# Presentation: 532479

## High Gradient RF Cavity R&D for Muon Cooling Channels 

**Derun Li**** **
**Beam Electrodynamics Group**
**Center for Beam Physics**
**Lawrence Berkeley National Laboratory**
- DOE High Energy Physics Review
- February 18-19, 2004

## Outline

**Introduction**
**High Gradient RF Cavity R&D Activities**
- 805 MHz cavity
- 201 MHz cavity
- Be window R&D
- LBNL Responsibility for MICE
- RF and coupling coil module
- Cooling channel integration
**Summary**

## Introduction

- Muon beam is unstable, and has short life-time (~ 2 s at rest)
- Muon beam is created with LARGE 6-D phase space
- Muon beam manipulation must be done **quickly,** including **cooling**
** ****Demand for highest possible RF gradient cavities**
- Technical challenges
- Requirements of RF cavity for a muon cooling channel
- High cavity shunt impedance and high accelerating gradient
- Gradient at 201 MHz:  ~ 17 MV/m 	[Kilpatrick: 15 MV/m]
- Gradient at 805 MHz: ~ 30 MV/m 	[Kilpatrick: 26 MV/m]
- Normal-conducting:** strong external magnetic fields** preclude use of superconducting RF
- Practical implementation for muons: **RF cavity with irises terminated by thin low-Z material windows **
- Higher shunt impedance
- Lower ratio of peak surface field to accelerating field
- Independent phase control, higher transit-time factor
- We started with experimental R&D at 805 MHz cavity

## 805 MHz Cavity Test Setup at Lab G of FNAL

**SC Solenoid**

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

**Input RF Waveguide**

**Up to 12 MW peak power**

## Collaboration on the 805 MHz Cavity Tests at Lab G, Fermilab

- Exceeded its design gradient of 30 MV/m at no magnetic field and reached up to 40 MV/m
- Thin Be windows with and without 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

**LBNL 805 MHz cavity had been under test for nearly two years**
**at Lab G, Fermilab**

##  201 MHz Cavity Concept

** ****201 MHz Cavity Concept**

- Spinning of half
- shells using thin Cu
- sheets and e-beam
- welding to join the
- shells

- Water cooling
- channels

- Cavity design accommodates
- different windows

- Extruding ports

## The Cavity Body Profile

- De-mountable pre-curved
- Be windows 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
- Be window radius and thickness: 21-cm and 0.38-mm
-   Nominal parameters for 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

## Spinning @ 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

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

- Stiffener ring

## Extruding Tests at JLab

- Extruding tests on a flat Cu plate
- going through e-beam joint

- Possible improvement:
- Anneal around the extruding area or
- Combination between pilot hole
- dimensions and lid heights, ...

*** ******Successful extruding recently!****    *

## 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
- 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

## Be Windows R&D

-  Ideal windows
-   Transparent to muon beams
-   Perfect electric boundary to RF field
-   No detuning of cavity frequency
-  Engineering solutions
-  Pre-stressed flat Be (low-Z) windows
-  **Pre-curved Be windows**
-  Grids

- A pre-curved Be window:
- 0.25 mm thick and 21 cm
- in radius

**Window **
**profile**
**evolutions**

## Curved S.S. Windows

- Succeeded in the S.S. window with Cu frame for 805 MHz cavity

- Pre-formed at room temperature by holding
- foil edge then brazing the Cu frames

- A finished curved S.S. window with
- brazed Cu frame

## Curved Be Window 

- Failed in forming Be window at room temperature

**The curved Be windows can be formed at higher temperature  at Brush-**
**Wellman Company.  Purchase order has been placed and the window **
**will be ready soon.   **

- The Be foil cracked during forming at
- room temperature (LBNL)

-    A successfully formed Be foil
- (Feb. 5th 2004 at Brush-Wellman)

## Components for MICE

- Cooling Channel

-  Envelope

- Absorber

- Tuning

- Bellows

- RF Cavity

- Cavities

- Raft

- Solenoid

- Magnet

- LBNL responsibility for MICE: **RF and coupling coil module**
-  _Eight 201 MHz RF cavities with Be windows_ for the cooling channel (D. Li)
-  _SC magnet_ (M. Green) and _system integration_ (S. Virostek)

- Coupling coils

- Eight 201 MHz cavities

- LH absorber

## Summary

**LBNL provides a leadership role in muon-beam R&D**
- RF cavity R&D
- Be window R&D
- SC magnets and system integration
**Expertise that LBNL provides to MC is well recognized**
**805 MHz cavity performs well at Lab G and provides valuable operation experience and experimental data that will benefit accelerator community**
**805 MHz cavity and the SC magnet will be moved to MTA, FNAL  **
** ****LBNL plays leading role and is responsible for eight 201 MHz RF cavities, two SC magnets and system integration**
**201 MHz test cavity fabrication progressing well; ready for testing this fall**