recent progress of rf cavity study at mucool test area n.
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Recent progress of RF cavity study at Mucool Test Area. Katsuya Yonehara APC, Fermilab. Ionization cooling channel. Magnet. Magnet. RF cavity. Absorber. Beam envelop. Longitudinal momentum is regained by RF cavity. μ beam. RF cavity is embedded in strong B field (> 2 T).

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ionization cooling channel
Ionization cooling channel



RF cavity


Beam envelop

Longitudinal momentum

is regained by RF cavity

μ beam

RF cavity is embedded

in strong B field (> 2 T)

After π → μ decay

& μ collection

Achievable smallest transverse

beam phase space is determined

by focus strength (β⊥) and Z &

A of cooling absorber

Perpendicular momentum

after cooling absorber

becomes smaller due

to ionization energy loss


Perpendicular momentum

before cooling absorber

NuFact'11 - K. Yonehara

problem b field effect on rf cavity
Problem: B field effect on RF cavity

A. Bross, MC’11

Data were taken in an 805 MHz vacuum pillbox cavity

Required E in

cooling channel


Gradient in MV/m

>2X Reduction @ required field

Peak Magnetic Field in T at the Window

NuFact'11 - K. Yonehara

mucool test area mta work space
Mucool Test Area (MTA) & work space

Multi task work space to study RF cavity under strong magnetic fields

& by using intense H- beams from Linac

MTA exp. hall

Entrance of MTA

exp. hall

200 MHz cavity

Compressor + refrigerator room

SC magnet

400 MeV H- beam transport line


NuFact'11 - K. Yonehara

illustrated standard model of rf breakdown event
Illustrated “Standard model” of RF breakdown event

RF cavity wall

RF cavity wall

3. High energy e- smashes

on cavity wall and

generates secondary e-

1. An “asperity” emits

a surface electron

2. Electron gains

kinetic energy

from E

4. Electron heats up

cavity wall

5. Repeat heating and

cooling wall material

induces wall damage

6. Some amount of wall

material is taken off

from wall and generates

dense plasma near surface

Show just dominant process

B field confines an electron beam and enhances breakdown process

as shown in slide 3

NuFact'11 - K. Yonehara

material search
Material search
  • High work function & low Z element can be a good material for cooling channel
    • Beryllium & Aluminum are good candidate

M. Zisman, Nufact’10

Simulated max grad in

an 805 MHz RF cavity with

Be, Al, and Cu electrodes

Beryllium button assembled

805 MHz pillbox cavity

Test will be happened in

this summer & fall

NuFact'11 - K. Yonehara

special surface treatment
Special surface treatment
  • By treating cavity surface by using superconducting cavity technique a field enhancement factor significantly goes down
  • In addition, we propose a very thin coat on the cavity wall by using Atomic Layer Deposition (ALD) method to reduce a field enhancement factor
  • Or, apply E × B force on the wall surface to defocus dark current
    • Test has been done
    • Investigation & analysis are on going

NuFact'11 - K. Yonehara

rf r d 201 mhz cavity test treating ncrf cavities with scrf processes
RF R&D – 201 MHz Cavity TestTreating NCRF cavities with SCRF processes

A. Bross, MC’11

  • The 201 MHz Cavity – 21 MV/mGradient Achieved (Design – 16MV/m)
    • Treated at TNJLAB with SCRF processes – Did Not Condition
  • But exhibited Gradient fall-off with applied B


NuFact'11 - K. Yonehara

fill up dense gas to slow down dark current
Fill up dense gas to slow down dark current

805 MHz High Pressure RF (HPRF) cavity has been successfully operated in strong magnetic fields

Maximum electric field in HPRF cavity

Schematic view of HPRF cavity

  • Gas breakdown:
  • Linear dependence
  • Governed by electron mean free path
  • Metallic breakdown:
  • (Almost) constant
  • Depend on electrode material
  • No detail study have been made yet

Metallic breakdown

Operation range (10 to 30 MV/m)

Gas breakdown

P. Hanlet et al., Proceedings of EPAC’06, TUPCH147

NuFact'11 - K. Yonehara

study interaction of intense beam with dense h2 in high gradient rf field
Study interaction of intense beam with dense H2 in high gradient RF field

ν= 802 MHz

Gas pressure = 950 psi

Beam intensity = 2 108 /bunch

RF power is lost

when beam is on

RF power is recovered

when beam is off

Ionization process

RF pulse length

(80 μs)

p + H2 → p + H2+ + e-

Beam signal (x8)

(8 μs)

1,800 e- are generated by

incident p @ K = 400 MeV

Does intense beam induce

an electric breakdown?

