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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). - PowerPoint PPT Presentation
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Recent progress of RF cavity study at Mucool Test Area
Katsuya YoneharaAPC, 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
NuFact'11 - K. Yonehara 3
Problem: B field effect on RF cavity
Grad
ient
in M
V/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
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
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
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
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
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
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
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 MeVDoes 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
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
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/sSF6 dopant fraction
0.0000 % 0.0001 % 0.001% 0.01%
Pic
kup
sign
al (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))
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