J-PARC Neutrino Primary Beamline Beam Induced Fluorescence Profile Monitor R&D M. Friend, S. Cao, K. Sakashita (KEK), M. Hartz (IPMU/TRIMUF), A. Nakamura, Y. Kosho (Okayama University) September 19, 2018
Outline • J-PARC and Neutrino Primary Beamline Overview • BIF Monitor R&D • Gas Injection System R&D • Optical System R&D 2 / 29
J-PARC Accelerator • J-PARC = Japan Proton Accelerator Research Complex • Accelerates proton beam to 30 GeV by: • 400 MeV Linac (linear accelerator) → 3 GeV RCS (Rapid Cycling Synchrotron) → 30 GeV MR (Main Ring) • Fast Extraction (FX) scheme into neutrino primary proton beamline 3 / 29
J-PARC FX/NU Beam Parameters • Currently : 485 kW with 2.48 s repetition rate (2 . 4 × 10 14 PPP) • 500+ kW achieved during beam tests • Plan to upgrade MR power supplies in 2020 to reach 1.3 s repetition rate • RF improvements can allow for further decrease to 1.16 s • Further improve beam stability, reduce MR beam losses to increase number of protons per pulse Bunch Structure: 8 bunches/pulse Bunch Timing: 80 ns/bunch (3 σ ) Bunch Spacing : 581 ns/bunch Pulse Timing : 4.2 µ s/pulse 4 / 29
Neutrino Primary Proton Beamline 5 / 29
Neutrino Primary Proton Beamline • Preparation section : normal conducting dipole and c quadrupole magnets • Used to bend and focus the proton beam extracted from the MR accelerator • Prepare the beam to be safely transported through the superconducting Arc section • Final Focusing section : normal conducting dipole and quadrupole magnets • Arc section : superconducting • Used to bend and focus the combined-function magnets proton beam correctly onto • Used to sharply bend the the neutrino production target beam towards the Super • Proton beam position, angle, Kamiokande direction size at the target must be carefully controlled 6 / 29
Neutrino Primary Proton Beam Monitors Beamline Final Focusing Section Beam Direction → • Beam monitors are essential for protecting beamline equipment and understanding proton beam parameters for neutrino flux MC • 5 CTs (Current Transformers) – monitor beam intensity • 50 BLMs (Beam Loss Monitors) • 21 ESMs (Electrostatic Monitors) – monitor beam position • 19 SSEMs (Segmented Secondary Emission Monitors) – non-continuously monitor beam profile + Profile Monitor R&D • 1 OTR (Optical Transition Radiation) Monitor – continuously monitors beam at neutrino production target • Working on non-destructive profile monitor R&D for continuous profile monitoring + long-term robustness at high beam powers 7 / 29
Destructive Profile Monitoring Segmented Secondary Emission Optical Transition Radiation Monitor (SSEM) Monitor (OTR) • Ti foil @target (50 µ m thick) • Optical Transition Radiation produced when charged particles travel between two • Protons interact with Ti foils materials with different (15 µ m/SSEM) dielectric constants • Secondary electrons emitted from • OTR light proportional to segmented cathode plane beam profile • Compensating charge in each • Light detected by rad-hard cathode strip is read out by ADC camera in low-rad area 8 / 29
BIF Project Goals Design and fabricate a non-destructive beam profile monitor to be used continuously in the neutrino primary extraction beamline • Beam loss should be reduced < 5 × 10 − 8 for one monitor • For < 100 µ Sv/hr on contact @1.3MW, 3months running • c.f. 1 NU SSEM (15 µ m Ti foils), where beam loss is ∼ 5 × 10 − 5 • Loss of 1 × 10 − 9 corresponds to ∼ 1 m of N 2 gas at 1 × 10 − 2 Pa • Monitor should work well at > 1 . 3MW (3 . 2 × 10 14 PPP) • Because monitor is designed for high-power, continuous operation, does not need to work well down to very low beam power • Minimum PPP of > 1 . 3 × 10 14 (250kW @2.48s repetition rate) • Aim for bunch-by-bunch profile measurement, but may be difficult • At least must make spill-by-spill profile measurement Profile reconstruction precision should be comparable to SSEM precision • SSEM position measurement precision is 0.07 mm • SSEM position measurement stability is ∼± 0.15mm • SSEM width measurement precision is 0.2 mm • SSEM width measurement stability is ∼± 0.1mm → Plan is to develop Beam Induced Fluorescence (BIF) monitor 9 / 29