→ No!

RF power reduction

due to beam

By comparing RF power reduction and light intensity in beam induced plasma with these at real RF breakdown, beam induced plasma density must be very thin.

RF power reduction

due to RF breakdown

Beam induced light

  • Observed plasma density in RF breakdown
  • = 1019 cm-3
  • Estimated beam induced plasma density
  • = 1014 cm-3

RF breakdown light

NuFact'11 - K. Yonehara

preliminary estimation of plasma loading effect in hprf cavity for cooling channel
Preliminary estimation of plasma loading effect in HPRF cavity for cooling channel

From RF amplitude reduction rate,

RF power consumption by plasma

can be estimated

ν= 802 MHz

Gas pressure = 950 psi

Beam intensity = 2 108 /bunch

Joule @ E = 20 MV/m

electrons@ t = 200 ns

Hence, energy consumption by one

electron is (including with initial beam intensity change)


Muon collider: ne per one bunch train = 1013μ × 103e = 1016 electrons → 0.6 Joule

Neutrino Factory: ne per one bunch train = 1012μ × 103e = 1015 electrons → 0.06 Joule

  • A 201 MHz pillbox cavity stores 8.5 Joule of RF power
  • > For MC, 0.6/8.5 of RF power reduction corresponds to 4 % of RF voltage reduction
  • > For NF, 0.06/8.5 of RF power reduction is negligible
  • Plasma loading effect in higher frequency pillbox RF cavity will be severe since the
  • cavity stores less RF power
  • > Need some technique to reduce plasma loading effect

NuFact'11 - K. Yonehara

improve performance of hprf cavity
Improve performance of HPRF cavity

Doping electronegative gas (SF6, NH3)

Induce plasma instability

by E × B force

Local electric field

due to plasma oscillation

Apply B⊥E

to induce E×B


(ex. Lifetime of wakefield plasma is O(fs))

This test will be done soon.

  • Other possible improvement:
  • Large charge capacitive RF cavity
  • Plasma loaded RF cavity has a big impedance change
  • > Modify Klystron (ex. multiple RF power injection) to match the impedance
  • Plasma loading in denser gas tends to be smaller
  • > Simply fill denser gas in the cavity to reduce plasma loading effect

NuFact'11 - K. Yonehara

  • MTA is a multi task working space to investigate RF cavities for R&D of muon beam cooling channel
    • Intense 400 MeV H- beam
    • Handle hydrogen (flammable) gas
    • 5 Tesla SC solenoid magnet
    • He cryogenic/recycling system
  • Pillbox cavity has been refurbished to search better RF material
    • Beryllium button test will be happened soon
  • E × B effect has been tested in a box cavity
    • Under study (result seems not to be desirable)
  • 201 MHz RF cavity with SRF cavity treatment has been tested at low magnetic field
    • Observed some B field effect on maximum field gradient
    • Further study is needed (large bore SC magnet will be delivered end of 2011)
  • HPRF cavity beam test has started
    • No RF breakdown observed
    • Design a new HPRF cavity to investigate more plasma loading effect

NuFact'11 - K. Yonehara

additional comments corrections from my presentation
Additional comments/corrections from my presentation
  • Slide 11: There were assumptions to estimate power deposition in an electron swarm
    • Use the same cavity length as an 805 MHz HPRF test cavity: 0.0177 m
    • Electron production ratio (103 electrons/muon) is estimated from 950 psi H2 gas @ room temperature
    • For NF, this number will be fine
    • For MC, we will put 2940 psi @ room temperature
    • Then, the number of electrons in the cavity will be 3 103 electrons/muon
    • E field drops 89 %
  • E amplitude reduction can be managed by tuning RF frequency
    • We do not use crest for cooling channel
    • Acceleration field can be tuned by adjusting RF frequency (ex. 200.000 MHz → 199.679 MHz for 60 ns muon beam pulse length)

NuFact'11 - K. Yonehara