MuCool Test Area Experimental Program Summary Alexey Kochemirovskiy The University of Chicago/Fermilab Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016)
Outline Introduction Motivation MTA facility 201 MHz MICE cavity program Gas filled cavity program Study of vacuum RF breakdown in 805MHz cavities Conclusion August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 2
Why are we interested in Muons? Muon accelerator R&D is focused on developing a facility that can address critical questions concerning two frontiers: The Intensity Frontier with Neutrino Factory generating well-characterized neutrino beams for precise high sensitivity study The Energy Frontier with a Muon Collider capable of probing multi-tev energies August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 3
Muon Ionization Cooling in called for Neutrino Factory (NuMAX) Proton Driver SC Linac Accumulator Share same complex Muon Collider Buncher Front End MW-Class Target Capture Sol. Decay Channel Buncher Phase Rotator Cooling Ini al Cooling Accelera on 0.2 1 GeV 1 5 GeV Accelerators: Single-Pass Linacs µ Storage Ring Proton Driver Front End Cooling Accelera on Collider Ring µ + 5 GeV µ 281m ν ν n Factory Goal: 10 21 m + & m - per year within the accelerator acceptance m-collider Goals: 126 GeV ~14,000 Higgs/yr Multi-TeV Lumi > 10 34 cm -2 s -1 µ + E CoM : SC Linac Accumulator Buncher Combiner MW-Class Target Capture Sol. Decay Channel Buncher Phase Rotator Ini al Cooling Charge Separator 6D Cooling µ Bunch Merge 6D Cooling Final Cooling Accelerators: Linacs, RLA or FFAG, RCS Higgs Factory to ~10 TeV µ + µ August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 4
Muon cooling concept Short lifetime of muon(2.2µs) require accelerating gradients of tens MV/m Gradient is restricted by RF breakdown, rate of which should be kept small It was experimentally shown that strong magnetic fields aggravates problem Study of RF breakdown in strong magnetic field is needed August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 5
Fermilab s Mucool Test Area (MTA) Facility built specifically for muon cooling hardware R&D Capacity to test 201 and 805MHz cavities in strong magnetic field H- beamline passes through the center of magnet bore Infrastructure for clean room assembly and inspection Cavity Magnet Beamline Extensive instrumentation for BD characterization Run control system to detect breakdown events and record relevant data streams August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 6
Data acquisition and run control system Trigger system for breakdown detection and run control Fast oscilloscopes to record time sensitive signals <- Labview Cavity pickups Light signal from optical fibers Scintillators ( X ray detection) Forward and reflected power Radiation detectors Vacuum pressure data Temperature sensors Acoustic spark localization Etc. Run control station at the linac gallery August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 7
MTA experimental programs 201MHz MICE cavity Gas filled cavity program 805MHz vacuum program August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 8
MTA experimental programs 201MHz MICE cavity Gas filled cavity program 805MHz vacuum program August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 9
International Muon Ionization Cooling Experiment (MICE) 201 MHz cavity module very similar to that of the full ionization cooling demonstration in MICE tested in MTA Design gradient of 10.3 MV/m, with cavities operated in fringe fields of multi- Tesla solenoid magnets MTA has a 5-T superconducting solenoid that provides operational conditions similar to MICE channel August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 10
201MHz MICE cavity Pillbox geometry (~1m in diameter, 40cm RF gap) External vacuum vessel Mechanical tuners Extensive instrumentation August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 11
MICE cavity program: operation results The experimental program complete Cavity has been tested under operating conditions similar to MICE Stable performance is demonstrated in both B=0T and B=3T external field configurations with copper and beryllium windows Zero breakdown rates at MICE design peak gradient (~10.3MV/m) Measured radiation rates are within limits for tracker backgrounds Gained a lot of experience in surface preparation, design and operation August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 12
MTA experimental programs 201MHz MICE cavity Gas filled cavity program 805MHz vacuum program August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 13
Helical Cooling Channel The main idea is to combine absorber and RF cavity together Gas filled RF cavities operating inside helix of solenoids Emittance exchange from dispersion in magnetic field 6D cooling Pressured gas inside the cavities acts both as an absorber for beam cooling an as RF breakdown mitigator Hydrogen is an ideal absorber due to large radiation length and stopping power August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 14
Gas breakdown vs metal breakdown Maximum gradient increases with gas pressure (Paschen curve) Gradient behavior gets dominated by metallic RF breakdown at high pressures Baseline 20MV/m gradient is demonstrated for gas pressure as low as 20 atm Gradient is not affected by external magnetic field (green vs magenta) Maximum stable gradient in hydrogen filled cavity for different electrode materials August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 15
Pressured gas cavity beam considerations Beam passing through the cavity must not trigger breakdown experiments at MTA showed it does not Resultant plasma loads the cavity which leads to degradation of accelerating gradient - needs to be minimized Factors effecting plasma loading: Gas pressure Dopant concentration Beam intensity Plasma loading effect in gas filled cavity August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 16
Dielectric Loaded Pressurized Cavity HCC design challenge: 325 & 650 MHz pillbox cavities do not fit in bores of present magnet technology Possible solution load cavity with dielectric to decrease resonant frequency Alumina - ideal dielectric candidate due to low losses Specially designed alumina donut insert to study effect on RF breakdown Dielectric Loaded High Pressure Cavity with alumina donut insert August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 17