BIF Design Principle Beam Induced Fluorescence (BIF) monitor • Uses fluorescence induced by proton beam interactions with gas injected into the beamline • Continuously and non-destructively monitor proton beam profile Basic details : • Protons hit gas (i.e. N 2 ) inside the beam pipe • Molecules are excited by interaction • Gas may or may not be ionized • Gas fluoresces when electrons fall to a lower energy state • Pattern of fluorescence light is proportional to profile of proton beam that excited the gas • Fluorescence light exits beampipe through viewport • Observe 1D vertical profile from the side and 1D horizontal profile from the bottom 10 / 29
BIF R&D Overview Now doing R&D for various components : • Gas injection : • For ∼ 1000 photons/spill, need to **locally** degrade vacuum level from ∼ 10 − 5 → ∼ 10 − 2 Pa while maintaining average vacuum level of ∼ 10 − 4 Pa at ion pumps and ∼ 10 − 6 Pa at SC section • Use pulsed gas injection to reach 1 × 10 − 2 Pa during each beam spill • Continue to use ion pumps throughout NU beamline for radiation safety reasons • Light transport and focusing : Must be radiation hard ( ∼ 10 Gy/month) • Light detection : • Must work in/near radiation environment • Must work down to very low light levels (maximize gain) • Should be fast to compensate for drift of any ions in proton beam space charge field • 2 methods of photon detection under development : 1 Focus light onto optical fibers which direct the light to MPPCs in sub-tunnel ← main option under development now 2 Focus light onto gatable image intensifier coupled to rad-hard CID camera ← backup plan; could use for 1/2 readout planes 11 / 29
BIF Number of Detected Photons • Number of detected photons ( N det ) should be � 1000 to reconstruct γ √ Gaussian profile with reasonable stat. error (1 / N = < 3%) • Note : Gaussian beam width is 4 ∼ 8mm at install position = N p N p • Number of detected photons is N det γ ǫ γ • N p = ∼ 2 . 5 × 10 14 protons per spill • Number of produced photons is N p γ = dE / dx × α γ × ∆ z × ρ • Energy loss dE / dx = 0 . 2 keVm 2 / g and number of photons per deposited energy α γ = 0 . 278 keV − 1 for N 2 (Nucl. Instrum. Meth., A492:7490, 2002) • If profile is integrated over ∆ z = 20mm along the beam direction, ideal gas law ( ρ = PM / RT = 0 . 011 P for N 2 ) gives number of produced photons N p γ = 1 . 2 × 10 − 5 P • Reasonable light collection acceptance × efficiency ǫ ≃ 4 . 1 ∼ 7 . 1 × 10 − 5 → Need pressure P = 5 ∼ 8 × 10 − 3 Pa to detect � 1000 photons → Must safely degrade vacuum to ∼ 10 − 2 Pa at BIF interaction point → Should maximize optical collection acceptance × efficiency in order to reduce required beamline pressure 12 / 29
Gas System Design Goals • Achieve 10 − 2 Pa during each beam spill while maintaining average vacuum of 10 − 4 Pa at ion pumps and 10 − 6 Pa at SC section entrance • Ion pump lifetime is proportional to average pressure and is ∼ 4 months @10 − 3 Pa or ∼ 3.5 years @10 − 4 Pa • 10 − 6 Pa required to prevent quench at SC section • Achieve gas non-uniformity of < 5% • Gas non-uniformity directly scales measured profile, so 5% fluctuation of gas density → 5% distortion of profile • Could correct profile by expected gas uniformity, if necessary, but prefer not to • Achieve gas injection with 20% pulse-by-pulse stability • Because BIF does not make a spill-by-spill absolute beam intensity measurement (just position and width measurement), absolute pulse-by-pulse gas stability doesn’t affect measurement • However, stability does need to be good enough to assure the required pressure to ± 20% during each beam spill • In order to ensure enough photons are produced • ie decrease of gas pressure of 20% would degrade statistical error √ 1 / N ∼ 3% → 3.5% 13 / 29
Test Vacuum Chamber Studies • Performed studies using 2 different small test vacuum chambers • 150mm diameter, 840mm and 1020mm length • Compared results of 2 chambers to understand gas flow dependence on chamber shape and benchmark gas injection simulations 14 / 29
Gas System Simulation • Ran simulation of steady-state gas injection by COMSOL (FEM), Molflow (MC) with same test vacuum chamber configuration • Difference in expected pressure for 2 chambers is small, but may be possible to resolve 15 / 29
Gas System Test vs Simulation • Confirmed that steady-state simulation of test chamber matches measured pressure in test chamber • Measurement and simulation are consistent for 2 different test chamber configurations at 2 different vacuum gauge positions for both simulation methods • Unfortunately, systematic error on vacuum gauge measurement is high Near Inlet Near Pump 16 / 29
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