# Presentation: 484745

## 201-MHz RF Studies 

**and Plans**
**Derun Li**
**Center for Beam Physics**
**Lawrence Berkeley National Laboratory**
**MUTAC Review at LBNL**
**April 8-10, 2008**

## Outline

**Introduction **
**brief review**
**Progress on 201-MHz test program **
**RF tests with curved thin Be windows**
**Curved Be windows installed**
**Cavity retuned to 200-MHz**
**Tested up to 19 MV/m**
**RF tests with stray magnetic fields of Lab-G magnet**
**Move the cavity next to Lab-G magnet**
**New transmission line**
**New Vacuum pump**
**Preliminary test results**
**Simulation study of RF cavity in strong magnetic field**
**Plans**
**More 201-MHz test with SC coupling coil (M. Green’s talk)**
**Beam test at MTA (A. Boss’s talk)**
**201-MHz cavities for MICE**
**Review and plans**
**Summary**

## Development of normal conducting 201-MHz cavity that can operate at a gradient of ~ 16 MV/m in a few Tesla magnetic fields environment
Prototype: exploring engineering solutions (challenges)
RF conditioning and operation without and with B fields
Last MUTAC review
The 201-MHz cavity reached 16 MV/m without external magnetic field
The cavity kept at 16 MV/m with ~ a few hundred Gauss stray magnetic fields from Lab-G magnet
We understand: operating a NC RF cavity (with thin and curved Be windows) at ~ 16 MV/m in strong B field could be very challenging, but to be confirmed experimentally
Test with stronger magnetic fields: move the cavity as close as possible to Lab-G magnet
SC coupling coil for MUCOOL: being fabricated at ICST, Harbin   

**Development of ****normal conducting**** 201-MHz cavity that can operate at a gradient of ~ ****16 MV/m**** in ****a few Tesla magnetic fields environment**
**Prototype: exploring engineering solutions (challenges)**
**RF conditioning and operation without and with *****B***** fields**
**Last MUTAC review**
**The 201-MHz cavity reached 16 MV/m without external magnetic field**
**The cavity kept at 16 MV/m with ~ a few hundred Gauss stray magnetic fields from Lab-G magnet**
**We understand: operating a NC RF cavity (with thin and curved Be windows) at ~ 16 MV/m in strong *****B *****field***** *****could be very challenging, but to be confirmed experimentally**
**Test with stronger magnetic fields: move the cavity as close as possible to Lab-G magnet**
**SC coupling coil for MUCOOL: being fabricated at ICST, Harbin  ***** ***

## 201-MHz Prototype Cavity

**The cavity design parameters **
**Frequency: 201.25 MHz**
** *****β***** = 0.87**
**Shunt impedance (*****VT******2******/P*****): ~ 22 MΩ/m**
**Quality factor (*****Q******0*****): ~ 53,500**
**Be window radius and thickness: 21-cm and 0.38-mm**
**Nominal parameters for cooling channels in a muon collider  or a neutrino factory**** **
**~ 16 MV/m peak accelerating field**
**Peak input RF power ~ ****4.6 MW**** per cavity (85% of Q****0****, 3******** filling time)**
**Average power dissipation per cavity ~ ****8.4 kW**
**Average power dissipation per Be window  ~ ****100 watts**** **

** ****The 201-MHz cavity at MTA**

## Test Setup at MTA 

**The 805-MHz and 201-MHz cavities at MTA, FNAL for RF breakdown studies with external magnetic fields.**

**805 MHz pillbox cavity**

**201 MHz cavity**

## Installation of Curved Be Windows

**Cavity surface inspection**

**Be window installation**

## RF Tests of Be Windows

**The cavity reached to ~ 19 MV/m quickly**
**Cavity frequency had to be retuned  **
**Cavity frequency was ****stable ****during the operation, however, we did observe frequency shift due to RF heating on the windows**
**Frequency shift of ~ 125 kHz (from 0 to ~ 19 MV/m, 150-micro-second pulse, 10-Hz repetition rate) in ~ 10 minutes, well within the tuning range ( ~ 110 kHz/mm, ******** 4-mm range)**
**With low external magnetic field**
**Cavity can be operated at ~ 19 MV/m with a few hundred Gauss stray field from Lab-G magnet**
**Tests with stronger external magnetic fields**
**SC coupling coil for MuCool**

