# Presentation: 258096

## NCRF R&D Programs and Plans

**Derun Li**
- Center for Beam Physics
- Lawrence Berkeley National Laboratory

- MUTAC Review at BNL
- April 28, 2004

## Collaborators

- R. MacGill, J. Staples, S. Virostek, M. Zisman
- Lawrence Berkeley National Laboratory
- J. Norem
- Argonne National Laboratory
- A. Bross, A. Moretti, Z. Qian
- Fermi National Accelerator Laboratory
- R. Rimmer, L. Philips, G. Wu
- Jefferson National Laboratory
- M. Alsharo’a, Y. Torun
- Illinois Institute of Technology
- D. Summers
- University of Mississippi
- W. Lau, S. Yang
- Oxford University, UK

## Outline

- Introduction
- Experimental study at 805 MHz
- Pillbox cavity with demountable windows
- Cavity iris termination: foils and grids
- Surface damage study with a button in the cavity
- 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
- Components for MICE
- Summary

## Demand for high gradient RF

- Technical challenges
- Muon beam is unstable, and has short decay time (~ 2 s at rest)
- Muon beam is created with LARGE 6-D phase space
- Strong external magnetic field needed to confine muon beams
- Muon beam manipulation must be done **quickly,** including **cooling**
- Requirements of RF cavity for muon beam
- High cavity shunt impedance, high gradient and high field
- Gradient at 201 MHz:  ~ 16 MV/m [Kilpatrick: 15 MV/m]
- Gradient at 805 MHz: ~ 30 MV/m [Kilpatrick: 26 MV/m]
- RF cavity without [beam] iris
- Higher shunt impedance
- Lower peak surface field
- Independent phase control, higher transit factor
**We choose RF cavity with irises terminated by windows ****or grids**

-    Highest possible gradient NC RF cavity !

## NC RF R&D programs

- Experimental studies at 805 MHz with and without external magnetic fields up to 4-Tesla
- Finished open 5-cell cavity test at Lab G
- Designed, fabricated and tested pillbox-like cavity with demountable windows at Lab G
- Will resume the study at MTA, FNAL soon
- Test of new materials/surface coating versus RF breakdown field
- Termination of cavity iris: grids and curved Be windows
- Prototype of 201 MHz cavity with curved Be windows
- Completed cavity design
- Cavity fabrication going well
- Significant progress in FEA modeling on Be windows and grids
- Ready to make 21-cm curved Be windows

## Test setup at Lab G, FNAL

- 12 MW klystron
- with capability of
- 50 s and 15 Hz

- Open 5-cell cavity

- Waveguide Input

- Waveguide directional
- coupler

- Superconducting
- solenoid up to 5 T

- 805 MHz pillbox cavity with windows

## Test setup for the pillbox cavity 

**SC Solenoid**

**805 MHz cavity**
**inside the solenoid**

**Input RF Waveguide**

**Up to 12 MW peak power**

**Pillbox cavity**

**Be (or Cu) windows**

**Waveguide + RF window**

**Coupler**

**Thermo-couples **
**   ****or view ports**

**Three more view ports on the equator**

**Superconducting magnet**

**film**

**Input RF Power**

**Up to three RF probes**

## 805 MHz cavity tests 

**Open 5-cell cavity reached 25 MV/m gradient (54 MV/m surface field) **
- Large dark current with surface and window damage
**Pillbox cavity test has exceeded its design gradient of 30 MV/m with no magnetic field and reached up to 40 MV/m**
**Thin Be windows with TiN-coated surface have been tested versus magnetic fields  up to 4 Tesla**
- No surface damage was found on the Be windows
- Little multipacting was observed; accelerating gradient limit is a function of the external magnetic field

**We have conducted experimental studies at 805 MHz for nearly **
**three years at Lab G, FNAL**

## Grid tube & button study

- ANSYS study of grid tube design and prototype for 805 MHz pillbox cavity (M. Alsharo’a’s Ph.D thesis)
- Tests at MTA soon
- Field enhancement between 1.4 & 3.6
- RF Heating on tubes

**Grid Model     Electric Field     Magnetic Field**

**Maximum Surface Field Enhancement**

- Button study
-  Completed design
-  ~ 70% of peak
-   surface field
-   enhancement with
-   ~ 0.5 MHz shift
-   Ready for
-    fabrication
-   New material
-   Different surface
-     coatings

- Demountable button

## Grids Prototype

- Prototype of grids
- (solid Aluminum)
- for 805 MHz cavity

- 16-cm

##  201 MHz cavity concept

- 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 Be
- windows, but also accommodates
- different windows or grids.

## The cavity body profile

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

- 2o tilt angle

- Spherical section at the equator to
- ease addition of ports (± ~ 6o)
- Elliptical-like (two circles) nose to
- reduce peak surface field

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

- Stiffener ring

- Bolted Be window

## The cavity parameters

-  The cavity design parameters
- Frequency: 201.25 MHz
-  β = 0.87
- Shunt impedance (VT2/P): ~ 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 a neutrino factory
- Up to 17 MV/m peak 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

## 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
- Thin, flat Be foils (pre-tensioned)
- Pre-curved Be foils
- Grids (Ph.D thesis of M. Alshaor’a at IIT)

## 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
- Ready to make 21-cm radius pre-curved Be windows for 201 MHz cavity

- ANSYS simulations: mechanical vibration modes

- Fabricated pre-curved Be window:
- 16-cm in diameter and 0.254 mm thick

## 201 MHz cavity fabrication: spinning at ACME

- Spinning a bowl

- Spinning tools

- An example of using spinning technique !

## RF & CMM measurements at LBNL

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

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

**Measured frequency: 196.97 MHz**
**(simulated frequency: 197.32 MHz)**

## E-Beam welding at JLab

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

- Stiffener ring

## Recent progress for the welding

- fmeasured = 200.88 MHz

## Preparation for equator welding

## Equator welding

## Extruding tests at JLab

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

## 201 MHz cavity status

- Four half shells have been formed by spinning
- Cu stiffener rings were e-beam welded to two half shells
- The shells were mechanically cleaned at JLab
- Shells are ready for machining prior to e –beam welding of equator joint
- Equator weld fixturing has been fabricated at LBNL
- Cavity nose piece rings (Univ. of Mississippi) have been brazed at LBNL
- Conceptual design of RF loop coupler
- E-beam welding of equator joint
- Extruding four ports
- Nose pieces
- Chemical cleaning and electro-polishing of the cavity
- Pre-curved Be windows
- The cavity should be ready for test in MTA at Fermilab this fall

## Components for MICE

-  Envelope

- Absorber

- Tuning

- Bellows

- RF Cavity

- Cavities

- Raft

-  _Eight 201 MHz RF cavities with curved Be windows_ for the cooling channel

- Coupling coils

- Eight 201 MHz cavities

- LH absorber

## Summary

**Good progress on NC RF R&D programs **
- Experimental study at 805 MHz
- 201 MHz cavity prototype
- Be window: FEA modeling and prototype
**805 MHz pillbox cavity tests progress well at Lab G and provide valuable operation experience and experimental data that will benefit accelerator community**
**Experimental study using the 805 MHz cavity will be resumed once the cavity and the SC magnet are moved to MTA, FNAL.**
**201 MHz test cavity fabrication progressing well; ready for testing this fall (2004)**
**The 201 MHz cavity has been used as baseline design for MICE    **