The Mechanism of Field Dependent Secondary Emission

Abstract

that using 7=1.7, the then accepted value, does not yield good quantitative agreement with experiment and suggested that better agreement would be obtained with a smaller value of 7. In a further paper 13 (appar-13 H. Messel, Proc. Phys. Soc. (London) A64, 726 (1951). ently overlooked by Treiman), Messel presented results adopting "the upper limit 7=1.1." Instead of the agreement between Messel's theory (with 7=1.7) and experiment being spurious, the facts are, as stated in the literature, that with 7=1.7 the theory is not in good quantitative agreement but with 7= 1.1 they are.In recent experimental investigations, it was found that secondary emission ratios as high as 10,000 to 1 could be attained utilizing field dependent secondary emission from magnesium oxide. Early tests showed the mechanism causing the high gains to be fundamentally different from the more standard secondary emission phenomenon.The hypothesis was made that the mechanism of field dependent secondary emission was a process similar to that of the "Townsend avalanche" occurring in gas discharges. As the surface of the dielectric film was bombarded with primary electrons, the high resistivity of the material in combination with the secondary emission current caused the surface to charge to the potential of the collector grid, producing a high field within the dielectric. Electrons released within the material could then gain enough energy to liberate additional electrons, and an avalanche type process resulted.Experiments were conducted to test this hypothesis and each proved to be consistent with the above theory. The main content of these experiments can be summarized in the following statements:(1) The high yields appeared to be independent of the base material. This indicated that the surface or volume effects were most important, implying that a Fowler type field emission from the base metal was not a significant factor.(2) In studying the secondary current as a function of field, the gas discharge equations were found to be correct. These equations predicted a straight line plot of the In ln5 vs 1/E, and in addition, gave a close estimate of the magnitude of the secondary emission ratio.(3) By means of retarding potential measurements, the energies and mean free paths of the emitted secondary electrons were determined. These data were in good agreement with the results in item (2).(4) The rise time for surface charging was determined by using square wave variations of bombarding currents, and was found to be consistent with the original hypothesis.

Robust and High Current Cold Electron Source Based on Carbon Nanotube Field Emitters and Electron Multiplier Microchannel Plate

“…the crystal size, porosity of the MgO. Due to porosity, most of the primary electrons can penetrate some distance into the volume of dielectric releasing more secondary electrons as compared to MgO deposition obtained from sol-gel which seems to be less porous [90,93]. SEY of MgO depends on various parameters such as crystal size, surface properties, porosity [90,93].…”

“…Due to porosity, most of the primary electrons can penetrate some distance into the volume of dielectric releasing more secondary electrons as compared to MgO deposition obtained from sol-gel which seems to be less porous [90,93]. SEY of MgO depends on various parameters such as crystal size, surface properties, porosity [90,93]. High SEY values were observed from single crystal MgO as the secondary electron loss mechanism are relatively less in them [58][59][60][61][62][63][64][65][66][67][68][69][70][71][72][73].…”

“…The simulation was performed for a channel with a diameter of ~250 µm and aspect ratio of 2; these dimensions were chosen as these were the ones which were achievable for alumina green tape with laser facility available at the lab. Even though the exact mechanism of high SEY from MgO-CNT system is not clearly understood, it is assumed as Townsend avalanche [93] that can be explained as follows.…”

“…Figure 4.22b shows the emission image with well defined emission spots that are due to focused electrons as MCP channels prevent electron spreading. Even though the exact mechanism of high SEY from MgO-CNT system is not clearly understood, it is assumed as Townsend avalanche [93] that can be explained as follows.…”

“…The minimum and maximum reported values of SEY for MgO/CNT are 400 and 22000, respectively [13][14][15][90][91][92], which is the highest of all the materials. Even though the exact mechanism of high SEY from MgO/CNT system is not clearly understood, it is assumed as Townsend avalanche [93] that can be explained as follows. Secondary electrons are generated from the MgO film when primary electrons bombard it.…”