The fusion diagnostic community require optical recording instruments with precise time resolution covering a dynamic range of many orders of magnitude. In 2012 the Laboratory for Laser Energetics, Photek and Sydor Instruments embarked on the re-design of an improved streak tube for fusion diagnostics. As a baseline, we started with the Photek STY streak tube because the tube body can accommodate a 35 mm long photocathode. Electron optical modelling was carried out by both Paul Jaanimagi in the US and by Photek in a parallel exercise. Many changes and modifications were made: the time resolution was improved to 5 ps, the usable cathode length was increased from 20mm to 32 mm under high extraction field operation and the off-axis spatial resolution was substantially improved compared to other tubes of this format. Several tubes have been built and tested in a Sydor ROSS-5800 streak cameras, and show greatly improved resolution (MTF).
The Diagnostic Development Group at the Laboratory for Laser Energetics has endeavored to build a stand-alone, remotely operated streak camera with comprehensive auto-focus and self-calibration capability. Designated as the Rochester Optical Streak System (ROSS), it is a generic streak camera platform, capable of accepting a variety of streak tubes. The system performance is limited by the installed tube's electron optics, not by any camera subsystem. Moreover, the ROSS camera can be photometrically calibrated.
We present a novel x-ray streak tube design that uses a modest extraction field at the photocathode, axial time-of-flight dispersion compensation, and transverse energy selection to achieve better-than-100-fs time resolution.
The penumbral imaging technique has proven to be ideally suited for neutron imaging. The French CEA has successfully installed a neutron imaging system at the LLE (Rochester-New York) in June 2000. Images of the 14MeV fusion neutrons produced in the target have been recorded in the range 1012 to 1014 with a two-point resolution of 45 micrometers. The detector used was a 15cm diameter circular array composed of plastic scintillator elements. For several of the CEA experiments, bubble detectors developed for General Atomics simultaneously recorded neutron images. The SIRINC (Simulation and Reconstruction Imaging Neutron Code) code has been used to unfold neutron images obtained both with the segmented scintillator detector and with the bubble detector. We first describe the experimental setup and detector designs, then compare the sensitivity, quantity of information, and signal to noise ratio for those two detectors. Then raw and unfolded images are presented. The spatial resolution obtained for the unfolded images are estimated and compared for the two detectors types.
Direct-drive laser-fusion ignition experiments rely on detailed understanding and control of irradiation uniformity, the Rayleigh-Taylor instability, and target fabrication. LLE is investigating various theoretical aspects of a direct-drive NIF ignition target based on an 'all-DT' design: a spherical target of approximately 3.5-mm diameter, 1 to 2 micrometers of CH wall thickness, and an approximately 350-micrometers DT-ice layer near the triple point of DT. OMEGA experiments are designed to address the critical issues related to direct-drive laser fusion and to provide the necessary data to validate the predictive capability of LLE computer codes. The cryogenic targets planned for OMEGA are hydrodynamically equivalent to those planned for the NIF. The current experimental studies on OMEGA address all of the essential components of direct- drive laser fusion: irradiation uniformity and laser imprinting, Rayleigh-Taylor growth and saturation, compressed core performance and shell-fuel mixing, laser- plasma interactions and their effect on target performance, and cryogenic target fabrication and handling.
The OMEGA laser at the University of Rochester's Laboratory for Laser Energetics is used for direct-drive inertial confinement fusion (ICF) experiments. To achieve highly symmetric implosions, it is necessary that all of the beams be power balanced. The definition of power balance must be tied to the physics of the ICF targets: in a given time interval what level of nonuniformity can be tolerated before the laser drive disrupts the implosion. We monitor the pulse shape on several of the OMEGA beamlines using multiplexed streak cameras. From a database of over 15,000 individual streaks we have determined that if all the OMEGA beams are energy balanced to the 1 percent level, the present system would have an overall power balance of better than 10-15 percent. This was determined by grouping individual traces by pulse shape and energy. A further improvement in power balance can be obtained by matching the electrical energy stored in Nd:glass amplifiers. This information is important because the first step in achieving power balance will be to energy balance the OMEGA laser. This research tells us what strategies for achieving energy balance will enhance power balance.
