Browsing by Subject "Electron beams"
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Item An electron beam controlled CO2 laser(Texas Tech University, 1977-05) Jasper, Jon ShippThe engineering design and operation of a double sided cold-cathode-electron-beam controlled CO2 laser is discussed. Basic theoretical considerations are discussed, including fractional electron power transfer to the various energy levels of the CO2molecule as a function of gas mixture and E/n. Two types of cold cathode designs are investigated, and experimental results are given. Operational parameters and results for single-sided and double-sided operation are discussed.Item An electron beam controlled diffuse discharge switch(Texas Tech University, 1985-05) Harjes, Henry CharlesIn recent years there has been an increasing interest in the development of fast, repetitive, opening switches which would allow the use of inductive energy storage in repetitively operated pulsed power systems. An opening switch concept that shows promise for such operation is the electron beam controlled diffuse discarge switch (EBCS). An analysis of this switch that demonstrates the importance of the switch gas composition is presented and several desirable gas properties are identified. An experimental facility for the study of promising gases in a repetitively operated EBCS is also described. It includes an e-beam generating tetrode that is able to deliver an e-beam with a maximum energy of 250 keV, a risetime of 10 ns, a maximum pulse duration of lus, and a variable repetition frequency up to 25 MHz in a lys burst. Results of experiments in various switch gas mixtures are presented. Current gains of approximately 10 and opening times < 100 ns have been observed.Item An electron beam triggered spark gap(Texas Tech University, 1981-08) McDonald, Kenneth FoxThe triggering of a high-voltage gas-insulated spark gap by an electron (e) beam has been investigated. Rise times of approximately 2.5 ns with sub-nanosecond jitter (~0.2 ns) have been obtained for 3 cm gaps charged at voltages as low as 50 percent of the self-breakdown voltage (varied up to 0.5 MV). The switch delay (including the e-beam diode) was 52 ns. The triggering e-beam pulse has a duration of 15 ns and a 0-50 percent rise time of 1.5 ns. The e-beam current is 0.5 kA, and the electron energy can be varied in the range from 80 to 145 keV. The working media were N2, mixture of N2 and Ar, and N2 and SF6 at pressures of 1-3 atm. Voltage, current and jitter measurements have been made for a wide range of gap conditions and e-beam parameters. The effects of varying the e-beam cross-sectional area and the beam energy are discussed.Item An investigation of electron beam initiated spark gap breakdown(Texas Tech University, 1979-12) Newton, Mark AllenA coaxial system was designed and constructed to investigate electron beam initiated discharges in high pressure gases. An electron beam was fired axially into a gap formed by an interruption in the center conductor of the coaxial system to initiate a discharge. Photographic techniques were used as diagnostics to examine the physical processes occurring in the discharge. Open shutter photographs revealed broad discharge channels with spatial dimensions approximately equal to the spatial dimensions of the electron beam. Streak photographs of the discharge provided evidence that an avalanche process was occurring in the discharge.Item Current density and time resolved emittance measurement of an electron beam extracted from a plasma edge cathode(Texas Tech University, 1995-12) Stemprok, RomanThe Plasma Edge Cathode concept can produce high electron current densities while avoiding or delaying plasma closure of the extraction gap of an electron source. A plasma beam is generated by a surface flashover from a spark plug. An obstacle partially intercepts the emitted plasma jet at a distance of approximately 200 mm from the plasma source and causes a stationary transverse plasma boundary from which electrons are extracted. The plasma should experience negligible extemal electric forces as long as the extraction is space charge limited. Thus, it should be possible to avoid the plasma closure of the extraction gap and to obtain an extracted electron current density over 100 AJcnfi. Experimental electron beam density measurements support this theoretical prediction. For an electron beam with a diameter of about 2.5 cm the extraction current is smooth and electron current densities up to 100 A/cm were obtained repeatedly with a duration up to 2.5 jis for an extraction gap of 4 mm. For a 7.5 cm diameter beam an electron beam current density was measured from 2 to 7 A/cm^ at an increased extraction gap of 15 mm. The pepper-pot