Progress of High Pressure Hydrogen Gas Filled RF Cavity Test Katsuya Yonehara Accelerator Physics Center, Fermilab MTA RF workshop Fermilab, November 15-16, 2010 November 15-16, MTA RF workshop – HPRF R&D

Prospect of HPRF cavity project Short term (~ FY12) Demonstrate cavity under high radiation condition Investigate beam induced plasma in the cavity Figure out beam loading effect and suppress it Make a new cavity to test the modification technology Provide information to high level management group for RF technology selection (including with safety issue) Long term (FY13 ~) If the HPRF cavity is chosen as the RF technology Make a realistic RF cavity - Test in cryogenic condition - Test in cooling channel fields - Test gas regulation system - etc… November 15-16, MTA RF workshop – HPRF R&D

Time table of HPRF beam test Nov, 2010Dec, 2010Jan, 2011 RF modification, assemble & calibration Wave Guide & Circulator Beam line Detector & DAQ Timing Calibration RF test without beam RF beam test 805 MHz button cavity test Time duration: 3~4 months MTA Radiation Shielding Assessment has been passed in the local radiation safety committee at Fermilab Beam will be available in January Maximum rate 60 pulse/hr November 15-16, MTA RF workshop – HPRF R&D

MTA beam line MTA experimental hall MTA solenoid magnet Final beam absorber 400 MeV H - beam Beam profile: Deliver 400 MeV H - beam in the MTA exp. hall to H - /pulse Tune beam intensity by collimator and triplet Reduce factor from full linac int. down to 1/40 or less MTA exp hall 400 MeV H - beam November 15-16, MTA RF workshop – HPRF R&D

Apparatus for beam test 400 MeV H - beam Elastic scattered proton from vacuum window (100~1000 events/pulse) No energized device within 15 feet from HP cavity due to hydrogen safety issue Beam must be stopped in the magnet due to radiation safety New apparatus: New HPRF cavity Beam extension line Collimator+Beam absorber Luminescence screen+CCD Beam counter RF circulator + damper etc… (Ti) Bound electrons of negative Hydrogen will be fully stripped in the vacuum window Ex) Thickness = 4.5 g/cm 3 × 0.1 = 0.45 g/cm 2 Stripping cross section ~ cm /47.9 × = = 5600 November 15-16, MTA RF workshop – HPRF R&D

RF modification & collimator 6 Current status (11/15/2010) New RF test cell has been delivered Now Cu plating New rail system has been delivered New collimator has been delivered Waiting for Cu electrode & Thread rod Collimator HPRF cell RF coax line beam Gas inlet RF inlet Support rail Luminescence screen November 15-16, MTA RF workshop – HPRF R&D

Waveguide & circulator New 800 MHz Waveguide 400 MeV proton beam Schematic picture from Al Moretti’s hand drawing Solenoid magnet stage 5-T solenoid magnet HPRF cavity RF Circulator + Damper HPRF (1540 psi, 11/13/09) RF modulation due to reflected RF power Current status (11/15/2010) Al has a hand drawing of new wave guide system Order pieces to extend wave guide Al found the RF power damper and tested November 15-16, MTA RF workshop – HPRF R&D

Beam extension line 400 MeV H - beam Elastic scattered proton from vacuum window (Ti) Current status (11/15/2010) Discussed with engineer Final drawing will come out soon They expected two weeks of construction period November 15-16, MTA RF workshop – HPRF R&D

Detector system 400 MeV H - beam Elastic scattered proton from vacuum window (Ti) Current status (11/15/2010) Luminescence screen+ CCD have been delivered We will use a telescope counter for beam monitor by using existing plastic counters Also use toroid coil for beam intensity monitor November 15-16, MTA RF workshop – HPRF R&D

Data acquisition system 10 SiPM/PMT RF Pickup 1 2 RF Forward 1 RF Return 1 Beam counter RF Klystron trigger System trigger Gate NameDiagnostic signal ADC input SiPM/PMT1Top plate Light trig. Fast Osc1 SiPM/PMT2Top plate Spectroscopic Fast Osc1 SiPM/PMT3Topl plate TPB/spare SiPM/PMT4Side wall Light trig. SiPM/PMT5Side wall Spectroscopic SiPM/PMT6Side wall TPB/spare RF Pickup1Electric signalFast Osc1 RF Pickup2Magnetic signalSlow Osc RF Forward1Upstream of Circulator RF Return1Upstream of Circulator Slow Osc RF Forward2Between Circulator&cavity RF Return2Between Circulator&cavity Slow Osc Toroid1In front of cavityFast Osc1 Toroid2MTA beamlineFast Osc2 Beam counterTelescopeFast Osc2 RF KlystronTTL From MCRFast Osc2 November 15-16, MTA RF workshop – HPRF R&D

Fine timing calibration ps Laser HPRF Optical feedthrough SiPM/PMT Digital Oscilloscope ½” Heliax for RF pickup signal ½” Heliax for Optical signal Current status (11/15/2010) Ordered optical feedthrough Preparing SiPM and optical fiber ps Laser is ready Goal: Timing calibration < 100 ps Acceptable polar angle ~ 45 mrad November 15-16, MTA RF workshop – HPRF R&D

