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Recent progress of RF cavity study at Mucool Test Area Katsuya Yonehara APC, Fermilab 1

Recent progress of RF cavity study at Mucool Test Area Katsuya Yonehara APC, Fermilab 1

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Page 1: Recent progress of RF cavity study at Mucool Test Area Katsuya Yonehara APC, Fermilab 1

Recent progress of RF cavity study at Mucool Test Area

Katsuya YoneharaAPC, Fermilab

1

Page 2: Recent progress of RF cavity study at Mucool Test Area Katsuya Yonehara APC, Fermilab 1

Ionization cooling channel

NuFact'11 - K. Yonehara 2

Perpendicular momentumbefore cooling absorber

Perpendicular momentumafter cooling absorber becomes smaller dueto ionization energy loss process

μ beam

AbsorberRF cavity

Magnet Magnet

After π → μ decay & μ collection

Longitudinal momentumis regained by RF cavity

RF cavity is embeddedin strong B field (> 2 T)

Beam envelop

Achievable smallest transverse beam phase space is determined by focus strength (β⊥) and Z & A of cooling absorber

Page 3: Recent progress of RF cavity study at Mucool Test Area Katsuya Yonehara APC, Fermilab 1

NuFact'11 - K. Yonehara 3

Problem: B field effect on RF cavity

Gra

dien

t in

MV/

m

Peak Magnetic Field in T at the Window

>2X Reduction @ required field

X

A. Bross, MC’11

Required E in cooling channel

Data were taken in an 805 MHz vacuum pillbox cavity

Page 4: Recent progress of RF cavity study at Mucool Test Area Katsuya Yonehara APC, Fermilab 1

NuFact'11 - K. Yonehara 4

Mucool Test Area (MTA) & work spaceMulti task work space to study RF cavity under strong magnetic fields

& by using intense H- beams from Linac

Compressor + refrigerator room

Entrance of MTAexp. hall

MTA exp. hall

SC magnet

200 MHz cavity

Workstation

400 MeV H- beam transport line

Page 5: Recent progress of RF cavity study at Mucool Test Area Katsuya Yonehara APC, Fermilab 1

Illustrated “Standard model” of RF breakdown event

NuFact'11 - K. Yonehara 5

1. An “asperity” emits a surface electron

RF cavity wall

2. Electron gainskinetic energyfrom E

RF cavity wall

3. High energy e- smashes on cavity wall and generates secondary e-

4. Electron heats upcavity wall

5. Repeat heating andcooling wall materialinduces wall damage

Show just dominant process

6. Some amount of wall material is taken off from wall and generates dense plasma near surface

B field confines an electron beam and enhances breakdown processas shown in slide 3

Page 6: Recent progress of RF cavity study at Mucool Test Area Katsuya Yonehara APC, Fermilab 1

Material search• High work function & low Z element can be a

good material for cooling channel– Beryllium & Aluminum are good candidate

NuFact'11 - K. Yonehara 6

M. Zisman, Nufact’10

Beryllium button assembled805 MHz pillbox cavity

Simulated max grad in an 805 MHz RF cavity with Be, Al, and Cu electrodes

Test will be happened in this summer & fall

Page 7: Recent progress of RF cavity study at Mucool Test Area Katsuya Yonehara APC, Fermilab 1

NuFact'11 - K. Yonehara 7

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

Page 8: Recent progress of RF cavity study at Mucool Test Area Katsuya Yonehara APC, Fermilab 1

NuFact'11 - K. Yonehara 8

RF R&D – 201 MHz Cavity TestTreating NCRF cavities with SCRF

processes The 201 MHz Cavity – 21 MV/m Gradient

Achieved (Design – 16MV/m) Treated at TNJLAB with SCRF processes – Did Not Condition

But exhibited Gradient fall-off with applied B

1.4m

A. Bross, MC’11

Page 9: Recent progress of RF cavity study at Mucool Test Area Katsuya Yonehara APC, Fermilab 1

NuFact'11 - K. Yonehara 9

Fill up dense gas to slow down dark current

Maximum electric field in HPRF cavity

Schematic view of HPRF cavity

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

Metallic breakdown

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

Gas breakdownOperation range (10 to 30 MV/m)

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

Page 10: Recent progress of RF cavity study at Mucool Test Area Katsuya Yonehara APC, Fermilab 1

NuFact'11 - K. Yonehara 10

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

Beam signal (x8)(8 μs)

RF power is lostwhen beam is on

RF power is recoveredwhen beam is off

RF pulse length(80 μs) p + H2 → p + H2

+ + e-

Ionization process

1,800 e- are generated by incident p @ K = 400 MeV

Does intense beam induce an electric breakdown?→ No!

RF power reductiondue to beam

RF power reductiondue to RF breakdown

RF breakdown light

Beam induced light

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. • Observed plasma density in RF breakdown = 1019 cm-3

• Estimated beam induced plasma density = 1014 cm-3

ν= 802 MHzGas pressure = 950 psiBeam intensity = 2 108 /bunch

Page 11: Recent progress of RF cavity study at Mucool Test Area Katsuya Yonehara APC, Fermilab 1

NuFact'11 - K. Yonehara 11

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

Joule @ E = 20 MV/m

Hence, energy consumption by one electron is (including with initial beam intensity change)

Joule

electrons@ t = 200 ns

Muon collider: ne per one bunch train = 1013 μ × 103 e = 1016 electrons → 0.6 JouleNeutrino Factory: ne per one bunch train = 1012 μ × 103 e = 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

ν= 802 MHzGas pressure = 950 psiBeam intensity = 2 108 /bunch

Page 12: Recent progress of RF cavity study at Mucool Test Area Katsuya Yonehara APC, Fermilab 1

NuFact'11 - K. Yonehara 12

Improve performance of HPRF cavity

0 5 10 15 20 25 30 35 40 45 500.0

0.2

0.4

0.6

0.8

1.0

1.2

RF offRF on

Emax

= 20 MV/m, p = 800 psi

r (300 K) = 1.05x10-7 cm3/s

SF6 dopant fraction

0.0000 % 0.0001 % 0.001% 0.01%

Pic

kup

sig

na

l (N

orm

aliz

ed

)

Time (s)

Beam on

Beam off

ppp ~ 1.25x1012, rb ~ r

col ~ 1 mm,

~ 16% transmission, dE/dx ~ 6 MeV/gcm2

Doping electronegative gas (SF6, NH3)

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

Induce plasma instability by E × B force

Local electric fielddue to plasma oscillation

Apply B⊥Eto induce E×Bforce

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

Page 13: Recent progress of RF cavity study at Mucool Test Area Katsuya Yonehara APC, Fermilab 1

NuFact'11 - K. Yonehara 13

Summary• 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

Page 14: Recent progress of RF cavity study at Mucool Test Area Katsuya Yonehara APC, Fermilab 1

NuFact'11 - K. Yonehara 14

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)