method was used to measure the emittance of the extracted electron beam. An extraction gap of 15 mm together with an extraction voltage less than 20 kV yielded a relatively smooth pattem on the scintillator. The images were recorded with a framing camera. Then they were digitized with a scanner and a computer code determined the emittance. A two-frame intensifier camera with an exposure time down to 5 ns was built to record the electron beam pattem. A 50 ns exposure time was used to determine the effective emittance. Approximately half of the 100 shots taken had a suitable exposure and separated spots for evaluation. At higher extraction voltages the pattem was irregular and indicated some instability of the plasma surface. The measured normalized emittance is on the order of EJ,^^ = 5 X 10"^m^ rad^. The normalized brightness was found typically to be B = 3 X10^ A m~^ rad"^. The trace space density measurements show a nearly parallel electron beam. A major difficulty with the emittance measurements was the limited reproducibility from shot to shot.Item Design and construction of a high power electron gun(Texas Tech University, 1984-05) Leiker, Gary RA high power electron gun capable of producing an e-beam over a 100 square centimeter area has been designed and tested. Grid control of the e-beam provides 100 ns, 250 keV electron bursts through the foil in 100 ns intervals. Results of voltage and current measurements of the electron beam, as well as measurements of its spatial and temporal uniformity, are presented.Item Electron beam controlled switching(Texas Tech University, 1995-05) Awrach, James MichaelNot availableItem Experimental investigation of a plasma edge cathode scheme for high current density, long pulse electron extraction(Texas Tech University, 1990-05) Grothaus, Michael GlennConventional cold cathode sources used for the extraction of long pulse electron beams are typically limited by plasma closure of the extraction gap. In the plasma edge cathode scheme, a nearly stationary plasma cathode is formed by partially intercepting a well directed plasma jet with a material edge. Electrons are extracted in a direction normal to the flow of plasma behind the obstacle. Under space charge limited extraction, the plasma boundary will experience negligible electric force since the electric field at the boundary will be reduced to near zero. As a result, if the boundary is initially stationary, it will remain so and plasma closure can be avoided. An electron current density in excess of 100 Acm^2 is theoretically possible from a plasma with density n = 4x10^13 cm^3 and kTe=1eV. Experimental investigation of the plasma edge cathode has confirmed the underlying concept and has resulted in an achieved current density in excess of 20 A/cm^2 for 8 µs. Geometric diode gap spacings between 0.4 cm and 1.5 cm were typically used resulting in the observance of closure velocities as low as 0.08 cm/µs. Both small (4.71 cm^2) and large (42.4 cm^2) area anode configurations were used. Variation of the gap potential and spacing verified the space charge limited nature of the extracted electron current density. The density and velocity of the plasma jet are inferred from biased flux probe measurements taken at various positions in the vacuum chamber. In addition, the electron temperature both in and downstream of the extraction region was measured as a function of time with a double Langmuir probe. Finally, a multi-pinhole imaging technique was employed to measure the intrinsic divergence and microscopic brightness of the electron beam as well as to infer the effective shape of the plasma boundary.Item A model for multi-wave beam-plasma interaction(2004) Evstatiev, Evstati Georgiev; Morrison, Philip J.A system that describes the interaction of an electron beam, plasma waves, and electromagnetic waves in a cold plasma is presented and studied. A multi- wave model is developed that allows for efficient computational and analytical study. The model is based on the slow amplitude and phase change approxima- tions. Using a Lagrangian approach, the continuous system of electron beam, background plasma, and waves is reduced to a finite degree-of-freedom system. This model, describes an efficient energy transfer mechanism between electro- magnetic waves and the plasma wave, via the particles trapped in the plasma wave. It is suggested that this energy transfer be used in plasma-based accel- erators to further increase the energy of the accelerated particles. Numerical and analytical studies of this mechanism are performed and an experimental test is proposed.Item Negative differential conductivity in e-beam sustained diffuse discharges for switching applications(Texas Tech University, 1986-05) Strickland, Bryan