progress of high pressure hydrogen gas filled rf cavity test katsuya yonehara accelerator physics...

Progress of High Pressure Hydrogen Gas FilledRF Cavity Test

Katsuya YoneharaAccelerator Physics Center, Fermilab

MTA RF workshopFermilab, November 15-16, 2010

November 15-16, 2010 1MTA RF workshop – HPRF R&D

Prospect of HPRF cavity projectShort 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…


Time table of HPRF beam test

Nov, 2010 Dec, 2010 Jan, 2011RF modification, assemble& calibrationWave 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


MTA beam line

MTA experimental hall

MTA solenoidmagnet

Final beam absorber

400 MeVH- beam

Beam profile:• Deliver 400 MeV H- beam in the MTA exp. hall• 1012 to 1013 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 MeVH- beam


Apparatus for beam test

400 MeVH- 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 befully stripped in the vacuum windowEx) Thickness = 4.5 g/cm3 × 0.1 = 0.45 g/cm2

Stripping cross section ~ 10-18 cm2

0.45/47.9 × 6.0 1023 = 5.6 1021

5.6 1021 10-18 = 5600


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RF modification & collimator

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 cellRF coax line

beam

beam

Gasinlet

RFinlet

Support rail

Luminescencescreen


Waveguide & circulatorNew 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 powerCurrent 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


Beam extension line

400 MeVH- beam

Elastic scattered protonfrom vacuum window

(Ti)

Current status (11/15/2010)• Discussed with engineer• Final drawing will come out soon• They expected two weeks of construction period


Detector system

400 MeVH- beam

Elastic scattered protonfrom 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


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Data acquisition systemSiPM/PMT 1

2

3

4

5

6

RF Pickup 1

2

RF Forward 1

RF Return 1

Beam counter

RF Klystron triggerSystemtrigger

Gate

Name Diagnostic signal

ADC input

SiPM/PMT1 Top plateLight trig.

Fast Osc1

SiPM/PMT2 Top plateSpectroscopic

Fast Osc1

SiPM/PMT3 Topl plateTPB/spare

SiPM/PMT4 Side wallLight trig.

SiPM/PMT5 Side wallSpectroscopic

SiPM/PMT6 Side wallTPB/spare

RF Pickup1 Electric signal Fast Osc1

RF Pickup2 Magnetic signal Slow Osc

RF Forward1 Upstream of Circulator

RF Return1 Upstream of Circulator

Slow Osc

RF Forward2 Between Circulator&cavity

RF Return2 Between Circulator&cavity

Slow Osc

Toroid1 In front of cavity Fast Osc1

Toroid2 MTA beamline Fast Osc2

Beam counter Telescope Fast Osc2

RF Klystron TTL From MCR Fast Osc2November 15-16, 2010 10MTA 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


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


Physics in HPRF plasma


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, 10-8 cm3/s are

chosen for this simulationScientific 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


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


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 1018~1019 electrons/cm3


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 pickup3 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


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Decay constants in HPRF and box cavity

09/24/2008

Breakdown τ in HPRF (various gas pressures) Breakdown τ in box cavity

Low pressure High 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


Critical issues for down selection

RF field must be recovered in few nano seconds1. 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

calculation3. Electrons recombine with positive ions and removed. If this is very fast

they don’t load cavity, if slow cause troubleBeam measurement will give the recombination rate

4. Solution: use electronegative gas(es) to capture electrons and form negative ions

Beam measurement will verify attachment rate5. A+e →A- heavy negative ions. How long do these hang around and do

they cause the breakdown voltage of the cavity to be loweredBeam measurement will give necessary answers

Feasibility including with hydrogen safety analysis also need to be answered

€

v = μE rf

€

J= nev

€

j×E rf = (neμE rf )E rf


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Collaborators

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)


progress of high pressure hydrogen gas filled rf cavity test katsuya yonehara accelerator physics...

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