201 MHz NC RF Cavity R&D

Derun Li

Center for Beam Physics

Lawrence Berkeley National Laboratory

WG3 at NuFact 2004July 28, 2004

WG3 at NuFact 2004 July 28, 2004

201 MHz NC RF Cavity R&DDerun Li

Osaka University, Osaka, Japan Page 2

Collaborators

R. MacGill, J. Staples, S. Virostek, M. Zisman

Lawrence Berkeley National Laboratory

R. Rimmer, L. Philips, G. Wu

Jefferson National Laboratory

D. Summers

University of Mississippi

W. Lau, S. Yang

Oxford University, UK

WG3 at NuFact 2004 July 28, 2004

201 MHz NC RF Cavity R&DDerun Li

Osaka University, Osaka, Japan Page 3

Outline• Introduction• 201 MHz cavity

– Cavity design– Fabrication status– Progress on curved Be windows

• FEA modeling and prototype for 805 MHz cavity

– 21-cm curved Be windows

• Experimental study at 805 MHzExperimental study at 805 MHz– Pillbox cavity with demountable windowsPillbox cavity with demountable windows– Cavity iris termination: foils and gridsCavity iris termination: foils and grids– Surface damage study with a button in the cavitySurface damage study with a button in the cavity

• Summary

WG3 at NuFact 2004 July 28, 2004

201 MHz NC RF Cavity R&DDerun Li

Osaka University, Osaka, Japan Page 4

IntroductionMuon cooling channels call for normal conducting RF cavities with

highest possible accelerating gradients– Muon beam: secondary, unstable and has short decay

time (~ 2 s at rest)

• Created with LARGE 6-D phase space → High gradient with large beam aperture (iris)

• Strong external magnetic field needed to confine muon beams

→ Normal conducting • Muon beam decays, manipulation must be done quickly,

including cooling→ High accelerating gradient

– Goal: design and engineering for • RF cavity with high shunt impedance, large beam aperture

and withstand high peak RF field→ Higher gradient for the same input RF power (less RF

power)→ No surface breakdown

WG3 at NuFact 2004 July 28, 2004

201 MHz NC RF Cavity R&DDerun Li

Osaka University, Osaka, Japan Page 5

Introduction (cont’d)

• Shunt impedance of an RF cavity

• High peak RF field: Kilpatrick number

• Required gradient at 201 MHz: ~ 16 MV/m

– Kilpatrick: 15 MV/m

• Required gradient at 805 MHz: ~ 30 MV/m

– Kilpatrick: 26 MV/m

axis beamon voltagengAccelerati :

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z

WG3 at NuFact 2004 July 28, 2004

201 MHz NC RF Cavity R&DDerun Li

Osaka University, Osaka, Japan Page 6

Choice of NC RF Cavity

• Conventional approach: NC “Ω” cavity with open iris– “High” shunt impedance for open iris cavities

– high peak surface field (high Epk/Eacc ~ 2)

– Shunt impedance reduces with the increase of iris

→ Very difficult (if not possible) to achieve the gradient required

for a muon cooling channel

• Taking advantage of muon beam’s penetration property, we choose a RF cavity with irises terminated by windows or grids– Pillbox like cavity

– Lower peak surface field (low Epk/Eacc ~ 1)

– Independent phase control, higher transit factor– High shunt impedance, but with large windows

Be

win

dow

WG3 at NuFact 2004 July 28, 2004

201 MHz NC RF Cavity R&DDerun Li

Osaka University, Osaka, Japan Page 7

201 MHz Cavity R&D

• Prototype of 201 MHz cavity with curved Be windows– Cavity design

• Body profile• Be windows, grids• Ports• Coupler• RF (ceramic) windows• Tuners

– Cavity fabrication going reasonably well • Cu sheets + spinning techniques• E-beam welding• Ports extruding• Cleaning, …

– Progress and status of cavity fabrication – Placed purchase order of 21-cm radius of curved Be

windows

WG3 at NuFact 2004 July 28, 2004

201 MHz NC RF Cavity R&DDerun Li

Osaka University, Osaka, Japan Page 8

201 MHz Cavity Concept

Spinning of half shells and e-beam welding

Water coolingchannels

Cavity design accommodatesdifferent windows

At NuFact 2003 !

WG3 at NuFact 2004 July 28, 2004

201 MHz NC RF Cavity R&DDerun Li

Osaka University, Osaka, Japan Page 9

201 MHz Cavity Design

Spinning of half shells using thin Cu sheets and e-beam welding to join the shells. Four ports across the e-beam joint at equator.

Cavity design uses pre-curved Bewindows, but also accommodatesdifferent windows or grids.

