# Presentation: 639984

## FNAL Director’s Review Proton DriverSpoke Cavity Development

*Kenneth W. Shepard*
*Physics Division*
*March 16, 2005*

## Spoke-loaded concept introduced  more than a decade ago

**“****Design and Test of a Superconducting Structure for High-velocity Ions”,** **J. R. Delayen, et al., ****LINAC 92**

**Developed for high-intensity ion beams**
**6.5 inch ID**
**855 MHz**
**= 0.5**
**The two-gap niobium structure was ****operated at 7.2 MV/m at 4.2 K**

## Cylinder     1⁄2 x 1⁄2 Meter

**Cylinder     ****1⁄2 x 1⁄2**** Meter**

**TE111 at 462 MHz**

**TM010 at 459 MHz**

## Cylinder, 1⁄2 x 1⁄2 Meter, plus a transverse spoke

**Cylinder, ****1⁄2 x 1⁄2**** Meter, plus a transverse spoke**

**TM010 at 430 MHz**

**TE111 at 454 MHz**

## Spoke-loading reduces cavity diameter, good properties for  = v/c  1/2

**Lowest RF eigenmode is now a TEM Mode, with the spoke acting like a ********/2 ****section of transmission line.  The resonance is at 340 MHz, somewhat higher than a nominal 300 MHz for a ****1⁄2**** meter length of line**

***Spoke-loading reduces cavity diameter, good properties for ************ = v/c ************ 1/2***

## We need a lower-frequency spoke cavity (~350 MHz) for the RIA driver linac

****=0.4

***We need a lower-frequency spoke cavity (******~******350 MHz) for the RIA driver linac***

****=0.3

**“****Development of Niobium Spoke Cavities for a Superconducting Light-ion Linac”**, **K. W. Shepard, et al., ****LINAC 98**

**Notes:**
And I’ll begin by discussing the spoke cavity development project.
These cavities are particularly interesting because they span a velocity region that is relatively unexplored in ion linacs.
These two cavities were built in the mid-to-late nineties with discretionary money as prototypes for the RIA driver linac
They both operate near 350 MHz and are constructed from solid, mostly eighth inch sheet, niobium
Initially the pieces were electropolished, the end plates were then welded on and the cavities received a very light chemical polish at JLAB.
More recently these cavities were reprocessed in new Buffered chemical polish and high-pressure rinse facilities and I will report experimental results following these treatments.
I’d also like to refer you to the contribution to the 1993 SRF workshop by Delayen et al. were here discusses his earlier 805 MHz spoke resonator.

## Kelly (ANL) and Tajima (LANL) improved performance of ANL niobium prototype spoke cavities with HPR & ultra-clean techniques                                PAC 2001

- Argonne Result **********0.4 cavity**

**Argonne, Spring 2001**

**Los Alamos, Spring 2001**

- Los Alamos Result **********0.3 cavity**

- Q

***Kelly (ANL) and Tajima (LANL) improved performance of ANL niobium prototype spoke cavities with HPR & ultra-clean techniques******                                ******PAC 2001***

**Notes:**
Most of the effort here has been concentrated on the beta 0.4 cavity. This was because multipacting which it was thought might
Be a critical issue in this unexplored region has so far turned out not to be. It also turned out that the people at Los Alamos had an application for a lower beta spoke
Cavity and they offered to process and test the beta 0.3 cavity.
A few weeks before we measured this result the Los Alamos group measured this very nice result for the beta 0.3 cavity. They have
Both a high Q and a quite high maximum accelerating gradient. 

## Los Alamos β-0.175, 350 MHz Spoke for intense beams (LEDA afterburner project)

**“****Test Results of Two LANL Beta = 0.175, 350-MHz, 2-Gap Spoke Cavities”, Tajima, et al., ****PAC 2003**

## IPN Orsay  0.35, 352 MHz single spoke for 100 MeV Proton Linac

**“****Status Report on Supercond-ucting Spoke Cavities at IPN“, Guillaume Olry, ****PAC 2003**

## At ANL, multi-cell 345 MHz spoke-loaded cavities are being developed for RIA

***β******=0.5 and 0.64 triple-spokes***

***β******=0.4 double-spoke***

## Cold-test results for the  =0.4 double-spoke for RIA

***Cold-test results for the ************ =0.4 double-spoke for RIA***

**Shepard, Kelly, Fuerst, presented at ****PAC 2003, SRF 2003**

**The cavity can operate cw at gradients up to 12 MV/m, producing more than 4.5 MV of accelerating potential**

## Cold-test results for the β=0.5 triple-spoke for RIA

**Shepard, Kelly, Fuerst, to be presented at ****PAC 2005**

**At the design gradient, 9.2 MV/m, the cavity produces more than 6 MV of acceler-ating potential**

## Spoke cavities have good mechanical properties

**The Lorentz transfer function for ******** 0.5 triple-spoke measured at ANL by U of I graduate student Zachary Conway   ( to be presented at ****PAC 2005****)**

**Measured mechanical properties indicate that pulsed operation should be straight-forward**

## Tuners and couplers being developed for RIA 

**RF Power Coupler for CW operation at 5 – 10 kW**
**(Need a new coupler for Proton Driver)**

**Prototype piezo-fast-tuner mounted on double-spoke cavity**

## The ANL 345 MHz Triple-spoke cavities can be re-tuned to 325 MHz by increasing the (20 cm) housing radius by only 8 mm (existing tooling can be used to form the parts)

**CST MicrowaveStudio model of 325 MHz triple-spoke for FNAL Proton Driver**

## Conclusions

**The development of superconducting spoke-loaded cavities is well-advanced: ready for production**
**Results to date indicate that pulsed operation is entirely feasible – suitable for Proton Driver**
**Remaining development is straightforward:**
- Prototype 325 MHz niobium cavities
- RF Power coupler for pulsed operation
- Fast-tuner system for pulsed operation