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Title Page

Abstract

Contents

Chapter 1. Introduction 21

1.1. Conventional RF- and plasma-based accelerators 21

1.2. Thesis outline 23

Chapter 2. Laser wakefield acceleration 26

2.1. Plasma oscillation 27

2.2. Electromagnetic wave propagation in plasma 28

2.3. Electron motion in electromagnetic field 30

2.3.1. Linear regime 31

2.3.2. Non-linear regime 31

2.3.3. Ponderomotive force 32

2.4. Gas ionization by laser field 34

2.5. Injection mechanisms 37

2.5.1. Wavebreaking and self-injection 37

2.5.2. Ionization injection 39

2.5.3. Density transition injection 40

2.6. Limitations in energy gain 41

2.6.1. Laser diffraction 41

2.6.2. Laser power depletion 45

2.6.3. Electron beam dephasing 46

Chapter 3. Development of the 20 TW/32 fs Ti:sapphire laser system 48

3.1. Introduction 49

3.2. Seed pulse generation from the mode-locked oscillator 52

3.3. Pulse stretching and measurement result 54

3.4. Setup of the multi-staged amplifier and output measurement 56

3.5. Pulse compression and measurement result 62

Chapter 4. Amplified spontaneous emission (ASE) reduction by spectral-matching 67

4.1. Introduction 67

4.2. Experimental setup 68

4.3. Experimental results for contrast ratio enhancement 71

4.4. Summary 75

Chapter 5. Development of the capillary gas/plasma targets and electron beam generation 76

5.1. Introduction 76

5.2. Multi-gas injected capillary for the density down-ramp 76

5.2.1. CFD simulations 77

5.2.2. Experimental setup and results 78

5.3. Leak-free capillary gas cell for up-tapered density generation 88

5.3.1. CFD simulations 88

5.3.2. Fabrication of the capillary gas cell 92

5.3.3. Experimental setup 95

5.3.4. Experimental results 100

5.3.5. Summary and conclusions 110

5.4. Experiments for electron beam generation 111

5.4.1. Experimental setup 111

5.4.2. Experimental results of electron beam generations 113

Chapter 6. Particle-in-cell simulations for a compact light source 119

6.1. Particle-in-cell (PIC) simulations 119

6.2. Radition simulation for the water window regime 127

6.3. Summary 129

Chapter 7. Summary and conclusions 132

References 134

List of Tables

Table 2.1. Ionization potential and the corresponding field intensity for the barrier suppressed... 36

Table 5.1. Experimental targets and driving pulse parameters. 115

Table 5.2. Electron beam output parameters. 115

List of Figures

Figure 1.1. Photographs of the conventional linear accelerators. (a) The 3rd and 4th gener-...[이미지참조] 21

Figure 2.1. Scheme of the laser-wakefield acceleration. 26

Figure 2.2. Plasma oscillation 27

Figure 2.3. Electron motions in the laser field. 32

Figure 2.4. Ponderomotive force by the laser field. 33

Figure 2.5. Wakefield by different laser intensity.(a) shows a clear sinusoidal wakefield dis-... 34

Figure 2.6. The initial bounding potential changes by the externally applied laser field. (a)... 35

Figure 2.7. Bubble/blow-out regime. (a) 1-D and (b) 2-D plasma density distribution when... 38

Figure 2.8. 3-D PIC simulation results for the ionization injection by OSIRIS. 39

Figure 2.9. Laser diffraction. 43

Figure 2.10. Laser power depletion. 45

Figure 2.11. Electron beam dephasing for untapered and positively tapered density distribution. 47

Figure 3.1. Overview of the laser system and the acceleration experiment chambers (Inset)... 48

Figure 3.2. Absorption and emission spectral bands of Ti:sapphire crystal. 49

Figure 3.3. Schematic of the chirped pulse amplification (CPA) with the reflection type gratings. 50

Figure 3.4. Laser system configuration. 51

Figure 3.5. Beam path of the oscillator. 52

Figure 3.6. Kerr-lens effect in Ti:sapphire crystal. 53

Figure 3.7. Output spectrum of the oscillator where the spectral band width is around 65 nm... 54

