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Status of Sep 09 HPRF Run

Status of Sep 09 HPRF Run. K. Yonehara. Scientific goal. Observe plasma/electron dynamics in high densities hydrogen gas under high electric fields Pre-breakdown light will indicate plasma formation process Time structure (fast → streamer, slow → accumulation)

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Status of Sep 09 HPRF Run

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  1. Status of Sep 09 HPRF Run K. Yonehara NFMCC Friday meeting, K. Yonehara

  2. Scientific goal • Observe plasma/electron dynamics in high densities hydrogen gas under high electric fields • Pre-breakdown light will indicate plasma formation process • Time structure (fast → streamer, slow → accumulation) • Light spectrum will indicate what recombination process is dominant • H+, H2+, H3+, polyatomic hydrogen • Systematic measurement • Define probability density function of breakdown near maximum electric field area NFMCC Friday meeting, K. Yonehara

  3. First breakdown model in GH2 filled RF cavity Plasma accumulation model E ① ② H2 electron Free electron is accelerated by external E and makes cascade if E/p > 14.5 V/cm/torr, that is a condition some electron kinetic energy to be more than 15 eV (ionization energy of H2) Generated free electron by some mechanism (Cosmic ray, Surface field emission, etc) ➂ ➃ Plasma reaches both electrodes and make a short circuit Electrons will be accumulated in several RF cycles if they can survive in one RF cycle (Assumelow recombination rate) NFMCC Friday meeting, K. Yonehara

  4. Simulation based on accumulation model Evolution of electron density in RF 25 MV/m is a threshold field to make a breakdown event in this pressure There is no electron absorption process In this simulation except for recombination process Recombination process (H2+ + e → H2) can be slow (~ms order) according to past study We expected that we can see a pre- breakdown light during accumulation process NFMCC Friday meeting, K. Yonehara

  5. Plasma formation PMT: Measure light in RF cell as a function of time (RCA8575; 5ns rise 20 ns decay) PU: Pick up electric field in RF cell as a function of time We can see an 800 MHz RF cycle PMT PU • Rise time of PU signal is 14 μs, that shows an RF Q value ~6,500 • RF pulse length is 20 μs, hence there is 6 μs RF flattop if there is no BD • A big light burst always happens when a breakdown takes place in RF cell • Timing delay on PMT signal, that is caused by cable delay, transition time etc • However, we do not see any pre-breakdown light (except for background light) NFMCC Friday meeting, K. Yonehara

  6. Compare with simulation model Timing delay comes from cable delay and PMT transition PMT PU Boundary E field PMT: rise time = 3 ~ 5 ns decay time = 60 ~ 80 ns PU: decay time = ~ 10 ns • Observed time constant is much shorter than the plasma formation in simulation • BD event seems to be a prompt, like streamer, rather than accumulation • PMT decay time can show reduction of electron density, that is shorter than the • expected recombination time • Simulation still indicates something, i.e. well reproduced SF6 result in simulation NFMCC Friday meeting, K. Yonehara

  7. Other evidence to determine plasma formation time Breakdown timing in different gas pressure RF power off 6 μs RF rise time BD timing (μs) 8:32 To 8:47 8:58 to 9:31 9:41 to 9:59 10:36 to 10:45 16:59 to 17:53 14 μs Pressure =820 psi Pressure =430 psi Pressure =1600 psi Pressure =1350 psi SF6 run Various pres. Start feeding RF power BD event # NFMCC Friday meeting, K. Yonehara

  8. Sorted BD timing BD timing (μs) 8:32 To 8:47 8:58 to 9:31 9:41 to 9:59 10:36 to 10:45 16:59 to 17:53 Pressure =820 psi Pressure =430 psi Pressure =1600 psi Pressure =1350 psi SF6 run Various pres. The probability of BD event seems to be uniformly distributed Plasma formation will be happened within 1 μs NFMCC Friday meeting, K. Yonehara

  9. Other electron absorption process? Electron will be absorbed in H2 with various processes This chain process diagram is made by Alvin Assume we have muon beam inject into HPRF cell (E=16 MV/m, GH2 pressure = 200 atm) NFMCC Friday meeting, K. Yonehara

  10. Development of polyatomic hydrogen Zeke Insepov We will be able to study those processes by comparing the observed light spectra in HPRF with the simulation NFMCC Friday meeting, K. Yonehara

  11. Prospect in optical measurement • High sense fast PMT • 0.5 ns rise time • Single photon • Measure pre-breakdown light • Study plasma formation • Spectrometer • Study polyatomic hydrogen structure • Study metallic breakdown by measuring Cu lines NFMCC Friday meeting, K. Yonehara

  12. Study breakdown probability function Pierric Hanlet Fitting curve will be a nth order polynomial function This will help us to determine the real maximum RF field Probability of BD This curve will be taken in the new DAQ system Observed E field in HPRF cell Number of BDs Probability of BD = Number of RF pulses NFMCC Friday meeting, K. Yonehara

  13. New data acquisition system Fast signals (optical signal, RF signal) are fed into fast digital oscilloscopes Oscilloscopes are hooked up the DAQ computer to synchronize timing NFMCC Friday meeting, K. Yonehara

  14. Modify HPRF test cell Spectroscopic measurement New magnetic probe NFMCC Friday meeting, K. Yonehara

  15. Preparation • Reconfigured test stand and equipments in MTA • Modified HPRF cell at Machine shop • Open one optical port and one magnetic pickup port • Re-coat Cu plate • Hydrogen safety • First inspection has been done • Next inspection will be happened after gas sealing test • Assemble electronics • Optical device • (Pressure gauge) • DAQ system • Ready to go NFMCC Friday meeting, K. Yonehara

  16. Configuration of MTA exp. Hall Real run Calibration run Use N2 gas for calibration Therefore, no hydrogen roof is required NFMCC Friday meeting, K. Yonehara

  17. Run plan Expected run time: 2 weeks • Gas species can be switched • in a short time except for SF6 • For SF6, it will take an hour to • switch gas • Choose gas species • H2, N2, He, H2+SF6 • Choose electrode • Cu • RF duration time • 20, 40, 80 with more pressure points • Changing electrode is a big issue • since surface conditioning is • required (~ 1.5 day w. calibration) Ex). Calibration with N2 = 1.0 day + Conditioning with GH2 = 2.0 days → Take BD probability function at boundary (see next slide) + Change RF duration = 1.0 day (at five specific pressures) + Open and inspect RF cell = 1.0 day + He run = 1.0 day + H2 and SF6 with Cu electrode = 1.0 day Total: 7.0 days NFMCC Friday meeting, K. Yonehara

  18. Beam test • Radiation safety document is still at Fermilab • Beam test will be happened in December in most optimistic scenario • We start designing a collimator to control beam intensity in the RF cell NFMCC Friday meeting, K. Yonehara

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