Laser plasma interaction and terahertz (THz) field emission

LASER PLASMA INTERACTION AND TERAHERTZ (THz) FIELD EMISSION

by

MONIKA SINGH Centre for Energy studies

Submitted

In fulfillment of the requirements of the degree of

DOCTOR OF PHILOSOPHY

to the

Indian Institute of Technology Delhi

June- 2013

Dedicated to

My Parents, Family and Thesis Supervisor

Certificate

This is to certify that the thesis entitled “Laser Plasma interaction and Terahertz (THz) Field emission” being submitted by Ms. Monika Singh to the Indian Institute of Technology Delhi, is worthy of consideration for the award of the degree of

‘Doctor of Philosophy’ in Centre for Energy Studies, and is a record of bonafide research work carried out by her. Monika Singh has worked under my guidance and supervision and that the results contained in this work have not been submitted in part or full to any other university or institute for award of any degree. I approve the thesis for the award of the aforesaid degree.

Prof. R. P. Sharma

Centre for Energy Studies Indian Institute of Technology Delhi

Hauz Khas, New Delhi-110016

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Acknowledgements

My Ph. D. work has been seen through to completion with the support and encouragement of many people. I would like to thank all those, who helped me and made this Ph.D. work possible and unforgettable for me.

In the first place, I feel privileged to express my sincere regards to my supervisor R.

P. Sharma, Professor and Head, Centre for Energy Studies, IIT Delhi, for his guidance, cool temperature, valuable suggestions, support, advice and constant encouragement throughout the course of my research work. The critical comments rendered by him during the discussions are deeply appreciated. I could not have imagined having a better advisor and mentor for my Ph.D. work. It is because of his deep knowledge of the subject that this work has taken the present shape. I hope that I could be as lively, enthusiastic and energetic as him. I could not be prouder of my academic roots and hope that I can in turn pass on the research values and the dreams that he has given to me.

I wish to express my sincere thanks to Dr. H. D. Pandey, Prof. S. C. Kaushik, Prof. A.

Chandra, Prof. T. S. Bhatti, Secretary CRC, Dr. R. Uma, , and all faculty members of CES for their valuable suggestions, advices and approval of my thesis work. I thank to other staff members of Center for Energy studies, IIT Delhi for their kind help and cooperation during my research work.

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I am particularly thankful to all my group members, Dr. Karuna Batra, Dr. Hemam Dinesh Singh, Dr. Prashant Kumar Chauhan, Dr. Sachin Kumar, Dr. Sanjay Kumar, Dr. Prerna Sharma, Dr. Ruchika Gupta, Naveen, Alok ji, Nidhi Sharma, B. K. Das, Nitin Yadav, Ramkishor Singh, Kalpesh modi, Nilesh Kumar Pathak, Anju Kumari, Sangita, Ashis Vyas, Ravindra Goyal, Swati Sharma, Prachi and Saba for their cooperation, and support during my research work. Heartful thanks are due to my friends Renuka, Tulsi Anna, Anuj Chauhan, Rakhi Malik, and Charu Diwedi who made easy for me to spend time in campus throughout this period.

This acknowledgement would remain incomplete without expressing my sincere feelings towards my parents, my husband, my son and other family members for their patience, and understanding during my research work. I owe it deeply to them for their encouragement and support, which gave me courage and confidence to materialize this research work.

Monika Singh

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Abstract

The present thesis is an attempt to generate the terahertz (THz) frequency radiation during laser plasma coupling by using numerous nonlinear processes like ponderomotive filamentation/selfcocusing of laser beams and parametric decay .This work has been studied under a variety of conditions, like, laser beams propagation in the plasmas, in paraxial as well as extended-paraxial ray approximation. The study includes the analytical modeling and numerical simulations for different effects in these processes. Most of the work deals with the generation of THz radiation by the nonlinear interaction of the preexisting low density ripple of electron plasma wave with filamented /self focused high intense laser beam in the paraxial and extended paraxial regimes respectively. By expanding the eikonal and other relevant quantities up to 4^th power of the radial distance, the extended – paraxial theory explains the phenomenon of the split profile of beam (ring formation) in the self-focusing region.