Figure 3.8. Stretched pulse duration measured by the fast photo-detector and 16 GHz oscil-loscope. 55

Figure 3.9. Beam path of the regenerative amplifier. 56

Figure 3.10. Spectral output of the regenerative amplifier with the band width of 41.5 nm at... 57

Figure 3.11. Beam paths of the pre-amplifier and the main-amplifier. 58

Figure 3.12. Output spectrum of the main amplifier. Spectral band width is around 38.5 nm... 59

Figure 3.13. (a) Pump beam spatial profile for pre-amplifier. (b) Amplified beam profile. 60

Figure 3.14. (a) and (b) Pump beam profiles when the second-harmonic crystals are aligned... 61

Figure 3.15. (a) Pump beam profile from the 90-degree rotated second-harmonic crystal. (b)... 61

Figure 3.16. Beam path in the compressor vacuum chamber for the high-power laser system. 64

Figure 3.17. Measured pulse duration by GRENOUILLE. (a) The retrieved FROG trace. (b)... 65

Figure 3.18. Measured temporal data of the intensity contrast ratio. Red line indicates the... 66

Figure 4.1. Scheme for controlling the spectrum of the seed laser pulse. The center wave-... 69

Figure 4.2. (a) Input spectra of the laser pulse into the regenerative amplifier. (b) Output... 70

Figure 4.3. Temporal intensity contrasts for different input spectral bandwidths with center... 73

Figure 4.4. Comparison of the temporal intensity contrasts for the spectrum-matched pulse... 74

Figure 5.1. 2-D CFD simulation results. (a) The blueish and reddish colors indicate high-Z... 77

Figure 5.2. (a) Experimental setup for the gas and laser-produced plasma diagnostics. (b) A... 79

Figure 5.3. (a) Raw phase map (interferogram) for the gas jet with a thin blade (red colored... 81

Figure 5.4. Density profiles of the nitrogen gas flow at different heights (p1~p5) in Figure 5.3... 83

Figure 5.5. Raw and analyzed phase maps for the multi-gas injected capillary gas cell. (a)... 85

Figure 5.6. On-axis plasma density profiles with different nitrogen backing pressures in the... 86

Figure 5.7. A typical 3-D CFD simulation results of Hydrogen gas pressure (density) dis-... 89

Figure 5.8. (a) Longitudinal pressure (density) distributions. Here, width of feedline 1 (350... 90

Figure 5.9. (a) The exploded view of the capillary target assembly. (b) The front view of the... 92

Figure 5.10. Detailed microscope pictures of the machined sapphire plates. (a) and (g) are... 93

Figure 5.11. Detailed pictures of the capillary gas cell. (a) Assembled capillary gas cell... 94

Figure 5.12. Experimental setup for the capillary plasma density measurement using a Mach-Zehnder interferometer. The main driving... 96

Figure 5.13. Pulsed valve cooling block. 98

Figure 5.14. Photograph of the interferometry setup in vacuum chamber. 99

Figure 5.15. Gas density temporal evolution in the capillary gas cell, where the valve is... 101

Figure 5.16. Gas density distribution near the capillary entrance, where the experimentally... 102

Figure 5.17. Interferograms for (a) a reference image without the plasma and (b) a signal... 103

Figure 5.18. The transverse density profiles at different longitudinal positions in the capillary... 104

Figure 5.19. Longitudinal plasma electron density profiles in the capillary gas cell, where... 105

Figure 5.20. 2-D EPOCH results of the laser's peak field amplitude evolution as laser propagates though hydrogen gas. Laser-induced... 108

Figure 5.21. (a) Laser spatial image at the capillary exit position without capillary and (b)... 109

Figure 5.22. Self-focused laser transmission images with different gas density gradient. Gas... 110

Figure 5.23. Experimental setup for the laser wakefield electron beam generation and detection systems. 112

