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Investigation of Gamma-Ray Time Shifts Caused by Capsule Areal

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Investigation of Gamma-Ray Time Shifts Caused by Capsule Areal INVESTIGATION OF GAMMA-RAY TIME SHIFTS CAUSED BY CAPSULE AREAL DENSITY VARIATIONS IN INERTIAL CONFINEMENT FUSION EXPERIMENTS AT THE NATIONAL IGNITION FACILITY AND THE OMEGA FACILITY by Elliot M. Grafil c Copyright by Elliot M. Grafil, 2015 All Rights Reserved A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Applied Physics). Golden, Colorado Date Signed: Elliot M. Grafil Signed: Dr. Uwe Greife Thesis Advisor Golden, Colorado Date Signed: Dr. Jeff Squier Professor and Head Department of Physics ii ABSTRACT This thesis describes work on Cherenkov based gamma detectors used as diagnostics at Inertial Confinement Fusion (ICF) facilities. The first part describes the calibration and commissioning of the Gamma Reaction History diagnostic which is a four cell Cherenkov de- tector array used to characterize the ICF implosion at the National Ignition Facility (NIF) by measuring the gamma rays generated during the fusion event. Two of the key metrics which the GRH measures are Gamma Bang Time (GBT) generated from the D(T; α)n thermonu- clear burn and Ablator Peak Time (APT) caused by (n; n0)γ reactions in the surrounding capsule ablator. Simulations of ignition capsules predict that GBT and APT should be time synchronized. After GRH commissioning, the array was used during first year of NIF op- eration in the National Ignition Campaign. Contrary to expectations, time shifts between GBT and APT of order 10s of picoseconds were observed. In order to further investigate the possibility of these time shifts in view of testing both instrument and code credibility an ICF shot campaign at the smaller OMEGA facility in Rochester was devised. It was performed during two full shot days in April of 2013 and 2014 and confirmed in principle the viability of the Cherenkov detector approach but raised additional questions regarding the credibility of the simulation codes used to describe ICF experiments. Specifically the measurements show that the understanding of temporal be- havior of GBT vs APT may not be properly modeled in the DRACO code used at OMEGA. In view of the OMEGA results which showed no time shifts between GBT and APT, the readout and timing synchronization system of the GRH setup at the NIF was reevaluated in the framework of this thesis. Motivated by the results, which highlighted the use of wrong optical fiber diameters and possible problems with the installed variable optical attenuators, the NIF equipment has been updated over the recent months and new timing tests will be performed during the next years. iii TABLE OF CONTENTS ABSTRACT . iii LIST OF FIGURES AND TABLES . vii LIST OF SYMBOLS . xix LIST OF ABBREVIATIONS . xx ACKNOWLEDGEMENTS . xxii DEDICATION . xxiii CHAPTER 1 INTRODUCTION . 1 1.1 Nuclear Fusion . 2 1.2 Achieving ICF Through NIF . 4 1.2.1 The NIF Facility . 4 1.2.2 ICF Capsules . 10 1.2.3 ICF Process At The NIF . 13 1.3 Diagnostic Development . 14 1.4 Ablator Time Dependance . 17 CHAPTER 2 CHERENKOV DETECTION OF ELECTROMAGNETIC RADIATION 19 2.1 Cherenkov Radiation . 19 2.2 History Of Cherenkov Detectors At ICF Facilities . 23 2.3 Evolution Of The GRH Detector At NIF . 25 CHAPTER 3 CALIBRATION OF GRH . 32 3.1 Calibration Experiments At HIGS . 32 3.1.1 Translational Scan Charecterization . 39 iv 3.1.2 Pressure Scan . 40 3.2 Detailed Geometric Simulation Of GRH and Comparison With HIGS . 50 3.2.1 Geant4 Simulation . 50 3.2.2 HIGS Comparison . 52 3.2.3 Simulated GRH's Gas IRF . 55 CHAPTER 4 GAMMA RAY TIMESHIFT BETWEEN D-T SIGNAL AND CAP- SULE ABLATOR . 60 4.1 Theory . 60 4.2 Simulations . 62 4.3 Ablator Timeshift Measurement At The National Ignition Facility . 69 4.3.1 GRH Diagnostic At The National Ignition Facility . 69 4.3.2 Timing Calibration At The National Ignition Facility . 79 4.3.3 Experimental Results At The National Ignition Facility . 81 4.3.4 Cross Cell Analysis Of GRH Diagnostic Data . 83 4.4 Verification Experiments At OMEGA . 87 4.4.1 OMEGA Facility . 87 4.4.2 GRH System At the OMEGA Facility . 90 4.4.3 GCD System At OMEGA Facility . 99 4.4.4 Cross Timing Between The GRH And The GCD . 102 4.4.5 Measurement Of Gamma-Ray Time Shift Caused By Time Dependent Ablator Arial Density . 105 4.4.6 OMEGA Ablator Timeshift Experimental Results . 110 4.5 Potential Explanations For the Discrepancy Between NIF and OMEGA timing data . 114 v 4.5.1 Mach-Zehnder Data Acquisition System . 115 4.5.2 Monte-Carlo Error Analysis of Mach-Zehnder System . 124 4.5.3 Instrumental Timing Error At The NIF . 133 CHAPTER 5 SUMMARY AND CONCLUSION . 140 5.1 Issues With HYDRA And DRACO Simulations Of ICF Implosions . 