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소장정보 검색

국회도서관 홈으로 정보검색 소장정보 검색
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Title Page

Abstract

Contents

Chapter 1. Introduction 15

Chapter 2. Basic Theory 19

2.1. Laser-Plasma Acceleration 19

2.1.1. Laser-Driven Plasma Wakefield 19

2.1.2. Ponderomotive Force 23

2.1.3. E-field Strength of Wakefield 24

2.1.4. Dephasing Length-Acceleration Distance Limitation 26

2.1.5. Optical Guiding 30

2.1.6. Appendix 37

Chapter 3. Experimental Results 38

3.1. Introduction 38

3.2. Mach-Zehnder Interferometry 39

3.3. Predicted Phase Shift-Cylindrical Symmetry 42

3.4. Experimental Set-up 44

3.5. Phase Shift Retrieval 47

3.6. Density Converting Formula 51

3.7. Experimental Result and Analysis 54

Chapter 4. Summary 60

References 62

List of Figures

Figure 1.1: Simulation shot of wakefield excited by intense laser pulse. The laser pulse is propagating through plasma while electrons in front of the pulse are radially pushed away by nonlinear force so called ’ponderomotive force’, explained later. The laser pulse plays a driver to raise plasma wave(wakefield), analogous to wave rippled back... 18

Figure 2.1: Mechanism of the laser plasma acceleration. Intense laser pulse is expelling electrons by ponderomotive force while heavy ions are relatively stationary. While the laser pulse is continuously expelling electrons in front of the pulse, electrons already pushed out are experiencing attrac- tive force from stationary ions, leading to... 22

Figure 2.2: When the laser pulse with spatially varing intensity is tightly focused on the plasma, electrons are ‘pushed’ out from the peak amplitude to the low amplitude region due toeld inhomogeneity. Since the force is explicitly dependent on the particle mass, heavy ion motions are neglected. 24

Figure 2.3: Illustration of how dephasing length is defined. Electrons gain higher velocity than speed of wakefield at the early acceleration stage due to powerful electric field over the bubble. Maximum energy is gained when electrons reach center of the bubble, after which is deceleration phase due to symmetric structure of the bubble 29

Figure 2.4: Maximum energy gain vesus initial electron density. Higher electron density gives stronger maximum electric field due to more electron par- ticipations to form the field, however, dephasing length(acceleration dis tance) is reduced faster due to decreased laser group velocity. Thus, the maximum energy gain is decreased as electron density increases. 30

Figure 2.5: Guiding through plasmas channel. Radially decreasing refraction index profile due to preformed plasma parabolic density profile, relativistic mass increment, expelling electrons by ponderomotive force plays a pos- itive lens such that the laser phase front is curving toward the axis. 33

Figure 2.6: Numerical solution of Eq. (2.21). r0 = 25㎛. It is noted that actual channal depth △n should be around to critial channel depth △nc for effective guiding.(이미지참조) 36

Figure 2.7: Critical channel depth △nc vesus initial spot size r0. If the actual channel depth is equal to the critical channel depth △nc for the given initial spot spot, than the spot size is maintained over the plasma channel. Otherwise, the size is oscillating as shown in Fig. 2.6(이미지참조) 36

Figure 3.1: Different interferometers used for plasma diagnostics. (a), (b) and (c) are indicating the Michelson interferometer, Mach Zehnder Interferometer and Nomarski interferometer, respectively. Unlikely to other interferometers, Nomarsky interferometer doesn't require two beam alignment, leading interferometer arrangement simplicity.... 40

Figure 3.2: Magnification view of Mach-Zehnder interfermeter after M 3. There is an angle between probe and reference beam to make fringes. If θ is zero, no fringe is observed. The larger θ gives thinner fringe thickness due to rapid additional phase shift increment speed along the detector... 41

Figure 3.3: Phase shifts from cylindrical symmetry density profiles. For ωp《 ωl(typical case), since phase shift is proportional to the integrated electron number along the beam path, center phase shift is always the highest. And the sign of phase shift is always negative since refraction index of plasma is less than 1, which is unique... 43

Figure 3.4: Experimental set-up. The laser, the discharge circuit and the CCD camera are synchronized with each other using DG-535 triggering boxes. The double-shielded BNC-type cables are used to connect to each instru- ments for screening background noises generated from the discharged plasma and the thyratron. The probe beam pulse... 46

Figure 3.5: A picture of interferometer installed inside the vacuum chamber. The translator is used to adjust two arm's length difference within temporal coherence length of ND:YAG laser, which is an several milimeters order. 47

Figure 3.6: Phase retrival process using MATLAB. Applying FFT(Fast Fourier-Transformation algorithm) on interferogram gives three peaks, DC component and side peaks corresponding to positive and negative frequency components for fringe pattern respectively. Applying Inverse-Fourier-Transformation after leaving only one... 50

Figure 3.7: Plasma cross section with cylindrical symmetry. dashed line represent contour of electron density 53

Figure 3.8: Example of applying Gonsalves's density converting formula from the extracted phase shift information [30]. X-Y symmetry is assumed thus the electron density profile along x and y direction are identical. In this figure, density pro les along two black arrowed line are the same. 53

Figure 3.9: The effect of beam pointing stability. The interferograms are taken without plasma under the same external condition. Due to the existence of beam pointing stability, the interferograms are inherently shifted with respect to each other. 55

Figure 3.10: The method to determine a minimum phase shift from a interferogram without clear reference phase map. The fringes over capillary are only effected by discharge plasma, the fringes are shifted with respect to outside fringes. By using this property, the average distance between 1 and 1’, 2 and 2’, 3 and 3’ and so on gives the minimum phase shift. 55

Figure 3.11: The measured density profiles along x axis at the center of y (or vice versa since X-Y symmetry). Low pressure discharged plasma has more stable density profile since higher ionization. For low ionization, colli sions and diffusions with neutral particles have negative effect for for mation of the plasma channel. 56

Figure 3.12: The comparison of plasma currents and the average densities over dis charge time. There is a delay between peak of current and maxium densitie for each figure since recombination process takes some time. The delays are reduced for higher current since recombination process is accelerated due to faster electrons colliding with ions. 57

Figure 3.13: The Electron density and temperature profile over radius. The fact that temperature is decreasing radially due to thermal conduction to the capillary wall while pressure over the plasma cross section is constant gives radially increasing density profile, according to ideal gas law. The image is from ’Simulations of a hydrogen-filled... 59

초록보기

Laser-driven electron acceleration with pre-ionized capillary waveguide is the next generation electron acceleration method. The capillary is a waveguide in which a laser pulse is guided over a long distance(typically several centi-meters order) without rapid divergence due to diffraction. The guiding is done if plasma has radially increasing density profiles, which means radially decreasing refraction index according to the plasma dispersion. The capillary was developed to form such a density profiles. By taking the importance of the laser guiding for effective acceleration into account, it is necessary to investigate actual spatio-temporal electron density profiles of the capillary plasma before the electron beam generation experiments.

In this thesis, the interferometric technique for studying electron density profiles within the capillary is introduced and the results are reported.

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