# Presentation: 939239

## Breakdown Study on the CLIC Designed T18 X-band Structure

- Faya Wang
- Jul.-31-2008

## Topics

- T18 RF Conditioning History
- Breakdown Study on T18
- Summary

## T18 Structure Profile

- 120°

-  Field
- Amplitude

- The structure is designed by CERN, built at KEK, assembled and bonded in SLAC and tested at SLAC (NLCTA).

| Freq.: GHz | 11.424 |
| --- | --- |
| Cells | 18+input+output |
| Filling Time: ns | 36 |
| Length: cm | 29 |
| Iris Dia. a/λ(%) | 15.5~10.1 |
| Group Velocity: vg/c (%) | 2.61-1.02 |
| S11/ S21 | 0.035/0.8 |
| Phase Advace Per Cell | 2π/3 |
| Power Needed <Ea>=100MV/m | 55.5MW |
| Unloaded Ea(out)/Ea(in) | 1.55 |
| Es/Ea | 2 |
| Pulse Heating ΔT: K (75.4MW@200ns) | 16.9-23.8 |
| High Power Test Time: hrs | 1400 |
| Total Breakdwon Events | 2148 |

## RF Conditioning Statistics 

- 	120MV/m at 70ns for 6hrs    (*152MV/m)
- 	120MV/m at 100ns for 76hrs (*152MV/m)
- 	120MV/m at 140ns for 47hrs (*152MV/m)
- 	110MV/m at 190ns for 41hrs  (*140MV/m)
-              120MV/m at 200ns for 21hrs (*152MV/m)
- 	120MV/m at 210ns for 24hrs (*152MV/m)
- 	110MV/m at 230ns for 78hrs (*140MV/m)
- ~1400 hours total conditioning from 14 Apr. 2008 to 3-Jul-2008
- ~2148 breakdowns (average 119 per cell)
- *:Max accelerator gradient in the structure

- RF Conditioning Statistics

- RF Conditioning Time: hrs

- 1400                                      600

## T18VG2.4_Disk structure RF process history begin at Apr.14 2008

- The gradient is the average unloaded gradient for the full structure.

- The BKD Rate is normalized to the structure length(29cm).

- The beginning 500hrs, maximum unloaded gradient is 110MV/m

## The following 900hrs, maximum unloaded gradient is 120MV/m

- Short pulse higher gradient condition

- Pulse shape dependence BKD study.

- BKD pulse width dependence study at 110MV/m.

- BKD gradient dependence study at 230ns pulse width

## Field Decay Time of Breakdown

- Field Decay time: the time for field collapsed to 5% of normal field level

- Field Decay Time for All Recorded 1100 Events

## BKD Rate Characteristics at Different Conditioning Time

- RF BKD Rate Gradient Dependence for 230ns Pulse at Different Conditioning Time

- RF BKD Rate Pulse Width Dependence at Different Conditioning Time

- After 900hrs RF condition BKD rate has a gradient dependence ~
- and pulse width dependence ~

## Unloaded Gradient at Different Conditioning Time

- For Constant Breakdown Rate

## The following test after 1200hrs  at 110MV/m@230ns shows BKD rate is very high up to 1.9e-5/pulse/m. 

- The following test after 1200hrs  at 110MV/m@230ns shows BKD rate is very high up to 1.9e-5/pulse/m.

- Then, set at 105MV/m@230ns for 140hrs ( 7 BKD Events )

- Red cross—cell position
- Blue dot - breakdown position

## BKD Distribution along Structure at Different Stage Based on the Time of  RF Signal

## Breakdown Distribution along the Structure in Cell Cluster

## Pulse heating related to breakdown study during the conditioning stage of 600~700hrs

## Pulse Heating BKD Study

- Pre pulse

- Main pulse

- After pulse

- SLED output pulse

- No hot cells during this test

| Pre pulse gradient of 100ns pulse width | Time: hr | BKD Events | BKD Rate
(1/pulse/m) |
| --- | --- | --- | --- |
| Single Pulse | 19 | 16 | 1.3e-5 |
| 81MV/m | 16 | 6 | 5.8e-6 |
| 97MV/m | 21 | 8 | 5.9e-6 |
| 111MV/m | 20 | 81 | 6.3e-5 |
| 119MV/m | 21 | 68 | 5.0e-5 |

- Main pulse is fixed to 119MV/m of 100ns pulse width

## Pulse Heating for a square pulse P(t)

- For step pulse case

- Number of pulse to damage the surface at certain pulse heating

- U is a constant, T0 surface temperature without pulse heating

- *V.F.Kovalenko, "Termophysical Processes and Electrovacuum
- Devices", Moscow, SOVETSKOE RADIO (1975), pp. 160-193.

- Red line: power level
- Blue line: pulse heating

- Bad Fit

## For a constant breakdown rate, we have: 

- *From pulse heating related damage caused breakdown:

- #From pulse heating on thermal fatigue:

- #S. V. Kuzikov & M. E. Plotkin,” Theory of Thermal Fatigue Caused by RF Pulsed Heating”, Int J Infrared Milli Waves (2008) 29:298–311

- For a constant breakdown rate, we have:

- *V.F.Kovalenko, "Termophysical Processes and ElectrovacuumDevices", Moscow, SOVETSKOE RADIO (1975), pp. 160-193.

- Stresses caused by temperature gradient

- Probability to break the link among atoms in a lattice

## RF Conditioning history in pulse heating scale

- Based on the experience on the NLC/GLC structure H60VG3, which has a pulse heating temperature of 43K at 65MV/m with 400ns pulse[1], it will not damage the surface on the scale of pulse heating of T18.

- [1] C. Adolphsen, ‘Normal-Conducting RF Structure Test Facilities and Results’, SLAC–PUB–9906, September 2003.

## Summary

- T18 has preformed very well during test, with a quiet low breakdown rate of less than 1e-6/pulse/m at 106 MV/m with 230 ns pulses at the end of conditioning, which maybe good enough for a collider at of 100MV/m, however, it doesn’t yet have all necessary features such as higher efficiency and wake field damping.