WG3 at NuFact 2004 July 28, 2004

201 MHz NC RF Cavity R&DDerun Li

Osaka University, Osaka, Japan Page 10

Cavity Body Profile

De-mountable pre-curved Be windows pointing in the same direction to terminate RF fields at the iris

2o tilt angle

Spherical section at the equator to facilitate fabrication of ports (± ~ 6o)Elliptical-like nose shape to furtherreduce peak surface field

6-mm Cu sheet permits spinning technique and mechanical tuners similar to SCRF ones

Stiffener ring

Bolted Be window

Spinning

E-Beam welding

Port extruding

Curved Be windows

WG3 at NuFact 2004 July 28, 2004

201 MHz NC RF Cavity R&DDerun Li

Osaka University, Osaka, Japan Page 11

The cavity parameters

The cavity design parameters (~1.2 m diameter, 0.43 m long)– Frequency: 201.25 MHz– β = 0.87

– Shunt impedance (Vacc2/Pw): ~ 22 M/m

– Quality factor (Q0): ~ 53,000

– Curved Be window radius and thickness: 21-cm and 0.38-mm (better performance with significant savings, compared to pre-tensioned flat Be windows)

Nominal parameters for a cooling channel in neutrino factory

– 16 ~ 17 MV/m accelerating field

– Peak input RF power ~ 4.6 MW per cavity (85% of Q0, 3τ filling)

– Average power dissipation per cavity ~ 8.4 kW– Average power dissipation per Be window ~ 100 watts

WG3 at NuFact 2004 July 28, 2004

201 MHz NC RF Cavity R&DDerun Li

Osaka University, Osaka, Japan Page 12

Spun half shells + RF & CMM measurements

CMM scans, RF frequency andQ measurements of half shells;Cu tape for better RF contacts.

201 MHz Muon Cavity Shell #1 CMM Profiles

0

4

8

12

16

20

0 10 20 30 40 50 60

Radial Dimension (cm)

Axi

al D

imen

sio

n (

cm)

3 CMM scans per half shell conducted at 0o, 45o, 90o, respectively.

Measured frequency: 196.97 MHz

(simulated frequency: 197.32 MHz)

WG3 at NuFact 2004 July 28, 2004

201 MHz NC RF Cavity R&DDerun Li

Osaka University, Osaka, Japan Page 13

E-Beam welding at JLab

Preparation for e-beam welding of thestiffener ring (left); after the e-beam Welding (above)

Stiffener ring

WG3 at NuFact 2004 July 28, 2004

201 MHz NC RF Cavity R&DDerun Li

Osaka University, Osaka, Japan Page 14

Recent progress for the welding

fmeasured = 200.88 MHz

WG3 at NuFact 2004 July 28, 2004

201 MHz NC RF Cavity R&DDerun Li

Osaka University, Osaka, Japan Page 15

Recent Progress

Po

rt extrud

ing

We have successfully developed techniques to extrude ports across e-beam welded joints.

WG3 at NuFact 2004 July 28, 2004

201 MHz NC RF Cavity R&DDerun Li

Osaka University, Osaka, Japan Page 16

What’s Next?

Cavity has been cleaned and ready for nosewelding and ports annealing in next two weeks

WG3 at NuFact 2004 July 28, 2004

201 MHz NC RF Cavity R&DDerun Li

Osaka University, Osaka, Japan Page 17

Status

• Cavity cleaning at J-Lab• 2~3 Months delay for using NASA e-beam welder

– Extruding ports– Cosmetic welding

• Continue engineering designs– Loop coupler: conceptual → engineering design – Supporting structure (vacuum): developed– RF windows (SNS type 4” coaxial window)– Tuner: mechanical

• Cavity test at MTA this year

WG3 at NuFact 2004 July 28, 2004

201 MHz NC RF Cavity R&DDerun Li

Osaka University, Osaka, Japan Page 18

Window for muon RF cavity

• Performance for an ideal window– Transparent to muon beams

Low-Z material

– Perfect electric boundary to RF field Good electrical conductivity

– Mechanical strength and stability No detuning of cavity frequency under RF heating

• Beryllium is a good material for windows– High electrical & thermal conductivity with strong mechanical

strength and low-Z

• Engineering solutions being explored so far– Thin, flat Be foils (pre-tensioned)– Curved Be foils– Grids (Ph.D thesis of M. Alshaor’a at IIT)

WG3 at NuFact 2004 July 28, 2004

201 MHz NC RF Cavity R&DDerun Li

Osaka University, Osaka, Japan Page 19

Curved Be window R&D– Designed, fabricated and tested pre-tensioned flat Be windows

• They work, but expensive; balance between thickness & RF gradient – Progress on FEA modeling and engineering design of all approaches

– Fabricated pre-curved windows of S.S. and Be for 805 MHz cavity

– Cu frames + curved Be foils: better performance with BIG savings

ANSYS simulations: mechanical vibration modesFabricated pre-curved Be window:

16-cm in diameter and 0.254 mm thick

WG3 at NuFact 2004 July 28, 2004

201 MHz NC RF Cavity R&DDerun Li

Osaka University, Osaka, Japan Page 20

More on Be Windows

• Model validation– Preliminary measurements on mechanical

vibration frequency of curved Be windows for 805 MHz cavity agree with FEA modeling

• Ti-N coatings at LBNL recently– Curved Be windows– Pre-tensioned Be windows

• Placed purchase order of 3 curved Be windows for 201 MHz cavity (21-cm radius, 0.38-mm thick)

• Baseline design for MICE cavities

WG3 at NuFact 2004 July 28, 2004

201 MHz NC RF Cavity R&DDerun Li

Osaka University, Osaka, Japan Page 21

Summary

• Good progress on NC RF R&D programs – Experimental study at 805 MHzExperimental study at 805 MHz

• Tests of Ti-N coated curved and pre-tensioned Be windows at MTA

• Tests on grids

– 201 MHz cavity prototype – Be window: FEA modeling and prototype

• Progress on 201 MHz test cavity fabrication; ready for testing this year (2004)

• Test plans are being developed• The 201 MHz cavity has been used as baseline design

for MICE