NORTHERN ILLINOIS UNIVERSITY



NORTHERN ILLINOIS UNIVERSITY

A POSSIBLE CURE OF PHASE SLIP FOR A NONLINEAR

PLASMA WAKEFIELD ACCELERATOR

A THESIS SUBMITTED TO THE GRADUATE SCHOOL

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR THE DEGREE

MASTER OF SCIENCE

DEPARTMENT OF PHYSICS

BY

DANIEL S. BOLLINGER

© 2005 Daniel S. Bollinger

DEKALB, ILLINOIS

MAY 2005

ABSTRACT

There has been much recent interest in accelerating electron beams with a plasma wakefield accelerator (PWFA). The advantages are a large accelerating gradient in a small space. In order for this to be realized, a drive beam must set up the wakefield, and a witness beam to be accelerated would then be injected in the high-gradient wake. The problem with this injection scheme is that the drive beam tends to erode as it traverses the plasma column. This erosion causes the phase of the wakefield to slip in the beam frame. This causes a difference in phase velocities between the witness beam and the accelerating portion of the wakefield. The purpose of this thesis is to characterize the plasma column of the PWFA experiment for the Fermilab/NICAAD Photoinjector Laboratory and to find an anode geometry that would minimize the phase slip. Three different anodes were tested and it was found that an anode with .953cm ID was the closest to an ideal geometry for controlling the phase slip.

ACKNOWLEDGMENTS

There are numerous people I would like to thank who have helped me during my career that has led to a wonderful educational experience and this thesis. First and foremost I would like to thank Court Bohn for all of his support and guidance, not only in the writing of this thesis but as a teacher and mentor; without him this thesis would not have been possible.

The experiment was performed on the plasma chamber that Nick Barov designed and performed wakefield acceleration experiments with at the FNPL facility. I would like to that Nick for allowing me to use his plasma chamber and all of his help and patience with teaching me not only plasma physics but how to be an experimentalist. Also, I would like to thank Wade Muranyi for all of his technical support and all of the people associated with the FNPL facility.

My attending graduate school would not have been possible without the support of Jerry Blazey. Finally, I would like to thank Linda Bagby for being so supportive and the best study partner that I have ever worked with.

DEDICATION

To my parents, Charles and Nancy; my daughter, Brittany; and to the love of my life, Kathy.

TABLE OF CONTENTS

Page

LIST OF TABLES ………………………………………………………… viii

LIST OF FIGURES ……………………………………………………….. ix

Chapter

1. INTRODUCTION …………………………………………………... 1

The Quest for Higher Accelerating Gradients …..………………. 1

FNPL Beamline …………………………………………............. 3

Summary of Plasma Wakefield Acceleration …….……………... 5

FNPL Plasma Source …….…………………………………........ 5

Hollow Cathode ……..…………………………………….. 6

Chamber …………………………………………………... 7

Measurements of Plasma Profile ……….……………………….. 9

2. PLASMA DIAGNOSTICS: LANGMUIR PROBE …………………. 12

Langmuir Probe Design ……..…………………………………… 12

Typical Single Probe …………….……………………….... 13

The Langmuir Curve …………….……………………….... 14

Electronics Design ………….…………………………….. 17

Data Analysis ………………………………………………….... 20

Chapter Page

How the Data Was Taken …………………………………... 20

Uncertainties ………….…………………………………….. 21

Random Uncertainties ……………………………… 21

Systematic Uncertainties……………………………. 22

Error Bars …………………………………………… 23

3. PLASMA DENSITY PROFILE DATA ……………………………... 24

Introduction ……..………………………………………………... 24

Transverse Profile ……….……………………………………..…. 24

Transverse Langmuir Probe ……….……………………….. 24

Confinement Solenoid Current ………..……………………. 25

Argon Pulse Width …………….…………………………… 27

Anode Power Supply Voltage …………….………………... 27

Characterizing Density vs Radius ………………………….. 28

Longitudinal Density Profile …………………………………....... 29

Introduction ………………………………………………… 29

Longitudinal Probe …………………………………………. 29

Anode Geometry ……………….…………………………... 30

Longitudinal Density Measurements ………………………. 31

Chapter Page

4. INTERPRETATION OF DATA: WITNESS BEAM ACCELERATION AND MINIMIZING PHASE SLIPPAGE………. 33

Introduction and Motivations ……….…………………………… 33

Convolution Integral ……………..………………………………. 35

NOVO Simulations …………….………………………………… 37

Plasma Parameters and β Μatching ……………………….. 38

Measured Longitudinal Density Results …………………… 40

“Scaled up” Longitudinal Density Results ………………… 44

Discussion ………………………….……………………………. 45

5. CONCLUSIONS ……………………………………………………. 47

What Next? ……………………………………………………… 47

On-line Measurements ……………………………………... 47

Complete Longitudinal Scan ……………………………….. 48

Particle-in Cell (PIC) Simulations …………………………. 48

Different Anode Geometries ……………………………… 49

Witness Beam Experiment ……………………………….... 49

REFERENCES ………………………………………………………….. 50

APPENDIX ……………………………………………………………… 52

LIST OF TABLES

Table Page

2.1 Random uncertainties ……………………………………………….….. 21

4.1 Measured plasma parameters …………………………………………… 42

4.2 Scaled-up plasma parameters ………………………………………...… 44

LIST OF FIGURES

Figure Page

1.1 Livingston plot showing center of mass energy vs year of

commissioning. ……………………………………………………….… 2

1.2 FNPL beamline ……………………………………………………….… 3

1.3 Photo of the FNPL beamline. ………………………………………..…. 4

1.4 Basic wakefield acceleration ……………………………………...……. 5

1.5 FNPL plasma chamber ……………………………………………...….. 6

1.6 Hollow cathode assembly …………………………………………….… 6

1.7 Hinged cathode assembly ……………………………………….....…… 8

1.8 Plasma pulse timeline ………………………………………………….. 9

1.9 Plasma column and location of Langmuir probes ………………….….. 10

2.1 Typical single Langmuir probe design …………………………………. 13

2.2 Langmuir probe that was used ……………………………………….… 14

2.3 Typical Langmuir probe I-Vcharacteristic ……………………………... 14

2.4 Langmuir characteristic log scale plot of region II …………………...… 16

2.5 Comparison of 1300μF/50 μF capacitor data ……………………...…… 18

2.6 Langmuir probe electronics box schematic …………………………..… 20

2.7 Scope trigger ……………………………………………………………. 20

3.1 Photo showing the transverse probe ………………………………….… 24

3.2 Photo of confinement solenoids ……………………………………...… 25

Figure Page

3.3 Density vs confinement solenoid coil current ……………...…………… 26

3.4 Density vs argon pulse width …………………………………………… 27

3.5 Density vs anode power supply voltage ………………………………… 28

3.6 Density vs radius, the curve fit is to guide the eye…………………...…. 29

3.7 Photograph of longitudinal Langmuir probe …………………………… 30

3.8 Typical anode geometry ……………………………………………...… 31

3.9 Density vs z for all three anodes ……………………..……….……....… 31

4.1 Wakefield acceleration of witness beam. ……………………….…….... 34

4.2 Plasma wakefield ……………………………………………………...... 34

4.3 Convolution integral solution. ………………………….………..…..…. 36

4.4 Scans of various PWFA parameters. ……………………....………..….. 39

4.5 Density as a function of distance from the OTR foil window…………... 41

4.6 z80max as a function of z for the measured longitudinal density

profile…………………………………………………………………… 43

4.7 z80max as a function of distance from the OTR window for the

"scaled-up density simulations. …………………………………….…. 45

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download