# Presentation: 490269

## Acceleration gradient limitations in room temperatureand superconducting accelerationstructures

- Nikolay Solyak
- Fermilab

## RT Breakdown studies in recent years

- NLC/GLC and CLIC type TW and SW accelerating structures
-     @ 11 GHz (SLAC) and 30 GHz (CERN)
- Waveguide, Single cell cavities (SW, TW),
- Special cavities (choke-cavity, TM01, PBG)
- Frequency dependence
- Stored energy, Power and Group velocity
- Pulse length dependence
- Geometry (low/high  impedance structure, slots, choke, phase advance, ...)
- Materials (Cu, CuZn,  Au, Mo, Ti, AL, W, SS, ...), coating
- Surface preparation,  Oxidation
- HP processing
- Surface and data analysis, Simulations and Theoretical models (field emission models,  plasma formation, pulse heating, etc...)

- Aggressive R&D program to understand gradient limitations and physics of breakdown (collaboration of many institutions: SLAC,CERN,KEK,...)

- Still we have no theory which can explain all data for all set of parameters and make a prediction (example: T53vg3MC (good)  T26vg3 (not good))

- (J.P.Delahaye)

## Test Results at 11 GHz

***power fit = E******30***

- T53VG3MC-11GHz

- HDS60S

## Summary on gradient scaling (CLIC)

- For a fixed pulse length

- For a fixed BDR

- New constrain - modified Poynting vector which is geometry independent and can be scaled among **all** (TW and SW) Cu structures.

*A model of the breakdown trigger based on the pulsed heating of the potential breakdown  site by the field emission currents*
*describes both TW and SW accelerating structure experimental results*
*Good agreement experiments with analytical estimate*

*A.Grudiev, W.Wuensch*

## Predictions for test structures

|  | TD18vg2.4 | T18vg2.4 | T28vg3 | TD28vg3 | CLIC_G |
| --- | --- | --- | --- | --- | --- |
| Sc=6.2MW/mm2 @ tpP=100ns |  |  |  |  |  |
| Average unloaded gradient [MV/m] | 109 | 106 | 105 | 103 | 120 |

- Prediction of average unloaded gradient at rect. pulse length of 100ns and BDR=1e-6 based on the results achieved in T53vg3MC: 102.3MV/m at 100ns and BDR=1e-6: Sc=6.2MW/mm2@100ns.

- Best tested CLIC type structure TD18vg2.4:
- Eacc ~ 105 MV/m at 100 ns

*From Alexei Grudiev*

## “Practical” Gradient limitations in RT accelerating structure

- Stored energy (SW), RF power and group velocity (TW)
*fraction of energy dissipated in tip vs. geometry?*
*< 1J energy dissipated in breakdown site*
*In TWS power absorbed in spot, before plasma built*
- Breakdown triggered by field emission -  *E**s *
*Fatigue, modification of boundaries and grains*
- Magnetic pulse heating – ***T~ (H**s**2**)(****)**1/4** *  *(plastic deformation limit: ****T~50**oC for annealed Cu and ~130oC**)*
- Possible other limitations (dark current, Multipactoring, couplers, windows, etc...)

## Growing tips

## Theory of Breakdown 

- A lot of experimental data is available, more will come soon
- Need more statistics for each tested geometry
- Few models are presented, but non of them is able to explain all experiments and predict gradient limits
- Do we need one theory to explain everything?
- Few breakdown mechanisms  few theories
- Clear criteria to define which mechanism is dominated for chosen parameters space
- Facts, which should be explained by theory
- Breakdown in TM01 cavity (no surface electric field)
- Factor of ~2 difference in electric field in low/ high impedance structures
- Gradient limitation in cavity with small gaps (choke cavity)
- ....

## Let’s try to scale gradient limit results obtained for room temperature structures to SC cavity:

RT:	Eacc~100 MV/m (Epk~200 MV/m) at 100ns.

SC:     Eacc~ 20 MV/m (Epk~40 MV/m) @ 1ms or
	         Eacc~  7 MV/m (Epk~14 MV/m) @ 1sec 

BDR – usually not considered for SC cavity
Frequency of SC cavity is lower  stored energy higher

Conclusion:  Physics of breakdown in SRF is different

- RT:	Eacc~100 MV/m (Epk~200 MV/m) at 100ns.
- SC:     Eacc~ 20 MV/m (Epk~40 MV/m) @ 1ms or
- 	         Eacc~  7 MV/m (Epk~14 MV/m) @ 1sec
- BDR – usually not considered for SC cavity
- Frequency of SC cavity is lower  stored energy higher
- Conclusion:  Physics of breakdown in SRF is different

- Scaling results from NC to SC structures

## Achieved Limit of SRF electric field

- No known theoretical limit
- 1990: Peak surface field ~130 MV/m in CW and 210 MV/m in 1ms pulse.
*J.Delaen, K.Shepard,”Test a SC rf quadrupole device”, Appl.Phys.Lett,57 (1990)*
- 2007: Re-entrant cavity: Eacc= 59 MV/m (Epk=125 MV/m,Hs=206.5mT).
*   **(R.L. Geng et. al., PAC07_WEPMS006) – World record in accelerating gradient*

- CW  4.2 K

- EP

## 1.3 GHz cavities – 4 production batch

## “Practical” gradient limitations for SC cavities

- Surface magnetic field – 200 mT (absolute limit?)
- Field emission, X-ray, starts at ~ 20 MV/m
- Thermal breakdown (strong limits for F>2GHz)
- Multipactoring (in cavity or couplers)
- Medium and high field Q-slopes (cryogenic losses)
- Lorentz detuning and microphonics (frequency change)
- Quality of surface treatment and Assembly

