2, scint. det. #8, NaI #16 60 Accelerating field on axis (MV/m)

April 9th, 2008D Huang, MUTAC08 LBNL15 E  B study: Concept Stress on emitter F ~ j  BStress on emitter F ~ j  B –stress can be ~ 10 GPa, sufficient to trigger fracture –Data of Mo & W buttons consistent with this model In open-cell cavity (studied in  2000, lower-right), E generally not parallel to B, whereas in pillbox cavity, E // BIn open-cell cavity (studied in  2000, lower-right), E generally not parallel to B, whereas in pillbox cavity, E // B In order to reveal the relationship between field emission and orientations of E and B field, E  B study is planned.In order to reveal the relationship between field emission and orientations of E and B field, E  B study is planned. Open Cell Pillbox

April 9th, 2008D Huang, MUTAC08 LBNL16 E  B study: Experiment schematic 805 MHz cavity A Cross-section view of the RF Solenoid from the side

April 9th, 2008D Huang, MUTAC08 LBNL17 Summary Experimental studies of various button materials in 805-MHz cavity have been carried out at MTA.Experimental studies of various button materials in 805-MHz cavity have been carried out at MTA. –Experiment setup and diagnostics worked well, ready for more extensive studies. –Coating loss on Fermilab-coated TiN_Cu button. LBNL-coated button #2 shows better behavior without visible loss –Mo seems to withstand higher accelerating field than W –More buttons to be tested: Another TiN_Cu coated by LBNL, electro- polished Cu, unpolished Cu & Be –X-ray radiation follows Fowler-Nordheim law –805 cavity automatic control program is planned to be tested Improved uniformity of test proceduresImproved uniformity of test procedures Reproducibility improvedReproducibility improved Initial E  B experiment setup using existing 805 MHz cavity is going to be done soonInitial E  B experiment setup using existing 805 MHz cavity is going to be done soon