High precision and high dynamic range measurements are required to properly characterize the pulseshape in ICF laser systems. The dynamic range and precision of the measurements that can be made with streak tubes is determined by the number of photoelectrons that can be transported to the recording screen per channel and per temporal sample of the signal. They are limited by the overall current that the tube can deliver without distorting the signal. In order to build tubes with the large dynamic ranges required by this application, we need to understand at what current density level the tube response becomes nonlinear. We present results of experiments made by using a laser illuminating several streak tubes at various intensities. We show that charge depletion in the photocathode can occur in pulsed mode and limit the signal to levels well below that where space charge induced nonlinearities appear. The use of scientific grade CCD's for recording the streak traces has allowed the introduction of a new method to measure the absolute current in the tube. A 2D PIC electron optics code has been used to simulate the propagation of the beam in the input diode (photocathode/accelerating electrode) and in the drift region (accelerating electrode/screen) of a bilamellar streak tube. We compare the numerical results with the experiments. We conclude by comparing the bilamellar and classical electron optics tubes and show the advantages of the former tube design for this application.
The photoelectron throughput in streak tubes may be understood by using the brightness theorem to couple the photoelectron emission from the virtual cathode, through the anode aperture, to the recording screen. The virtual cathode is generated by the immersion lens formed by the photocathode-accelerating electrode structure. The throughput is limited by the anode aperture that acts as a system field stop. We have calculated the throughput for a variety of streak tubes, given the photoelectron energy distribution of some typical photocathodes. We conclude that higher throughputs are obtained when using a slot, rather than a mesh, for the accelerating electrode. The variable magnification design of the Philips P850 streak tube allows one to optimize the throughput for arbitrary photoelectron energy distribution.
Detailed measurements of the time dependence of the neutron flux from the implosion of DT- and/or DD-filled targets are required to better our understanding of inertial confinement fusion. Past efforts at developing fast neutron detectors have generally suffered from a lack of sensitivity and/or insufficient time resolution. In this paper we report on a new streak camera diagnostic for directly time-resolving the neutron burnwidth for ICF implosions. The technique uses the (n,p) reaction in CH2 to convert the neutron signal to a proton signal, which is proximity coupled to a CsI secondary electron emitter and is subsequently recorded with a standard LLE large-format x-ray streak camera. An x-ray signal is recorded simultaneously with the neutron-produced signal and provides an accurate timing fiducial for burn-time measurements. We have recorded usable signals from the implosion of DT-filled targets producing yields of 3 X 10 10 neutrons, with a target to photocathode distance of 30 cm. The calculated time resolution is better than 20 ps for 14 MeV neutrons and 10 ps for 2.45 MeV neutrons. Our technique for recording the neutron flux can also be extended to high-speed framing cameras, currently capable of 35-ps-duration gate times. The framing cameras will permit the simultaneous recording of the burnwidth and the neutron energy spectrum. Also, time-resolved neutron imaging of the core will be possible for DD yields > 1012.
The achievement of high density implosions of direct-drive laser fusion targets with the 24-beam TJV-OMEGA laser
system places very stringent requirements on the irradiation uniformity on target. Non-uniformities in the irradiation pattern
must not exceed -1% mis. One of the prerequisites for establishing the level of uniformity attained is the very precise
measurement of the laser power balance. This entails that the laser power in each of the 24 beamlines be diagnosed
simultaneously with a precision of 1%, over a dynamic range of 1000. The nominal laser pulse shape is a 600-ps FWHM
Gaussian, but this can vary between individual beamlines due to beam-to-beam differences in the nonlinear processes of
frequency conversion or gain saturation in the laser amplifiers.
We have set up a pulse-shape measurement system in which we pick off a small fraction of the energy in each
OMEGA beamline, linearly attenuate it, and couple the light into a multi-mode optical fiber for transport to a 24-channel
streak camera. The signals are multiplexed in 2 groups of 12 beams each. The streak-camera output is amplified with a dual
intensifier system and recorded on a photometric quality, cooled CCD camera. In this paper, we will report on the system performance and progress to date.
A dual channel picosecond resolution streak camera receiver system must be space qualified for the GLRS instrument. This study has focused on the requirements and characteristics of the streak camera tube and its associated electronics with some analysis of the input and output interfaces to the streak camera. The major tradeoffs considered and the baseline streak camera design are discussed. A streak tube design is proposed with an internal high gain microchannel plate and fiberoptic coupling to a solid-state self-scanned CCD array readout assembly. Concerns regarding the reliablility of an avalanche transistor based sweep circuit and the radiation resistance of a CCD camera are highlighted for further study. 1.
Common streak camera phosphors such as P-i 1 and P-20 have conversion efficiencies that depend on the duration of the excitation pulse. This reciprocity failure leads to the poor sensitivity that has been reported for streak camera systems. An alternative phosphor P-46 which has a very short persistence time appears to have a more linear response. In this paper we report on the conversion efficiency of these phosphors as measured with a photometrically calibrated CCD as well as a study of the rise and fall times of the phosphorescence. Excitation pulse durations range from 20 ps to 300 ms. 1.