Comparative studies of high-gradient Rf and Dc breakdowns [Elektronische Ressource] / Jan Wilhelm Kovermann

Sujets

• Physik

Informations

• Publié par: rheinisch-westfalischen_technischen_hochschule_-rwth-_aachen
• Publié le: 01 janvier 2011
• Nombre de lectures: 12
• Langue: English
• Poids de l'ouvrage: 3 Mo

Extrait

COMPARATIVE STUDIES OF HIGH-GRADIENT
RF AND DC BREAKDOWNS
Von der Fakultät für Mathematik, Informatik und Naturwissenschaften der RWTH
Aachen University zur Erlangung des akademischen Grades eines Doktors der
Naturwissenschaften genehmigte Dissertation
vorgelegt von
Diplom-Physiker Jan Wilhelm Kovermann
aus Vreden
Berichter: Universitätsprofessor Dr.A.Stahl
Dr.W.Wuensch (CERN)
Tag der mündlichen Prüfung: 17.12.2010
Diese Dissertation ist auf den Internetseiten der
Hochschulbibliothek online verfügbar.Contents
1 Introduction 1
1.1 The need for a linear collider in HEP . ............................ 1
1.2 The CLIC accelerator . . ................................... 4
1.3 CLIC accelerating structures: 100 MV/m as a feasibility issue . . .. 6
1.4 State of the art in accelerator performance limitation .................... 6
1.5 Study of the breakdown phenomena . ............ 6
2 Introduction to accelerating structures 7
2.1 Travelling wave structures and performance limiters . ............. 7
2.1.1 Overview of a travelling wave accelerating structure . . . ............. 7
2.1.2 Periodic loading and Floquet’s theorem . . . ........ 8
2.1.3 Finite length travelling wave structures . . ................. 9
2.1.4 Surface and volume field distribution ............ 10
2.2 High power limits and scaling laws . . .................... 11
2.2.1 The Fowler-Nordheim field emission law . . ........ 11
2.2.2 The Kilpatrick criterion . . . . ........................ 13
2.2.3 The P/C criterion ............... 14
2.2.4 The modified Poynting vector S ....................... 14c
2.2.5 Fatigue related limitations . . ............ 16
2.3 CLIC high gradient studies . . . . . ..................... 18
2.3.1 Rf tests . ....................... 18
2.3.2 Dc tests ........................ 18
2.3.3 Breakdown rate test data analysis . . ........................ 19
3 Theoretical description of breakdowns 21
3.1 Onset phase . . . ....................................... 21
3.2 Burning phase . ........ 24
3.3 Cratering phase . ....................................... 24
4 Experimental facilities and instruments 25
4.1 The 30 GHz structure test facility at CERN . ........................ 25
4.1.1 Test stand controls and operation . . ..... 27
4.1.2 Calibration of the 30 GHz test stand . ........................ 29
4.2 The X-band structure test facility at SLAC . ..... 30
4.3 Accelerating structures used in presented experiments .................... 32
4.4 The CERN dc spark setup................................... 33
4.4.1 The dc sparc vacuum and mechanical system.... 33
4.4.2 The dc spark electrical system ............................ 34
4.5 Instruments for breakdown detection and physics exploration . . .. 37
4.5.1 Rf test stand instrumentation . ............................ 37
4.5.2 The optical spectrograph . . ..... 40
4.5.3 Setup for time-resolved spectroscopy ........................ 43
iii CONTENTS
5 Power and energy measurements 45
5.1 Low power rf measurements of structures .......................... 45
5.2 Rf waveforms and power balance during breakdowns . . . . .... 46
5.2.1 Rf power waveform reconstruction and calibration check . . . ........... 50
5.3 Dc I-V waveforms and power balance during breakdowns . . . ....... 52
5.4 Dc breakdown equivalent circuit model . ...................... 53
5.4.1 Extension of the model . ........... 53
5.4.2 New circuit model validity check .......................... 54
5.4.3 Simulation results of the new model . . ....... 55
5.5 Emission currents in dc and in rf structures.......................... 58
5.5.1 Field emission current emission in dc . ....... 58
5.5.2 Electron and ion currents emitted by breakdowns . . . ............... 58
5.6 Surface damage by breakdowns . .................. 60
6 Optical spectroscopy in rf and dc 63
6.1 Optical spectroscopy during breakdown . .......................... 63
6.1.1 Spectroscopy of dcwns on copper samples . .... 63
6.1.2y of dc breakdowns on molybdenum samples . . . ........... 67
6.1.3y of rfwns in copper structures . . ....... 68
6.1.4 Estimation of plasma parameters for copper dc breakdowns . ........... 70
6.1.5 Reproducibility of rf and dc spectra . . . .............. 72
6.1.6 of line ratios in dc and rf.............. 73
6.1.7 Similarities and differences of dc and rf copper breakdown spectra . .... 77
6.2 Estimation of surface treatment durability by spectroscopy . . ............... 77
