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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 62, NO. 6, JUNE 2015
Anode Materials for High-Average-Power Operation in Vacuum at Gigawatt Instantaneous Power Levels Curtis F. Lynn, Member, IEEE, Jonathan M. Parson, Member, IEEE, Michael C. Scott, Member, IEEE, Steve E. Calico, Member, IEEE, James C. Dickens, Senior Member, IEEE, Andreas A. Neuber, Fellow, IEEE, and John J. Mankowski, Senior Member, IEEE Abstract— The thermal behavior of several electrically conducting solids under high incident electron fluence in high vacuum was evaluated. At electron energies of up to ∼200 keV, the depth-dose relationship for electron penetration into the materials was considered, and the resulting energy deposition profile from the surface was revealed to extend to a maximum of ∼175 µm below the surface depending on the anode material. Black body radiation is considered as the major mechanism that balances the power deposited in the material on the timescales of interest. Comparing the radiated power density at the sublimation temperature for different materials, metallic/nonmetallic, revealed that pyrolytic graphite anodes may radiate over 20 times more power than metallic anodes before failure due to sublimation. In addition, transparent pyrolytic graphite anodes (with a thickness on the order of several tens of micrometer) potentially radiate up to 40 times that of metallic anodes, since heating by the electron beam is approximately uniform throughout the thickness of the material, thus radiation is emitted from both sides. Experimental results obtained from titanium and pyrolytic graphite anodes validate the thermal analysis. Index Terms— Carbon, cold cathode tubes, electron beams, microwave tube.
I. I NTRODUCTION
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OLD cathode vacuum diodes have applications in laser pumping and a variety of high-power microwave sources due to their ability to supply average current densities on the order of tens of A/cm2 to several hundreds of A/cm2 . Over recent years, research into these diodes has focused on various cathode materials [1]–[5] and geometries [6]–[8] in order to optimize the electron beam uniformity. Carbon fiber cathodes have shown many favorable characteristics, including low electric field threshold for the onset of electron emission, excellent emission uniformity, minimal plasma expansion velocity, and lifetimes exceeding 106 shots [1]–[5]. In addition, investigations into multidimensional space charge limited flow
Manuscript received March 11, 2015; revised April 3, 2015; accepted April 14, 2015. Date of current version May 18, 2015. This work was supported by the Test Resource Management Center through the Test and Evaluation/Science and Technology Program within the U.S. Army Program Executive Office for Simulation, Training and Instrumentation under Contract W900KK-12-C-0053. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the Test Resource Management Center Test and Evaluation/Science and Technology Program and/or PEO STRI. The review of this paper was arranged by Editor M. Thumm. The authors are with the Center for Pulsed Power and Power Electronics, Texas Tech University, Lubbock, TX 79409 USA (e-mail: curtis.lynn@ ttu.edu;
[email protected];
[email protected]; steve.e.
[email protected];
[email protected];
[email protected]; john.
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2015.2424076
have demonstrated methods for minimizing edge emission from the cathode surfaces [6]. While advances in cold cathode performance greatly improved diode performance, information regarding optimal anode materials in open literature is quite limited. Published work has concentrated primarily on measuring the effects of gas evolution and plasma formation as well as expansion on diode performance [9]–[11]. It is well accepted that the anode plasma formation and expansion initiates bipolar flow and eventual impedance collapse within vacuum diodes [9]. The effects may be minimized by the surface treatments and/or using a mesh anode to minimize the anode surface area [9], [11]. Anode materials previously studied include, copper [9], Mo [12], Ti [11], stainless steel [9], [11], and graphite [13], among others. While anode and cathode plasma formation presents a hard limit on the duration of a single pulse, their effects are often negligible when operating a high-power vacuum diode for short duration pulses (