140 5.2 Instrumental Error In The GRH At The NIF . 142 5.3 Conclusion . 144 5.4 Future Work . 146 REFERENCES CITED . 148 APPENDIX A - DERIVATION OF LAWSON CRITERION . 165 APPENDIX B - KINDLE GRH GEOMETRY DEFINITION . 168 APPENDIX C - KINDLE PMT GEOMETRY DEFINITION . 192 vi LIST OF FIGURES AND TABLES Figure 1.1 The National Ignition Facility (NIF). Diagram shows the two laser bays containing a total of 192 beams routed to the target chamber (silver sphere), where inertial confinement fusion (ICF) experiments take place [1]..........................................1 Figure 1.2 One of forty-eight Preamplifier Modules (PAM) being inspected [2]. 5 Figure 1.3 One of two laser bays that houses the amplifiers for 96 of the 192 laser beams [3]. 6 Figure 1.4 The NIF target vacuum chamber. The Final Optics Assembly (FOA) attached to laser ports can be seen at the top and bottom. Unoccupied square aluminum laser ports meant for direct drive are seen in the center. Also in the center, circular diagnostic ports are visible. The blue borated concrete forms a protective layer around the aluminum vacuum chamber. Floors are removed digitally via Photoshop [4]. 8 Figure 1.5 Cryogenic target positioning system (CryoTARPOS) holding the hohlraum (silver cylinder) and capsule inside the hohlraum. Five seconds before a shot, the triangle shrouds opens exposing the cryogenically cooled (<19 K) capsule to the target chamber environment [5]. 9 Figure 1.6 The NIF hohlraum (a) Exploded schematic of the hohlraum and thermo- mechanical package. (b) Picture of a nominal NIF hohlraum [6]. 10 Figure 1.7 X-ray image of a typical Cryo D-T capsule. The various layers that make up the capsule shell can be seen [7]. 11 Figure 1.8 Monte Carlo N-Particle Transport Code simulation done by L. Dauffy of the photon spectrum for the National Ignition Facility. Simulated spec- trum is of a Cryo D-T capsule composed of CH. 16 Figure 1.9 The Gamma Reaction History array currently installed at the National Ignition Facility [8]. 17 vii Figure 1.10 Measurement of the gamma rays produced from a single NIF ICF event performed on June 20th 2011 (shot N110620-002-999). The four GRH detectors were set at 2.86 MeV (red) dominated by signals from the capsule ablator, 5 MeV (green) dominated by signals from the hohlraum, 8 MeV (purple) and 10 MeV (blue) both of which are dominated by signals from the thermonuclear burn. According to theory, these peaks should be time aligned and not separated. 18 Figure 2.1 Huygens' Principle applied to Cherenkov radiation. A particle (red dot) that is traveling to the left is emitting an equally spaced in time wave. Due to the particle traveling faster then the wave, a wavefront (blue line) is formed which can be subsequently viewed as the emission source (blue arrows). 20 Figure 2.2 The angle θ that the Cherenkov wave front makes with respect to the par- c ticle velocity. Since vthreshold> n , vparticle is the hypotenuse of the triangle when Cherenkov radiation is formed. 21 Figure 2.3 Plot of the Frank-Tamm formula for a variety of particle velocities. 23 Figure 2.4 Picture of the inside of the U.S. Geological Survey's TRIGA Reactor lo- cated in the Denver Federal Center. The blue glow is caused by Cherenkov radiation generated by relativistic particles interacting with the surround- ing water [9]. 24 Figure 2.5 Schematic of the Gamma Cherenkov Detector. Gamma radiation enters from the right until it interacts with a Compton converter plate (red). There the gamma ray is converted into an electron which travels through a gas cell. Cherenkov light is emitted which is then focused onto the PMT through Cassegrain optics (green) [10]. 24 Figure 2.6 The Gamma Cherenkov Detector undergoing preperations for deployment at the OMEGA facility. 26 Figure 2.7 Data from the OMEGA Facility, taken on 04/16/13 by the Gamma Cherenkov Detector using a Mach-Zehnder data acquisition system. Once the sys- tem has been timed, a measurement of Gamma Bang Time can be per- formed. This is done by measuring the difference in time between the initial Cherenkov signal generated by the D-T reaction and a timing fidu- cial(not shown). As the neutrons spread out they interact with some of the surrounding material generating gammas. This signal persists until the neutrons directly interact with the PMT, generating a spike in signal, until the neutron front passes through. 27 viii Figure 2.8 Schematics of the Gamma Reaction History (GRH) Detector. (a) Side view of a entire GRH detector. (b) Internal optic components of a GRH detector [11]. 28 Figure 2.9 Gamma Reaction History diagnostic deployed at the National Ignition Facility surrounded by the Gamma Reaction History group.
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