EdwardIn e-beam sustained diffuse discharges in gas mixtures which contain small additives of electronegative gases, the discharge characteristic (current density versus reduced field strength) may exhibit negative differential conductivity (NDC) depending on the source function and the concentration of attacher gas. In discharges exhibiting negative differential conductivity, electron depleted domains of high electric field intensity are formed in the discharge. The results of electrical and optical measurements performed on an e-beam sustained diffuse discharge in a gas mixture of 2% C2F6 in 1 atm Ar are presented and compared with theoretical predictions. The steady state current density (J) versus reduced field strength (E/N) exhibits a strong negative differential conductivity in an E/N range of 2.5 Td < E/N < 5 Td. Time resolved photographs taken in this E/N range show distinct luminous layers perpendicular to the discharge axis.Item Optimization of the source function in an electron-beam sustained discharge containing attaching gases(Texas Tech University, 1986-08) Reinking, Gregory FranklinIn pulsed power work, large amounts of stored energy are frequently required. This energy may be stored more efficiently with respect to volume and weight with inductive storage rather than capacitive storage. In order to effectively transfer inductively stored energy to a load, a fast opening switch is necessary. Of the many opening switch methods under study, the externally sustained diffuse discharge switch has several advantages. In electron-beam sustained discharges, electrons are generated by the high energy beam electrons and depleted through recombination and attachment. The use of attachers is necessary to achieve fast opening times. This project considers the two processes, electron generation and depletion, which affect the conductivity of gas discharge switches. The optimization of electron generation is experimentally investigated through the introduction of a low ionization (Penning) additive to the switching gas mixture. The process of electron depletion is investigated theoretically with Monte Carlo Code calculations. These calculations concentrate on the thermalization process of the Initial fast electrons and the interaction of the thermalizing electrons with the attachers used in the gas discharge. The presented results provide information that is required to select suitable gas mixtures to optimize the efficiency of diffuse discharge switches.Item Reflex-triode geometry of the virtual cathode oscillator(Texas Tech University, 2003-12) Lara, Matthew BAn eight-stage four-hundred kilovolt Marx generator, in connection with a 60 nanosecond pulse-forming line, is constructed and utilized as a pulsed source to power a triode version of the Virtual Cathode Oscillator (Vircator). Eight . I uF capacitors, charged to 50kV each, are switched in series by dry-air pressurized spark gaps. The energy from the bank charges a 23 ohm oil transmission line, breaking a peaking gap when the maximum voltage is reached, delivering a 60 ns-400kV pulse to the diode. The design of the "Reflex-Triode" geometry Vircator is based upon claims of high efficiency by Didenko et al.[l] A previously constructed TTU Vircator includes a unique E-beam source, the "brush" cathode; in which a circular array of pins is used as an explosive field emission source to produce relatively high beam currents. The anode consists of a round wire mesh through which the E-beam passes, generating a dense cloud of negative charge known as a "Virtual Cathode." This initial phase of testing is composed of basic operation of the entire system and baseline output power and efficiency measurements.Item The effect of space charge induced by an electon beam on spark gap operation(Texas Tech University, 1981-08) Tzeng, YonhuaAn investigation into the effect of electron beam induced space charge on the insulating property of a gas in a spark gap is presented. The characteristics of the gas transition from insulator to conductor show strong dependence on the amount and location of the space charge introduced. Investigations of the delay time and the characteristics of the conducting channel have been made. The delay time from the injection of the electron beam to the collapse of the gap voltage ranges from 10^-9 to 10^-3 second. From open shutter photography, we observe that the character of the conducting channel is quite varied. Dark, diffuse, filamentary, or diffuse followed by filamentary (single or multi) channels have been observed, depending on the space charge conditions. The fundamental processes leading to the collapse of insulating property of the gas for various experimental conditions are discussed.