Study delay & distortion of signal in long cable Giulia Collura (Summer student) made a fast laser circuit by using avalanche trangister Realize 0.6 ns timing resolution system Laser diode was located in parallel to R3 Similar test is planed by using < 10 ps Laser system Initial RF pickup signal Final signal after 300 ft ½’’ Heliax cable She also investigated the distortion of signal in cable No big signal distortion is observed Phase shift due to dispersion was observed November 15-16, MTA RF workshop – HPRF R&D

Physics in HPRF plasma November 15-16, MTA RF workshop – HPRF R&D

Expected beam loading effect in HPRF cavity Simulated RF pickup signal in HPRF cavity with high intensity proton beam passing though the cavity M. Chung et al., Proceedings of IPAC’10, WEPE067 Beam loading effect: Beam-induced ionized-electrons are produced and shaken by RF field and consume large amount of RF power Such a loading effect was estimated as a function of beam intensity Recombination rate, cm 3 /s are chosen for this simulation Scientific goals: RF field must be recovered in few nano seconds Measure RF Q reduction to test beam loading model Study recombination process in pure hydrogen gas Study attachment process with electronegative dopant gas Study how long does heavy ions become remain in the cavity November 15-16, MTA RF workshop – HPRF R&D

Electronegative dopant gas SF6 and other Fluorine base gas has a large attachment cross section However, SF6 is dissociated and forms F - that damages the cavity surface Besides, cavity cannot operate in cryogenic temperature lower than the boiling point of SF6 (209 K in STP) NH3 has 1/1000 lower attachment cross section than SF6 but it forms in H2 + N2 mixture gas due to beam energy deposition Question is how much amount of N2 need Damaged Cu electrode November 15-16, MTA RF workshop – HPRF R&D

Diagnostic of plasma dynamics RF pickup signal in breakdown process Equivalent resonance circuit Resonance circuit of normal RF cavity consists of L and C Breakdown makes streamer It produces additional L and R Resonance frequency is shifted by them Spontaneous emission: Solid line is a least square fitting of Lorentz function by taking into account all points Timing delay due to lifetime of de-excitation Broadened Balmer line is observed Stark effect well-explains resonance broadening Plasma density ~10 19 electrons/cm 3 November 15-16, MTA RF workshop – HPRF R&D

Breakdown event in HPRF and vacuum (box) cavities Box cavity (0T, 0degree, 10/08/10) Spec light (515.3 nm) X-ray Trig light RF pickup B= 0T B= 3T Spec light (515.3 nm) X-ray Trig light RF pickup 3 T, 0 degree (2010, 0820) X-ray disappears in strong B fields Trig light (white light) intensity is generally lower in stronger field Decay shape of RF pickup signal is modulated by external fields Spectroscopic measurement sometimes failed due to human error, not physics November 15-16, MTA RF workshop – HPRF R&D

Decay constants in HPRF and box cavity 18 09/24/2008 Breakdown τ in HPRF (various gas pressures) Breakdown τ in box cavity Low pressureHigh pressure Time constant in HPRF cavity is 10 times faster than the box (vacuum) cavity (Please be aware that the RF decay function in box cavity in strong fields are not really exponential) Clear RF frequency shifts are observed in the HPRF but not in the box cavity Both indicate the plasma density in HPRF cavity is much denser than the box cavity November 15-16, MTA RF workshop – HPRF R&D

Critical issues for down selection RF field must be recovered in few nano seconds 1.DC to 800 MHz, Hydrogen breaks down at E/P = 14. It indicates we can use DC data as a framework to explain results. Need higher frequency measurements to test frequency dependence 2.Electrons move with a velocity,. Current. Power dissipation due to electrons in phase with RF and dissipate energy through inelastic collisions = Measurements with beam verify mobility numbers and verify our loss calculation 3.Electrons recombine with positive ions and removed. If this is very fast they don’t load cavity, if slow cause trouble Beam measurement will give the recombination rate 4.Solution: use electronegative gas(es) to capture electrons and form negative ions Beam measurement will verify attachment rate 5.A+e →A - heavy negative ions. How long do these hang around and do they cause the breakdown voltage of the cavity to be lowered Beam measurement will give necessary answers Feasibility including with hydrogen safety analysis also need to be answered November 15-16, MTA RF workshop – HPRF R&D

Collaborators 20 Alan Bross, Al Moretti, David Neuffer, Milorad Popovic, Gennady Romanov, Vladimir Shiltsev, Alvin Tollestrup, Katsuya Yonehara (Fermilab) Chuck Ankenbrandt, Gene Flanagan, Rolland Johnson, Grigory Kazakevitch, James Maloney, Masa Notani (Muons, Inc) Ben Freemire, Pierrick Hanlet, Daniel Kaplan, Yagmur Torun (IIT) Moses Chung (Handong Global Univ.) Giulia Collura (Politecnico di Torino) Andreas Jansson (European Spallation Source) Leo Jenner (Univ. College London) Ajit Kurup (Imperial College) Giovanni Pauletta (Univ. of Udine) November 15-16, MTA RF workshop – HPRF R&D