## New Vacuum Pumping System

**Getter pumping system:**
** ****1500 l/s Getter pump**
** ****Backed by a 20 l/s Ion pump**
** ****Rough down by a with a**
**    ****Turbo-pump and dry pump**
** ****Vacuum valves & Ion gages**
** ****System has been running **
**     ****over a month without**
**     ****needing a re-charge**
**  ****Has achieved base vacuum**
**     ****level of 2.5x10****-8**** Torr**

**Roughing and Ion pump stand**** **

**Vacuum flexible line connection ****to the Getter pump**

## Tests with Magnetic Fields

**Tests with Magnetic Fields**

**The 100-cm long coaxial section was replaced by 20-cm one**
**Separation of the nearest curved Be window from the face of Lab-G magnet is 10-cm (before was 110-cm)**
**Maximum magnetic field near the Be window ~ 1.5 Tesla (5 Tesla in magnet)**

**The 201-MHz cavity**

**Lab-G magnet**

## Preliminary Test at B=0

**Radiation (mRem/hr)**

**Cavity Gradient (MV/m)**

**Preliminary Test at B=0**

**Represent  Chipmonk #1, #2 and #3, respectively**

**Radiation data compared with a fitting with to **
**gradient to ~ 14****th**** power**

**Zero B field**

## Radiation at 0.5-T Field

**Cavity Gradient (MV/m)**

**Represent  Chipmonk #1, #2 and #3, respectively**

**Radiation at 0.5-T Field**

**Radiation data compared with a fitting with to **
**gradient to ~ 14****th**** power**

**0.5-T B field to the **
**nearest Be window**

## Preliminary Test Results

**Multi-pactoring was observed at the entire magnetic field range up to 3.75-T (~ 1.1-T at nearest Be window)**
**A strong correlationship exists between cavity vacuum and radiation levels**
**We have achieved ~ 14 MV/m at 2.5-T (~ 0.75-T to the nearest curved thin Be window)**
**The test results are very encouraging, data analysis is being conducted **
**We may need to commission (condition)**
**     ****the 201-MHz cavity longer to reduce the **
**     ****multi-pactoring before we can properly **
**     ****measure and study the magnetic field **
**     ****effects in the stray field**
**Numerical studies with external **
**     ****magnetic fields and SC coupling coil**

## Numerical Study with B Field

**Preliminary studies, in collaboration with Dr. Zenghai Li at SLAC using Omega-3P and Track-3P codes**
**Cavity with flat windows:  5 MV/m on axis; 2-T uniform external magnetic field; scan of a few points from one cavity side**

**E field contour**

**Trajectories without external B field**

**Trajectories with external B = 2-T field**

## 201-MHz Cavity for MICE

**Eight 201-MHz cavities + with materials for two more spare cavities **
**Baseline design: 201-MHz for MUCOOL, but**
**Cavity body profile needs to be modified**
**Resonant frequency**
**Better estimation of spring back after spinning**
**Port extruding**
**Port interface is different from the MUCOOL cavity**
**RF coupler and ceramic window**
**The same window as for the MUCOOL cavity: Toshiba ceramic windows for SNS**
**Curved Beryllium windows**
**Improved design to better control silver alloy flow during the brazing**
**Tuners and interface with RFCC module**
**Post-processing: water cooling pipes, cleaning (EP & water rinsing), low power measurement, tuning, assembly and shipping  **

## MICE Cooling Channel

- Courtesy of S. Q. Yang, Oxford Univ.

