# Presentation: 576850

## Superconducting Materials Testing With a High-Q Copper RF Cavity

- Sami Tantawi, Valery Dolgashev, Gordon Bowden, James Lewandowski, Christopher Nantista
- SLAC
- Ricky Campisi*1, Tsuyoshi Tajima*2, Alberto Canabal*2
-  *1ORNL, *2LANL

- Work supported by the U.S. Department of Energy under contract DE-AC02-76SF00515.

- SRF Materials Workshop, FNAL, 23-24 May 2007

## Abstract

- SRF Materials Workshop, FNAL, 23-24 May 2007

## Finding the Right Cavity Shape

- TE01n pillbox
- (field same on top and bottom)

- SRF Materials Workshop, FNAL, 23-24 May 2007

## The Mushroom Cavity

- 57.1% peak

- 54.5% peak

- |Hs|

- contour (inches)

- bottom plate

- r = 0.98”

- |E|

- TE013-like mode

- |H|

- Q0 = ~44,000
- (Cu, room temp.)

- material sample

- _Features:_
- No surface electric fields (no multipactor)
- Magnetic field concentrated on bottom (sample) face (75% higher than anywhere else)
- Purely azimuthal currents allow demountable bottom face (gap).

- _Why X-band_ (~11.424 GHz)?:
- high power & rf components available
- fits in cryogenic dewar
- small (3”) samples required

- axis

- peak

- SRF Materials Workshop, FNAL, 23-24 May 2007

## Other Nearby Resonance Modes:

- 4.0% mode separation from axisymm. TM mode.

- 1.4% mode separation from non-axisymm. mode.

- TE131

- TM32?

- WC150
- (3.81 cm diam.)

- Couple on axis with pure axisymmetric tranverse electric mode TE01 in circular waveguide.

- SRF Materials Workshop, FNAL, 23-24 May 2007

## Mechanical Design

- copper plug

- temperature sensor

- WC150

- Cu

- Sample Dimensions

- ss Conflat flanges

- G. Bowden

- SRF Materials Workshop, FNAL, 23-24 May 2007

## WR90-WC150 Compact High-Purity  TE01 Mode Launcher

- WR90-WC150 Compact High-Purity  TE01 Mode Launcher

- height taper (.400”.800”)

- planar TE10TE20 converter

- WC150

- WR90

- V. Dolgashev

- SRF Materials Workshop, FNAL, 23-24 May 2007

## Cryogenic Dewar

- mode converter

- cavity

- different cavity

- vacuum can removed
- (cavity immersed in liquid He)

- inner diameter ~1 ft.

- Assembly

- WR90 waveguide

- SRF Materials Workshop, FNAL, 23-24 May 2007

## “Cold” Tests (Room Temperature)

|  | HFSS (Cu) | Copper | Niobium |
| --- | --- | --- | --- |
| fr | 11.424 | 11.4072 | 11.4061 |
| QL | 30,991 | 29,961 | 20,128 |
|  | 0.4383 | 0.4611 | 0.2728 |
| Q0 | 44,575 | 43,775 | 25,619 |
| Qe | 101,694 | 94,944 | 93,906 |

- “Cold” Tests (Room Temperature)

- HP 8510C Network Analyzer

- Nb sample mounted in bottom flange

- room temp. measurements

- SRF Materials Workshop, FNAL, 23-24 May 2007

## Processing Cold Test Data

- A “Q circle” is fit to the corrected S11 data in the complex plane to determine fr, , and QL.
- From these Q0 and Qe are derived.

- Temperature measured with a carbon-glass resistor (low end) inserted into a hole in bottom of cavity and from frequency shift (higher temperatures).

- Complex S11 is measured with 1601 points in 1 megaherz around resonance.

- SRF Materials Workshop, FNAL, 23-24 May 2007

## Copper Sample

- Q0: 44,000  190,300

- Q0: 25,600  330,700

- Niobium Sample

- QL

- Qe

- Q0

## Transition of Cavity Q0 During Warmup from Liquid He Temperature (reactor grade Nb)

- 9.3 K

- Transition of Cavity Q0 During Warmup from Liquid He Temperature (reactor grade Nb)

- SRF Materials Workshop, FNAL, 23-24 May 2007

## MgB2 film (~500 nm) by STI

- 4.2K

- SRF Materials Workshop, FNAL, 23-24 May 2007

## MgB2 surface resistance calculated from the difference betw. Cu and MgB2 tests (preliminary)

- SRF Materials Workshop, FNAL, 23-24 May 2007

## Needed Power for Nb RF critical field

- H field along radius of bottom face

- 56.76%

- U=8.72265×10-17 J
- |Hmax|2 = 4.1048×10-6 A2/m2
- |Hmax|2 = *A*U
-  *A* = 4.7059×1010 A2/m2/J

- If |Bc|=180 mT, then |Hc|=143.25 kA/m,
- Uc  0.43607J = 8.754×10-7s Pc for a 1.5 s flat input pulse.
-  Pc  498 kW

- Vertical axis is stored energy divided by incident power

- SRF Materials Workshop, FNAL, 23-24 May 2007

## New test setup for inexpensive accurate characterization of high-field RF properties of materials and processing techniques

- High Power Test of Niobium at 4.2K(reactor grade Nb)

- SRF Materials Workshop, FNAL, 23-24 May 2007

## High Power Testing

- An initial high power experiment was performed, but data has not yet been fully analyzed.
- A feedback code is used to make the low level RF drive track the resonant frequency by flattening the phase during cavity discharge.
- Quenching of Nb sample seen as a roll off of Q as power was raised.

- Our experimental setup is still maturing.  We will add a high-power circulator to isolate the klystron from the cavity reflection.  We will also add a silicon diode and a Cernox temperature sensor.

- We will soon test a sample of MgB2 from STI.   Some samples from STI were low-power tested at  LANL and has shown lower surface resistance than Nb at 4K.

- SRF Materials Workshop, FNAL, 23-24 May 2007

## Conclusions

- We have designed and fabricated a compact, high-Q rf cavity optimized for economically testing the rf properties of material samples and their dependence on temperature and field by means of frequency and Q monitoring.
- We’ve performed low power cryogenic tests with copper, niobium and MgB2 samples.
- High power tests are under way.
- A similar cavity will be used for pulsed heating material testing.

- SRF Materials Workshop, FNAL, 23-24 May 2007