Plasma Density Transition Trapping of Electrons in Plasma Wake Field Accelerators

196 Pages·2004·2.97 MB·English

University of California
Los Angeles
Plasma Density Transition Trapping of
Electrons in Plasma Wake Field Accelerators
A dissertation submitted in partial satisfaction
of the requirements for the degree
Doctor of Philosophy in Physics
by
Matthew Colin Thompson
2004
(cid:1)c Copyright by
Matthew Colin Thompson
2004
The dissertation of Matthew Colin Thompson is approved.
Chandrashekhar Joshi
Claudio Pellegrini
David Saltzberg
James B. Rosenzweig, Committee Chair
University of California, Los Angeles
2004
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To my family, for always believing ...
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Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 The Need for Advanced Accelerators . . . . . . . . . . . . . . . . 2
1.2 Plasma Based Accelerators . . . . . . . . . . . . . . . . . . . . . . 4
1.2.1 The Plasma Wake Field Accelerator . . . . . . . . . . . . . 9
1.2.2 The Laser Wake Field Accelerator . . . . . . . . . . . . . . 11
1.2.3 The Plasma Beat-Wave Accelerator . . . . . . . . . . . . . 12
1.2.4 The Self-Modulated Laser Wake Field Accelerator . . . . . 12
1.3 Conventional Electron Beam Sources . . . . . . . . . . . . . . . . 13
1.3.1 Thermionic Direct Current Guns . . . . . . . . . . . . . . 16
1.3.2 Radio Frequency Photoinjectors . . . . . . . . . . . . . . . 20
1.3.3 Magnetic Compression . . . . . . . . . . . . . . . . . . . . 25
1.4 Injection and Timing . . . . . . . . . . . . . . . . . . . . . . . . . 29
1.5 Plasma Based Electron Beam Sources . . . . . . . . . . . . . . . . 33
1.5.1 Random Phase Injection Sources . . . . . . . . . . . . . . 34
1.5.2 Laser Stimulated Injection Sources . . . . . . . . . . . . . 36
1.5.3 Plasma Density Transition Trapping . . . . . . . . . . . . 37
1.6 Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2 Theory of Particle Trapping and Acceleration . . . . . . . . . . 41
2.1 Trapping in RF Photoinjectors . . . . . . . . . . . . . . . . . . . . 41
2.2 Plasma Accelerators and Plasma Trapping . . . . . . . . . . . . . 45
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3 Numerical Simulations of Plasma Density Transition Trapping
Regimes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
3.1 The Strong Blowout Scenario . . . . . . . . . . . . . . . . . . . . 51
3.2 The Weak Blowout Scenario . . . . . . . . . . . . . . . . . . . . . 55
4 Scaling of the Transition Trapping System . . . . . . . . . . . . . 61
4.1 Driver Charge Scaling . . . . . . . . . . . . . . . . . . . . . . . . 61
4.2 Wavelength Scaled Sources . . . . . . . . . . . . . . . . . . . . . . 63
5 Design of the Transition Trapping Experiment . . . . . . . . . . 68
5.1 Design Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
5.2 The Argon Pulse Discharge Plasma Source . . . . . . . . . . . . . 70
5.2.1 General Source Design . . . . . . . . . . . . . . . . . . . . 71
5.2.2 Plasma Cathode Heater Design . . . . . . . . . . . . . . . 73
5.2.3 Plasma Source Operation and Characterization . . . . . . 77
5.3 Creation of the Plasma Density Transition . . . . . . . . . . . . . 79
5.3.1 Transition Production Using Metal Screens . . . . . . . . . 81
5.3.2 Baffled Screens . . . . . . . . . . . . . . . . . . . . . . . . 84
5.3.3 Density Transition Characterization . . . . . . . . . . . . . 86
5.4 SimulationofTrappingPerformanceUnderRealisticExperimental
Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
5.4.1 Realistic Density Profiles . . . . . . . . . . . . . . . . . . . 91
5.4.2 Impact of Transverse Magnetic Field . . . . . . . . . . . . 94
5.4.3 Imperfect Driving Beams . . . . . . . . . . . . . . . . . . . 95
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5.5 Comparison of Design Goals and Achieved Results . . . . . . . . . 97
6 Execution of the Transition Trapping Experiment . . . . . . . . 99
6.1 The FNPL Facility . . . . . . . . . . . . . . . . . . . . . . . . . . 99
6.1.1 Cathode Drive Laser . . . . . . . . . . . . . . . . . . . . . 102
6.1.2 L-Band RF Photoinjector . . . . . . . . . . . . . . . . . . 103
6.1.3 Nine-Cell Superconducting Accelerating Cavity . . . . . . 104
6.2 The Trapping Experiment Beamline . . . . . . . . . . . . . . . . . 105
6.2.1 Vacuum Window . . . . . . . . . . . . . . . . . . . . . . . 105
6.2.2 Diagnostic Positions . . . . . . . . . . . . . . . . . . . . . 108
6.2.3 Beam Transport . . . . . . . . . . . . . . . . . . . . . . . . 109
6.3 Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
6.3.1 Transverse Profile Diagnostics . . . . . . . . . . . . . . . . 111
6.3.2 Screen Position Determination . . . . . . . . . . . . . . . . 112
6.3.3 Charge Diagnostics . . . . . . . . . . . . . . . . . . . . . . 114
