Index of content:
Volume 77, Issue 7, July 2006
- NUCLEAR PHYSICS, FUSION AND PLASMAS
77(2006); http://dx.doi.org/10.1063/1.2216792View Description Hide Description
A triggered multiple-gap spark gap switch has been developed and tested under atmosphere. By means of an trigger circuit, the multiple-gap switch can be used very reliably. For the same switching voltage, with increasing the number of gaps from 2 to 6, the switching current rise time is reduced from , and the energy efficiency is increased from 87% to 92%. An eight-gap switch was also tested, and the switching current rise time is much smaller than the usable rise time of the current probe . One interesting application of the multiple-gap switch is to improve the switching performance in the multiple-switch and transmission lines based pulsed power circuit. To verify this application, a six-gap switch was tested. In contrast to a single-gap switch, the output current rise time was improved from by the six-gap switch.
77(2006); http://dx.doi.org/10.1063/1.2213169View Description Hide Description
In inertial confinement fusion implosion experiments with the primary-neutron yield as low as , the method of yield ratio is proposed to diagnose the areal density at Shenguang II laser facility. Considering the detection efficiency and the time response, a new detector for detecting the secondary-neutron signal is developed, which locates away from the target. According to Monte Carlo -particle simulation, -thick lead shield was placed in front of the detector to shield x rays. In the 2004 experiments, the highest primary-neutron yield is , which is an order lower than expected. Inspite of this fact, a secondary-neutron signal is measured for the first time at the Shenguang II laser facility, which proves the method’s feasibility. The method will be used in the experiments at the prototype of Shenguang III laser facility.
Technique for measuring pellet mass loss through a curved guide tube using two microwave cavity detectors77(2006); http://dx.doi.org/10.1063/1.2219748View Description Hide Description
Two microwavecavity mass detectors have been used to measure the mass loss of deuterium pellets transported through a curved guide tube. The test tube was a mock-up of the pellet injection guide tube for the proposed ITER experiment, which will be used to transport pellets, including deuterium-tritium (D-T), from the pellet acceleration device to the inner wall (or magnetic high-field side) of the large tokamak for pellet injection and core fueling of plasmas. An accurate estimate of the mass loss is particularly important for D-T injection, because the inventory of the radioactive isotope (T) for ITER is limited and accountability and recycling will be crucial issues. In the laboratory, frozen cylindrical pellets of nominal diameter were shot through the stainless steel test tube ( in length and inside diameter), with each end equipped with a microwavecavity. As the pellet passes through each tuned microwavecavity, the peak output signal from the electronics is directly proportional to the pellet mass. An absolute calibration of the cavities, which can be problematic, is not needed for the nondestructive technique described here. Instead, a cross calibration of the two cavities with pellets of varying masses provides the relationship to determine mass loss more precisely than any other technique previously reported. In addition, the individual output signals from the cavities can be used to identify intact pellets (a single signal peak) or broken pellets (multiple signal peaks). For the pellet speed range tested in this study , the mass loss for intact pellets was directly dependent on the pellet speed, with mass loss at . The microwavecavities and the associated electronics, as well as some basic theory, are described; calibration and experimental data are presented and discussed.
77(2006); http://dx.doi.org/10.1063/1.2198730View Description Hide Description
Microelectromechanical systems(MEMS) have led to the development of a host of tiny machines and sensors over the past decade. Plasma physics is in great need of small detectors for several reasons. First of all, very small detectors do not disturb a plasma, and secondly some detectors can only work because they are very small. We report on the first of a series of small (sub-Debye length) probes for laboratory plasmas undertaken at the basic Plasma Science Facility at UCLA. The goal of the work is to develop robust and sensitive diagnostic probes that can survive in a plasma. The probes must have electronics packages in close proximity. We report on the construction and testing of probes that measure the electric field.
Real-time recovery of the electron density from interferometric measurements affected by fringe jumps77(2006); http://dx.doi.org/10.1063/1.2219731View Description Hide Description
Optical interferometers are normally used in magnetically confined plasmas to measure the refractive index of the plasma by comparing the phase shift variation between a reference and a probe laser beam, from which the line-integrated electron density can be derived. Unfortunately, interferometric measurements are affected by fringe jumps, which are basically the erroneous phase difference determination due to the loss of signals or a phase difference bigger than . The multiple causes include the refraction, wavelength of the laser radiation used, sensitivity, and time resolution of the measurements. On the other hand, the plasma density has become an essential piece of information for many real-time control schemes, which can therefore be completely jeopardized by fringe jumps. To overcome this problem at JET two main approaches can be adopted. The first approach consists of performing a real-time correction of the affected chords, eliminating the spurious effect of the fringe jumps, and providing a corrected line integral of acceptable quality. This is done at JET by complex algorithms that have inputs of various interferometry and polarimetry measurements.
A second approach can be adopted based on the observation that, for many real-time experiments, an approximate estimate of the density profile is sufficient. In JET, it was demonstrated that the density profile of the vast majority of configurations could be determined with sufficient accuracy by using only the line-integrated density profile provided by two chords; one external and one internal. The various solutions were tested and results compared in order to verify the most suitable one for the various plasma configurations and operational scenarios. A “general purpose” version of the correction algorithm was implemented and is now normally running during JET operation.
Optimized method of producing washers of titanium hydride for plasma gun using occluded hydrogen gas77(2006); http://dx.doi.org/10.1063/1.2227648View Description Hide Description
An optimized way of producing washers of titanium hydride for the application to a plasma gun using the occluded gas is presented. The amount of gas (equivalently, gas pressure ) is entirely preadjusted in a gas reservoir of a simple instrument. The temperature of a furnace is completely feedback controlled. Data show that when is the order of , needs to be higher than about in order to successfully produce washers of titanium hydride. Results on compressive strength of the loaded washers suggest that an appropriate ratio of atoms of hydrogen to titanium is less than .