Statistics of breakdown and conditioning in pulsed dc and r - PowerPoint Presentation

Slide1

Statistics of breakdown and conditioning in pulsed dc and rf systems

Anders Korsback, Jorge Giner

Navaro

, Robin Rajamaki and Walter WuenschSlide2

Motivation

Statistics

Physics – The statistical properties of breakdown may give us insight into the evolution of the surface under pulses, the underlying trigger mechanism and what happens to the surface after breakdown.

Practical – We try to operate the structures ever closer the gradient limit but we demand reliability and life-time. The statistical properties may give the essential life functions.

Conditioning

Physics – Determining what exactly gets better as a structure conditions could give us insight into etc.

Practical – Conditioning is long, months at 50 Hz, and consequently expensive. How can we shorten this process or replace it with another one?Slide3

Breakdown statistics: RF and DC

Anders Korsbäck

CERN / University of Helsinki

Robin Rajamäki

CERN / Aalto University

Jorge Giner Navarro

CERN / University of ValenciaWalter WuenschCERN

WW note: The first time I saw a BD interval plot was by W. FaraboliniSlide4

What is breakdown statistics?

The operational history of an accelerating structure tested in Xbox-1 is shown. Instead of accumulating breakdowns at a constant rate, it shows a “staircase” structure on many scales in a self-similar way.

Hence, a single (overall) breakdown rate, i.e. n

breakdowns

/n

pulses

, is clearly insufficient to describe what’s going on. It doesn’t say anything about when breakdowns happen…

…in relation to the overall history

…in relation to each otherSlide5

Two-Rate Statistics

Or, to visualize what was just explained, let’s return to the operational history vector and to number of pulses to breakdown:

□: non-breakdown pulse,

■

: primary BD,

■

: follow-up BD□□□□□□■□■□□■□□□□□□□□□□□□□□□□□■■

□

■■

□□□□□□□□

■

■

□

■

□□□□

7

2 3 18 1 2 1 9 1 2

2, 3, 1, 2, 1, 1, 2

7, 18, 9

Red

numbers are values for nr of pulses to BD for primary BDs,

blue

for follow/up BDs.

Red

and

blue

are individually Poissonian, giving a two-exponential probability density when put togetherSlide6

Comparison of rf and dc

rf

dc

KEKSlide7

Breakdown positioning in CLIC prototype RF accelerating structures

CLIC workshop 2016

R

. Rajamäki*,

W. Farabolini, J. Giner Navarro, T. Argyropoulos, B. Woolley, W. Wuensch19.01.2016

*Aalto university / CERNSlide8

Introduction

What?

Localize BDs in RF accelerating structures

Why?

Structure diagnosticsBreakdown studiesHow?RF power and phaseDirectional couplerStructure vibrationsAccelerometer

Electron emissionFaraday cupSpectrometerPhotonsPMT/ cameraX-raySlide9

Structure diagnostics (1/2)

y-axis projection

x-axis projection

BD

cell

mapTD26CCSlide10

Method

comparison (3/3)

1.

2.

3.

Observations:

Methods are generally in agreement

Non-symmetric spread

Peak of correlation method

What about possible expanations?Slide11

Breakdown migration?

Upstream migration

Downstream migration

i.

ii.

iii.

Courtesy of W. FaraboliniPossible migration scenario

Mainly

upstream

migrationSlide12

Spatio-temporal correlations

Vertical lines = artefacts of conditioning algorithm

Breakdowns arriving shortly after

each

other

occur close to each other.i.ii.iii.

 Slide13

Newest pulsed dc data

Long pulsed dc run with electrodes prepared with same procedure as rf structures.Slide14

Heat treatment and joining

KEK/SLAC

Tsinghua U.

SINAP

CERNSlide15

BDR as a function of fieldSlide16

And corresponding distributionsSlide17

Performance summary at CLIC specifications

 Slide18

Conditioning

Accelerating structures do not run right away at full specification – pulse length and gradient need to be gradually increased while pulsing. Typical behaviour looks like this:

4 million pulses per day at 50 Hz

Pulse length steps

BDR falls during flat E runSlide19

Scaled gradient vs cumulative number of PULSES

Scaled gradient vs cumulative number of BREAKDOWNS

Comparing conditioning

Pulses

Breakdowns

 Slide20

Newest pulsed dc data

Long pulsed dc run with electrodes prepared with same procedure as rf structures.Slide21

Longer term operation

7 months @ 50 Hz

30

days @ 1 kHz

pulsed dc

rfSlide22

Long term evolution of BDR

rf

pulsed dc

PulsesSlide23

Effect of venting system

Test vent of dc system, 3 daysSlide24

Antti Meriläinen

1,2

, Robin Rajamäki

3

, Ivan Kassamakov1,2, Walter Wuensch3, Kenneth Österberg1,2 and Edward H

æggström1

1) Department of Physics, University of Helsinki 2) Helsinki Institute of Physics3) CERNDynamic Vacuum Meter is at CERN19.1.2016Slide25

Dynamic Vacuum Meter

50 cm

Dr. Walter

Wuensch

, Introduction to CLIC, Collaboration meeting at HIP 19.10.2010

19.1.2016

BreakdownVacuum tube or AS-elementp(t)Quartz crystal microbalance (QCM) as referencex = 5 ± 0.5 cm

R

Beam

= 5 mm

R

QCM

= 6.5 mm Slide26

19.1.2016

Electrode Design

For 0.1 mm

gap

, r ≥

0.3 mmRDesign = 0.5 mm ≥ 0.3 mmRDesignSlide27

Cad Design

19.1.2016Slide28

19.1.2016

Optics and ElectrodesSlide29

19.1.2016

DVM & DC

Spark

Estimated signal for conditioning

process

Estimated signal for Breakdown

Cu atoms releaseCu atoms releaseBreakdownSparkSlide30

New HV

pulser

15 kV, 1 kHz Marx generator from ISEL, Lisbon.

Fast rise and fall time!Slide31

Electrode pipeline

Running

Hard copper, with acoustic sensors - running

Available

Cu,

CuAg, stainless steel - SLAC Nb

3-D printed TiSmall ridge for optical accessAbout 10 pairs of diamond machined CuCeramic spacersUnder preparationVoids - HelsinkiDiamond like coating - PSISlide32

Nb

electrode made in central workshop.

We propose to build this cool-able to cryogenic temperatures.