6.3.4 Streak Camera . . . . . . . . . . . . . . . . . . . . . . . . 115
6.3.5 The Broad Range Vacuum Spectrometer . . . . . . . . . . 117
7 Experimental Results and Analysis . . . . . . . . . . . . . . . . . 122
7.1 Plasma Focusing . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
7.2 Drive Beam Characterization . . . . . . . . . . . . . . . . . . . . 124
7.3 Drive Beam Deceleration . . . . . . . . . . . . . . . . . . . . . . . 132
7.4 The Search for Trapped Electrons . . . . . . . . . . . . . . . . . . 133
7.5 Comparison of Experimental Results with Simulation . . . . . . . 135
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8 Conclusions and Future Directions . . . . . . . . . . . . . . . . . . 139
8.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
8.2 Future Directions . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
8.2.1 Continuation of the Low Density Trapping Experiment . . 141
8.2.2 Underdense Plasma Lens Experiment . . . . . . . . . . . . 143
8.2.3 Prospects for High Density Trapping Experiments . . . . . 143
8.2.4 Foil Trapping . . . . . . . . . . . . . . . . . . . . . . . . . 144
A Derivation of the Hamiltonian for Particles in Electromagnetic
Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
B Heat Transport in the Radiation Coupled Regime . . . . . . . . 150
C Analysis of Electrostatic Probe Signals . . . . . . . . . . . . . . . 154
D Optical Design of the Broad Range Vacuum Spectrometer . . 159
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
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List of Figures
1.1 Plot of the plasma wave breaking field and skin depth over the
densities of interest for advanced accelerators. . . . . . . . . . . . 8
1.2 Areaoccupiedbytheelectronbeamintracespaceatthreedifferent
locations along a beamline. . . . . . . . . . . . . . . . . . . . . . . 14
1.3 Idealized electron beam sources. . . . . . . . . . . . . . . . . . . . 16
1.4 Schematic of an example thermionic DC electron gun . . . . . . . 17
1.5 Simplified diagram of an RF photoinjector . . . . . . . . . . . . . 20
1.6 The mechanism through which beams are longitudinally focused
in photoinjectors by differential phase slippage. . . . . . . . . . . 24
1.7 The mechanism of magnetic compression . . . . . . . . . . . . . . 26
1.8 A typical set of chicane magnets. . . . . . . . . . . . . . . . . . . 27
1.9 Comparison of RF and Plasma accelerator acceptance windows . . 29
1.10 Suppression of laser/RF jitter via beam compression . . . . . . . 31
1.11 Configuration spacediagram of theplasma densitytransition trap-
ping mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
1.12 Diagram of the plasma electron rephasing caused by the plasma
density transition . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.1 Strong blowout configuration space (r, z) . . . . . . . . . . . . . . 52
3.2 Trappedparticleenergyversustemporalpositionwithinthebunch
for the strong blowout case. . . . . . . . . . . . . . . . . . . . . . 53
3.3 Weak blowout configuration space (r, z) . . . . . . . . . . . . . . . 56
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3.4 Plasma density and drive beam current profiles. . . . . . . . . . . 58
3.5 Trappedparticleenergyversustemporalpositionwithinthebunch
for the weak blowout case. . . . . . . . . . . . . . . . . . . . . . . 59
3.6 Trapped particle radial position versus temporal position within
the bunch for the weak blowout case. . . . . . . . . . . . . . . . . 60
4.1 The effects of driver beam charge scaling on trapped beams. . . . 62
5.1 Schematic of the plasma density transition experiment plasma
source. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
5.2 CAD rendered pictures of the plasma source . . . . . . . . . . . . 72
5.3 Schematic of the plasma source heater. . . . . . . . . . . . . . . 76
5.4 Photograph of the plasma column. . . . . . . . . . . . . . . . . . 78
5.5 Measured plasma column density . . . . . . . . . . . . . . . . . . 79
5.6 Simulated dependence of captured charge on transition length . . 80
5.7 Plasma density screen diagram . . . . . . . . . . . . . . . . . . . . 81
5.8 PIC simulation of density screen performance . . . . . . . . . . . 83
5.9 Blocking of wake particles by the density screen baffle. . . . . . . 85
5.10 Effect of the beam-baffle distance on trapping . . . . . . . . . . . 86
5.11 Photograph of the plasma density transition apparatus. . . . . . 87
5.12 Measured density transition - long scale. . . . . . . . . . . . . . . 88
5.13 Measured density transition - short scale. . . . . . . . . . . . . . 89
5.14 Comparison of gaussian and linear plasma density profiles. . . . . 93
5.15 Impact of drive beam length on trapped charge. . . . . . . . . . 96
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