Dielectric Loaded High Pressure cavity - performance Gas breakdown observed up to ~10atm pressure Comparison with baseline no-insert performance shows gradient limited by alumina No clear dependence of gradient on alumina purity August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 18
High pressure gas cavities: summary Demonstrated that high pressure gas filled cavities can be operated in external multi-tesla magnetic fields Baseline 20MV/m gradient in multi-tesla field is reachable with hydrogen pressure as low as 20atm Gas breakdown and metallic breakdown curves investigated for different gas species/dopants and metal material Established breakdown limits for alumina of 11-17MV/m depending on alumina purity and gas composition August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 19
MTA experimental programs 201MHz MICE cavity Gas filled cavity program 805MHz vacuum program August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 20
Why do we see the deterioration of cavity performance in strong B fields? Dark current: electron field emission from surface imperfections B field focuses dark current into beamlets Beamlets cause pulsed heating of the surface Pulsed heating leads to surface degradation Breakdown is triggered Potential mitigations: Surface treatment Use higher radiation length materials ( for ex. Be) Decrease impact energy density of electrons D.Stratakis, J.Gallardo, R.Palmer, Nucl. Inst. Meth. A 620 (2010), 147-154 August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 21
Mitigation technique: surface treatment 201 MHz MICE cavity - electro polished, clean room environment Conditioned to design gradient of 10.3MV/m with no breakdowns Modular Cavity - chemically polished, clean room environment. 45MV/m in 0T field reached All Seasons Cavity stainless steel with copper coating August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 22
Mitigation technique: geometry Box cavity: magnetic insulation E B Magnetic field deflects dark current electrons to low E field regions Works well only for angles very close to 90 o Small shunt impedance compared to pillbox cavities August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 23
Mitigation technique: geometry All Seasons Cavity: longer RF gap length Electron energy as a function of cavity length 15 cm Θ = 0 o Θ = 30 o Θ = 60 o August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 24
Mitigation Technique: geometry Grid windows Allow the beam to pass though Increase shunt impedance of the cavity Allow dark current beamlets to exit cavity volume August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 25
Performance of various 805MHz cavities No surface prep for this cavity Factors that affect fit quality: Condition history Local field enhancement around coupler regions Surface treatment August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 26
Pillbox 805MHz modular cavity Unique design features: End walls can be un-mounted easily Allows for end wall material swap Low E fields in the coupler region Allows for careful control over experimental conditions Evaluate different materials Perform frequent inspections to track surface state Goal: to build a coherent picture of processes inside the cavity during breakdown August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 27
Surface inspection after B=3T run Endplate after B=0T run with peak gradient of 50MV/m Endplate after B=3T run with peak gradient of 12MV/m First time inspections were carried out separately after run at zero magnetic field and run at high magnetic field All clearly visible damage was inflicted during B=3T run (!) August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 28
Inspection after B=3T run: damage microstructure Characteristic diameter of a BD damage ~1.5mm with melted core up to ~70um deep Traces of splashing Damage is much more violent after B=3T run, although stored energy is 16 times lower than in B=0T run Splashing traces around BD damage Typical breakdown damage 2mm 0.5mm 5mm 0.2mm Crater depth ~ 30µm August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 29
Inspection after B=3T runs: damage pattern Perfect 1-to-1 correspondence between 354 volcanos on each endplate (new result) - supports the model of BD being induced by focused dark current Coupler Damage distribution is denser in high E field region Mystery: detected 136 sparks, but observed 354 damage sites Endplate Center Map of volcanos 30 August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016)
Next set of measurements will be with Beryllium endplates What we expect to observe: Radiation length of Beryllium (~35cm) for electrons is higher than of copper (~1.4cm) 2.6 mm-thick window Mitigation of breakdown triggers on the surface Better gradient performance There are several measurements enabled by Beryllium: Direct measurement of dark current (Faraday Cup) Measurement of transverse emittance of dark current beamlets (film/glass) Inner surface of Beryllium endplate *Beryllium copper configuration is also being discussed August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 31
Conclusion We have gained a lot of hands-on experience operating cavities in external magnetic fields at MTA facility Applying advanced surface preparation techniques resulted in meeting design gradients for 201MHz MICE cavity We have demonstrated > 20 MV/m operation of 805 MHz vacuum cavities at B = 5 Tesla. That is an encouraging result towards demonstrating the feasibility of vacuum cavity-based ionization cooling channel Using RF cavities with high-pressure gas, we have demonstrated a general solution to the cooling problem A lot of interesting results coming from studying the problem of RF breakdown in strong magnetic field The work is still ongoing, including data analysis and highpower tests August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 32
Thank you slide Thank you! August 23, 2016 Alexey Kochemirovskiy NuFact'16 (Quy Nhon, August 21-27, 2016) 33