july 28, 2004k. yonehara, nufact'04 osaka1 high pressure rf cavities for muon beam cooling...

July 28, 2004 K. Yonehara, NuFact'04 Osaka 1

High pressure RF cavitiesfor muon beam cooling

Katsuya Yonehara

Illinois Institute of Technology

NuFact04 in Osaka 7/28/04


Muon accelerators

• Muon colliders (Energy frontier machine)– No limitation by synchrotron radiation

• Radiation (beam-strahlung) factor (m/me)2 ~ 40,000

– 1/10 energy/footprint of proton colliders• Energy of interaction is full energy of produced

particle since s are fundamental particles.

• Neutrino factories (Muon storage ring)– Exciting new physics


Six muon projectsPromoted by Muons, Inc., July, 2004

• High-pressure gaseous hydrogen RF cavity• MANX (Muon collider And Neutrino factory

eXperiment)• 6-Dimensional helical cooling channel

– see MuCoolNote0284

• Hydrogen cryostat• Phase ionization cooling• Cryogenic pulsed power compressor

See “http://www.cap.bnl.gov/mumu/conf/MC-040127/johnson-MC.pdf “for below three items

http://www.cap.bnl.gov/mumu/conf/MC-040127/johnson-MC.pdf


Muon ionization cooling

Step 1: Beam energy loss dE/dx in transverse and longitudinal directions

Step 2: Longitudinal energy replaced by RF electric acceleration field

# A strong solenoidal magnetic field is required to make a low in cooling material.

Cooling material

Step 1

Step 2

This method is only available for muons.

Py

Pz


Properties of gaseous hydrogen

• Best cooling material– Highest (X0 dE/ds)2

• High heat capacity– Cools Be RF windows effectively

• Low critical temperature– 33 K


Advantages of high pressuregaseous hydrogen in an RF cavity

• Dense GH2 suppresses high-voltage breakdown– Small mean free path inhibits avalanches

(Paschen’s law)

• Gas acts as an energy absorber– Needed for ionization cooling


Phase I research

• Build cryogenic HP RF test cell– Need special sealing technology on every joint

(ex. RF feed line, pickup coil, etc)– Good pressure seal and RF seal

• Measure RF breakdown voltage vs. pressure– The data should follow Paschen’s law, relating

breakdown voltage to gas density, over a range of temperatures, and pressures.

– Compare helium and hydrogen.


Collaborators

R. E. Hartline, R. P. Johnson, M. Kuchnir,

T.J. RobertsMuons, Inc.

C. M. Ankenbrandt, A. Moretti, M. PopovicFermi National Accel. Lab

M. Alsharo’a, D.M. Kaplan, K. YoneharaIllinois Institute of Tech.


HP RF cavity

Electrode (“knob”)replaceable

Metal sealing (use Aluminum gasket) RF feed line

All surfaces arecopper plated


Test Cell


Experiment


Frequency shift

P vs f, GH2 @77K 11/19/03

frequency = -7E-07P2 - 0.0263P + 805

794

796

798

800

802

804

806

0 100 200 300 400

P (PSIA)

TC

fre

qu

ency

(M

Hz)


11/19/03 Lab G Results, Molybdenum Electrode

H2 vs He RF breakdown at 77K, 800MHz

0

10

20

30

40

50

60

70

80

0 100 200 300 400 500 600

Pressure (PSIA)

Max

Sta

ble

Gra

die

nt

(MV

/m)

Linear Paschen Gas Linear Paschen Gas Breakdown RegionBreakdown Region

Metallic Surface Metallic Surface Breakdown RegionBreakdown Region

Waveguide BreakdownWaveguide Breakdown

Hydrogen Hydrogen

HeliumHelium

Fast conditioning: 3 h from 70 to 80 Fast conditioning: 3 h from 70 to 80 MV/mMV/m


Results

• 80 MV/m surface gradient achieved• Fast conditioning (Last 15 MV/m in 3 h)• Note that the resonant frequency diminishes with

pressure since the dielectric constant depends on density. As the klystron was tuned to follow the cavity resonant frequency, a resonance in the wave guide caused breakdown before the power reached the cavity i.e. the dip in the previous plot.


Plan I: Observe breakdown• Study metal surface breakdown

– Change electrode material• Cu, Mo, Cr, Be (ref. Perry Wilson)

• Optical approach– Use glass fiber optics

• Good transmission efficiency from UV to VIS region• Insensitive to temperature

– Use photo diode• Good sensitivity from UV to VIS region• Insensitive to magnetic field

– Use C fiber ferrule for optical feedthrough• Sealing test has been done under 1600 PSI GHe• Yet to do the same test at cryogenic temperature


Plan I setup

NPT-SWG1/8

C fiber ferrule

Fiber optics(D 500 microns)

Change electrodes materials


Plan II: Test with proton beam

• What is breakdown voltage with ionizing radiation? We expect;– Fast ion recombination in HP GH2

• Much shorter than the RF period

– RF breakdown is suppressed• Extrapolating from our measurement ~700 MV/m at

one half of LH2 density


Hopes for HP RF cavity

• Higher gradients than with vacuum• Less dependence on metallic surfaces

– Dark currents, x-rays diminished– Very short conditioning times already seen

• Easier path to closed-cell RF design– Hydrogen cooling of Be windows

• Can be used for 6D cooling and acceleration– Homogeneous absorber concept– Implies HF for muon acceleration (1.6 GHz)


Present activities for HP RF Phase II project

• Studying RF breakdown with Cu, Mo, Cr, Be electrodes 50:85:112:194 (Perry Wilson)

• Planning Test Cell for Operation in the 5 Tesla solenoid at 1600 PSI (~ 110 atm) and 77K

• Working on MTA Beamline– Want radiation test of GH2 RF in 2005

• Constructing simple MTA beam line (reference; MuCoolNote0287, 0294)


MANX project

• This is a first project using the high pressure gaseous RF cavities

• Muons, Inc. are funding phase I


MANX is GH2 version of MICE

Scifi Tracker Regions

Matching coils

Spectrometer solenoid 2

Cooling solenoids 1 & 2

High Pressure H2

RF cavities


Summary for HP GH2 RF

• GH2 an enabling technology for machines

– High gradient RF for less-expensive, more efficient beam cooling

– Emittance exchange with homogeneous absorber• 6D cooling makes Muon Collider possible

– Maybe ionization cooling parametric resonances for higher luminosity

• 6D cooling for less expensive acceleration for Neutrino Factory


6-Dimensional helical cooling channel

Design and feasible study of 6D HCC by using ICOOL and G4BL


Helix cooling channel

Bcouples with vz

Bzcouples with v

HPGH2 filled RF cavity

Helix + Solenoid coilsOne segment


Helical Dipole Magnet

Warm spin rotatorfor AGS ring at BNL


Simulation results in ICOOL 1

Reference orbit

Particle orbit

z

xy

Bx, By

z


Simulation results in ICOOL

No cooling material Multiple scattering on

No multiple scattering

transverse longitudinal 6-d

N = 500 muons


Possible origin of RF off-phase


Summary for 6D HCC• Analytical investigation by Ya S. Derbenev and

R.P. Johnson– Cooling factor: ~106

• First cooling effects were observed in ICOOL– Cooling factor: >106 without multiple scattering– : ~20 with multiple scattering– Need tilting RF cavities

• Progress on G4BL– Install ICOOL helix field and tested– Test basic parameters (compare with ICOOL): dE/ds,

Multiple scattering angle, range: a few % discrepancy

july 28, 2004k. yonehara, nufact'04 osaka1 high pressure rf cavities for muon beam cooling...

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