**LH****2**** absorbers**

**Eight**** 201-MHz RF cavities**

## Cavity Design Parameters

** ****The cavity design parameters**** **
**Frequency: 201.25 MHz**
** ****β**** = 0.87**
**Shunt impedance (VT****2****/P): ~ 22 MΩ/m**
**Quality factor (Q****0****): ~ 53,500**
**Be window radius and thickness: 21-cm and 0.38-mm**
**  ****Nominal parameters for cooling channels in a muon**
**      ****collider  or a neutrino factory and ****MICE**
**~ 16 MV/m ****(****~ ****8 MV/m****) peak accelerating field**
**Peak input RF power ~ ****4.6 MW**** (~ ****1 MW****) per cavity (85% of Q****0****, 3******** filling)**
**Average power dissipation per cavity ~ ****8.4 kW**** ****(~ ****1 kW at 1 Hz repetition rate and 1 ms pulse length****)**
**Average power dissipation per Be window **
**   ****~ ****100 watts**** ****(~ ****12 watts****)**

## MUCOOL Cavity Review

-  **Design and engineering at LBNL**
** ****Half shells spun at Acme in Minnesota **
** ****Parts made in Univ. of Mississippi and LBNL**
** ****E-beam welding & port-pulling, cleaning and **
**  ****EP at J-Lab, NASA**
** ****Coupler tests at SNS, Oak Ridge National Lab**
** ****Final assembly and high power tests at MTA, **
**  ****FNAL (March-2006) and reached 16-MV/m **
**  ****without external magnetic field **

## Cavity Body

**The frequency for MuCool cavity is slightly lower than 200-MHz, but within the bandwidth of RF source**
**Mechanical (fixed) tuners available and tested in air**
**MICE cavities  **
**Target frequency is 201.250-MHz**
**One RF source (tube) powers more than one cavity**
**Each cavity will be made (spinning) to frequency very close to 201-MHz **
**Modifying the existing mold used for MUCOOL cavity**
**A new mold (could be new materials)**
**3D simulations to predict the frequency shifts by ports, curved Be windows and thermal contraction (LN operation)**
**Conceptual tuner designs (fine tuning)**

## Ports Extruding and Coupler

**Local annealing of ports**

**Extruded port**

**Cavity ports being extruded (pulled)**

**Development of the technique**

**Cavity ports being extruded (pulled)**

**Power coupler and RF window**

## Curved Be Windows

**We have made two windows so far**
- 21-cm and 0.38-mm thick
***Good*** braze (between annular frames and foil)
- Achieved ~ 95 % of the designed profile
- Thin Ti-N coatings
**The windows have been tested**
** **

## Tuners for MICE Cavities

**Clocking of tuner position between adjacent cavities avoids interference**
**Actuators offset from cavity center plane due to width of coupling coil**
**Soft connection only (bellows) between tuner/actuators and vacuum vessel shell**

**Four 201-MHz cavities in each RFCC module; one tuner assembly **
**(six sets of tuners) on each cavity.**

## Tuner Concept for MICE Cavity (Sectional View)

- Ball contact only

- Dual bellows
- feedthrough

- Tuner actuator
- (likely air)

- Pivot point

- Fixed (bolted)
- connection

## Post-Processing

**Cavity cleaning, assembly and integration**
**Low power measurements and shipping**

## WBS Plan has been developed 

**WBS Plan has been developed**** **
**First four cavities ready in 2010**

## Summary

**Curved Be windows were installed and tested**
**Reached ~ 19 MV/m with stray magnetic fields of a few hundred Gauss**
**Reached to ~ 14 MV/m with ~ 0.75-T (@ nearest Be window) magnetic field by moving next to Lab-G magnet**
**Preliminary results are encouraging****  **
**Numerical studies of the cavity in magnetic fields started in collaboration with SLAC**
**Plans for building eight 201-MHz cavities for MICE are developed**
**Four cavities will be delivered by 2010**
**Need MuCool coupling coil to provide the needed strong magnetic field for further studies**