It is observed that the focusing of the laser beams become faster in extended paraxial regime than simple paraxial regime. In this thesis, THz generation schemes have been proposed under the effects of the laser beams propagation in plasmas within paraxial as well as extended-paraxial regimes. The high intense laser beam and preexisting density ripples of electron plasma wave exert a ponderomotive force on the electron.

This beat frequency ponderomotive force produces a nonlinear current which generate the THz radiation. THz radiation amplitude can be tuned by changing the external magnetic field.

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Further, the three wave parametric decay process provides a significant method to generate an electromagnetic radiation in the form of magnetosonic wave (MSW). The first one deals with decay of upper hybrid wave (UHW) as pump into extraordinary wave (laser) and MSW and another one deals with the reverse process of first. In this reverse process the pump wave (Laser beam) which is considered in the extraordinary mode, propagating perpendicular to the background magnetic field, decays into UHW and MSW. The power and instability of generated radiation grow faster because of the magnetic field.

In the last chapter of thesis, excitation of the THz radiation has been proposed by cross- focusing of two spatial-Gaussian lasers in a periodic density plasma where the ponderomotive force imparting a drift on electrons with the finite transverse component. The rippled plasma density helps to get the phase matching condition and the resonant excitation of THz radiation. By this analytical model, the nonlinear current density, emitted THz radiation amplitude and the efficiency of THz generation have been evaluated, respectively.

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Contents

Certificate i

Acknowledgements iii

Abstract iv

Contents vi

List of Figures x

Chapter 1 Introduction 1-30 1.1 THz sources based on laser plasma interaction 4

1.2 Importance of density ripples 5

1.3 Effective presence of transverse, static electric field in plasma 6

1.4 Self-focusing/Filamentation process 6

1.4.1 Thermal Self-focusing 8

1.4.2 Ponderomotive Self-focusing 8

1.4.3 Relativistic Self-focusing 9

1.5 Cross focusing of two laser beams 10

1.6 Effect of the external magnetic field 10

1.7 Methodology 12

1.7.1 Paraxial ray approximation 12

1.7.2 Extended-paraxial ray approximation 12

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1.7.3 Moment theory approach 13

1.7.4 Variational method 14

1.8 Objective and Outline of the thesis 15

Chapter Wise Summary 18

List of publications 22

References 24

Chapter 2 Interaction of high power laser beam with magnetized plasma and THz Generation 31-50 2.1 Introduction 31

2.2 Filamentation of Gaussian Laser beam in a 32

Nonlinear Magneto plasma 2.3 THz Wave Dynamics 37

2.4 Results and Discussion 42

2.5 Conclusion 43

References 48

Chapter 3 Generation of THz Radiation from Laser beam Filamentation in a Magnetized Plasma 51-67 3.1 Introduction 51 3.2 Filamentaion of Gaussian Laser Beam in Nonlinear Magneto plasma

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in the extended- paraxial region 53

3.3 THz Generation Mechanism 57

3.4 Results and discussion 60

3.5 Conclusion 61

References 66

Chapter 4 Electromagnetic field emission by the Upper Hybrid wave (UHW) 68-86 4.1 Introduction 68

4.2 Basic Equations 69

4.3 Extraordinary (Laser) wave Dynamics 71

4.4 MSW wave Dynamics 73

4.5 Stability analysis 79

4.6 Result and discussion 81

4.7 Conclusion 82

References 86

Chapter 5 Generation of THz radiation by Laser Plasma interaction 88-106 5.1 Introduction 88

5.2 Basic Equations 89

5.3. UHW Dynamics 91

5.4 THz wave Dynamics 92

5.4.1 THz wave amplitude 95

5.4.2 THz Radiation Power and Efficiency 97

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5.5 Stability Analysis 98

5.6 Results and discussion 100

5.7 Conclusion 102

References 106

Chapter 6 THz generation by cross-focusing of two laser beams in a rippled density plasma 108-125 6.1 Introduction 108

6.2 Cross-focusing of two coaxial laser beam 110

6.3 THz radiation generation 113

Bio-data of the Author 131