Figure 5.24. Electron energy spectrometer with the permanent dipole magnet. 113

Figure 5.25. Electron bunch shape image. 116

Figure 5.26. Electron energy spectrometer with the permanent dipole magnet with 7-mm-long capillary target. 117

Figure 5.27. Electron energy spectrometer with the permanent dipole magnet with 5-mm-long capillary target. 118

Figure 6.1. Plasma density profile for simulations. A high-power laser propagates in the x-... 120

Figure 6.2. The energy spectra of electron beams for n₂/n₁ ＝ l(non-tapered), 1.5,... 121

Figure 6.3. Energy gain of the test electrons as density tapering ratio (n₂/n₁) changes. The... 122

Figure 6.4. The phase changing position is going forward due to the shortening of the cavity in the higher plasma density, and finally... 124

Figure 6.5. The electron beam in the higher density gradient experiences the greater wake-... 126

Figure 6.6. Scheme of the tunable synchrotron radiation source system. 127

Figure 6.7. Synchrotron radiation spectra with various electron beams from PIC. The water... 128

Figure 6.8. Comparison of the radiation tunability between the change of the electron beam... 131

초록보기

Advanced particle accelerator concepts based on the high-power laser and the plasma interactions have been studied for the last three decades. They have been considered as suitable alternatives to the RF-based conventional accelerators. Femto-second (fs) and tera-watt (TW) scale lasers have become widely available and easily accessible even in small laboratories since 1980s. On the other hand, development of a proper plasma target is an important issue for more stable electron injections and higher energy electron beam generations.

This thesis presents results of the numerical and experimental works aimed to develop the new gas/plasma target and its detailed diagnostics for the improved environment of the laser-plasma acceleration. The experimental study was undertaken using the 20 TW and 32 fs laser system in the Laser-Plasma Accelerator Laboratory (LPAL) at Gwangju Institute of Science and Technology (GIST).

The laser system was established based on the chirped pulse amplification (CPA) technique with the Ti:sapphire oscillator as a fs seed generator. Three amplification stages including one regenerative amplifier and two multi-pass amplifiers were installed. The regenerative amplifier was optimized by suppressing the amplified spontaneous emission (ASE) value with the spectral-matching technique. This technique can provide an improved temporal contrast ratio for the amplified laser pulse. The two-staged multi-pass amplifier includes the 4-pass pre- and 4-pass main-amplifiers with 20- and 25-times amplification rate, respectively. The final pulse compression was done with the reflective grating pair down to 32 fs of the temporal pulse duration. This laser system also partially used for the experimental characterization of the newly designed gas and plasma capillary targets.

The transverse interferometers were employed as the main tools of the gas and plasma diagnostics. In particular, a Nomarski interferometer with a nano-second (ns) probe pulse for the neutral gas distribution and a Mach-Zehnder interferometer with a fs probe pulse for the laser-induced plasma were used.

In the laser wakefield acceleration, improving the electron injection rate and overcoming the energy gain limitations are the important issues. First, the density transition injection method has been studied and used for more controllable electron injection. I suggest a new scheme using the multi-gas injection for the stable density down-ramp generation in the relatively low density range and compare the detailed characteristics with the blade installed gas jet which has been used to generate a sharp density down-ram p. Second, the limited energy gain is mainly caused by the acceleration phase mi s-matching (dephasing) between the laser-induced wakefield and the accelerated electron beam. The linearly increasing density distribution is suggested to suppress this dephasing issue. Therefore, a density tapered gas cell capillary has be en designed based on the numerical simulation of three-dimensional computational fluid dynamics. The performance characteristics such as the fill-up time, density distribution, and so on, are also experimentally tested. The positively tapered plasma density was properly generated in the density range of 1018 ~ 1019 /cm-3 which is suitable for the laser wakefield accelerator. The density value was measured by the transverse Mach-Zehnder interferometer.

Some additional preliminary works such as the first experimental results of the electron beam generation with the square cross-section capillary target are also presented.

The particle-in-cell (PIC) simulation results are also given for the energy-tunable electron beam generation with the tapered density profile which is particularly aimed to the compact and tunable light source applications.

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