## From Alex Gurevich

## Energy gap is a function:  = (T,Es)

- Energy gap is a function: *** = ****(T,Es)*

- Magnito-optical Imaging – A.Polyanskii & P.Lee

## Limit in gradient and Magnetic field 

- Re-entrant single-cell

- K. Saito

## Pushing up SRF gradient limit

- New Geometry

- J.Sekutowicz, V.Shemelin, K.Saito

## Traveling Wave Accelerating Structure

-  ~ 25% (max 42%) higher accelerating gradient (vs. TESLA cavity)
-  Shorter cells (105 deg phase advance) to improve transit-time factor
-  No limitation (up to 10m) in cavity length
-  Need tuning to cancel reflected wave

*A.Kanareykin, N.Solyak, V.Yakovlev, P.Avrakhov*

## Slide 20

## Field Emission

- Caused by macro-particles at the surface
- Starts typically >20 MV/m
- Exponential Q-drop
- X-ray
- Dark current
- Disappear after HPR
- Clean assembly

## Thermal breakdown

- Particle and hole on thesurface

- Sub-mm size defects lead to quench

- Temperature Mapping

## Large Grain  :  Hot   Spots

- Reduced after baking
**Q-slope restored by 40 V anodization**

- Large-grain single-cell

***Large Grain  :  Hot   Spots***

*G. Ciovati - LINAC **(2006)*

## Conclusion

## Thermal Breakdown in pure Niobium

- Solution for *k=const (thermal conductivity)*

## Test result of 3.9 GHz accelerating cavity 

- 3-cell cavity built at FNAL

-  Final cavity preparation done at FNAL (BCP,HPWR)
-  Residual resistance   *R_res ~ 6 n***
-  Achieved: *H_peak = 103 mT**, * *E_acc = 19 MV/m*
* **(Goal: H**peak**= 68 mT,  Eacc=14 MV/m) *
- Magnetic field is likely limited by thermal breakdown
-  No Field Emission
-  *Q ~ 8*10 **9 **at **E_acc =15 MV/m*
* *Maximum accelerating field not depend on Temp

- 9-cell/3.9GHz:   *E**acc**= 21 MV/m*
- TESLA:             *E**acc**= 24 MV/m*

*H = 103 mT *

- No Temperature dependence – why?

## 1. Non-linear Surface Resistance

*For 2 cases: **β**·h<<1 and **β**·h>>1 it gives:*

- The non-linear BCS resistance (RF pair breaking) in the *clean limit* is given by

*Where:*

- *P.Bauer et.all, “Discussion of possible evidence for non-linear BSC resistance in SRF cavity”, SRF 2005 workshop

*Δ* *~1.5 meV* is the superconducting energy gap

- Surface Resistance used in models
- Non-linear case - blue

- - Linear BCS:

## 2. Kapitza Resistance of Niobium surface

- *A. Aizaz, “Improved Heat Transfer in SRF Cavities, NSCL, MSU, 2006
- ** Bousson et. All, “Kapitza Conductance and Thermal Conductivity of Materials Used For SRF Cavities Fabrication”, SRF workshop, 1999

- Summarized parameters *a*, n obtained experimentally**.

## 3. Thermal Conductivity of the bulk Niobium

- Measured values of thermal conductivity* (left) and values used in models for 3 cells and 9 cells case(right)
- * A. Aizaz, “Improved Heat Transfer in SRF Cavities, NSCL, MSU, 2006

## Results for 3.9 GHz cavity

- 3-cell cavity

- 9-cell cavity

*I. Gonin - FNAL*

## Multipacting

- MP-exponential increase of electrons under certain resonance conditions

- M.Liepe

## Multipacting in ICHIRO cavity

## Example: MP in HOM coupler

- Gradient Limitation due to MP in HOM coupler.
- Result: Coupler was destroyed

## Reproducibility problem – Scattering of gradient

- Gradients achieved over time in DESY cavities

- ILC Goal

- K.Saito

## ILC Word-wide S0 / S1  R&D program

- Aim to develop procedure which will provide 85 % acceptance yield in cavity performance
- S0 – single cavity
- S1 – 9-cell cavity
- Fabricate 4x4 cavities and demonstrate yield at 4x4 test each (statistics)

## S0 - Single-cell R&D program (KEK)

- ILC BCD receipt
- final EP (~20μm) after a heavy material removal ( > 100μm).
- HPR (70kg/cm2,1hr)
- Bake (120OC,48hr).
- Eacc = 46.5± 8.0 MV/m (17%)

- EP FLASH receipt
- EP(20μm)+EP(3μm, fresh acid)
- HPR (70kg/cm2,1hr)
- Bake (120OC,48hr)
- Eacc = 46.7±1.9 MV/m (4%)

## Project Cost -vs- Linac Gradient 

- Project Cost -vs- Linac Gradient

- Gradient ( MV/m)

- Relative Cost

- ILC Main Linac

- ~ Eacc2  - Cryogenic losses

- ~ Eacc Linac/Tunnel length

## Summary

- Gradient limitation in RT structure is not fully understood yet, but...
- mechanisms are identified
- multi-parametric studies underway
- better understanding of physics
- Modeling and prediction of gradient limits in real structure
- Gradient limitation in SC cavities
- Few mechanisms are studied
- Critical magnetic field
- FE, thermal breakdown, MP, Q-slope, ...
- New ideas to push gradient limits
- Geometry optimization (LL, Re-entrant, TW structure)
- multilayer
- Surface treatment and Cleaning are the key problems in SC cavity to reach stable performance.