6.3 Optical spectroscopy for structure failure analysis .......... 79
7 OTR emission from rf structures and dc spark gap 81
7.1 OTR from low energy electrons . . .............................. 81
7.2 OTR emission spectra from copper in the dc spark setup . . .... 82
7.3 Line emission of neutral molybdenum in the OTR spectrum . . ............... 83
7.4 OTR spectra from rf structures . .............. 84
7.5 β measurements using OTR . . . ...................... 86
7.6 Images of light emission after breakdowns in the dc setup . . .... 89
7.6.1 Source of the observed after-glow .......................... 89
8 Time-resolved optical spectroscopy of rf and dc breakdowns 91
8.1 Power and light intensity in rf and dc . . . .......................... 91
8.2 Time-resolved spectroscopy of breakdowns in the dc setup . .... 95
8.2.1 Consistency between integrated and time-resolved spectroscopy . . . ....... 95
8.2.2 Shape reproducibility of time-resolved signals in dc . . ........... 97
8.2.3 Time-resolved spectrum of dc copper breakdowns . ........ 99
8.2.4 Ted waveforms of CuI lines and continuum in dc . ....... 100
8.3 Time-resolved spectroscopy of breakdowns in rf structures . . ........... 102
9 Summary and conclusion 105
A List of physical properties of copper and molybdenum 109
B Detailed breakdown spectra 111
Acknowledgements 141
Curriculum vitae 143Nomenclature
Acronyms
ADC Analog-to-digital converter
ASTA Accelerator Structure Test Area
BDR Breakdown rate
CCD Charge-coupled Device
CLIC Compact LInear Collider
CTF2 CLIC test facility 2
CTF3 CLIC test facility 3
DAQ Data acquisition system
FC Faraday cup
FFT Fast Fourier transformation
GE Grating efficiency
GLC Global Linear Collider
HV High voltage
ILC International Linear Collider
IR Infrared
LEP Large Electron-Positron Collider
LHC Large Hadron Collider
LINAC Linear accelerator
LTE Local thermodynamic equilibrium
NEG Non-evaporable getter
NIST National Institute of Standards and Technology
NLC Next Linear Collider
NLCTA Next Linear Collider Test Area
OTR Optical transition radiation
PETS Power extraction and transfer structure
PMT Photomultiplier
QE Quantum efficiency
iiiiv CONTENTS
SEM Scanning electron microscope
SLAC Stanford Linear Accelerator Center
TLM Two line method
UHV Ultra-high vacuum
UV Ultraviolet
Constants
−12 As Vacuum permittivity, 8.854187· 100 Vm
−22
Reduced Planck constant, 6.582118· 10 MeV s
c Speed of light, 299792458 m/s
−19e Electron charge, 1.602176· 10 CChapter 1
Introduction
”God made the bulk, surfaces were invented by the devil.”
Attributed to Wolfgang Pauli
The experimental work presented in this thesis was done in order to compare the physics of breakdowns
occuring in high-power rf accelerating structures with a similar breakdown phenomenon observed in high-
gradient dc spark gaps. What motivated this comparison was the need to benchmark different structure
materials and surface treatments for their breakdown characteristics under equal electric field gradients.
Since tests with dc spark gaps are less expensive in cost and time compared to rf tests, the relevance of the
dc results towards their application in rf structure design was questioned and the comparison presented in
this thesis was triggered.
The rf breakdown is a key issue in the CLIC project, a multi-TeV linear collider which is being designed
for high-energy physics research at the energy frontier. A main component of CLIC are the high-gradient
accelerating structures, with a 100 MV/m accelerating gradient in order to keep the overall site length be-
low 50 km. 140000 of these structures will be needed to build CLIC, but a breakdown in only one of these
structures is capable of deviating the beam and reducing the luminosity of the full complex. It is therefore
a CLIC feasibility issue to develop rf structures running with the nominal accelerating gradient and at the
same time with a very low breakdown probability. The experimental work done in this thesis will also
help to get a better understanding of breakdown physics and will be used to benchmark breakdown models.
Finally, the goal of the overall breakdown research is to understand the phenomenon in order to optimize
high-power rf structure design and maximize performance.
1.1 The need for a linear collider in HEP
Since spring 2010, the LHC at CERN has been routinely colliding proton beams with a 7 TeV center-of-
134mass energy. This energy will be increased to 14 TeV with a nominal luminosity of 10 in the coming2cm s
years. At the same time the detectors have started taking data, the corresponding analysis results are fol-
lowing the amount of integrated luminosity. The LHC experiment aims at finding proof of the predicted
standard model Higgs part