No 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.

The design and experimental fiidinga of a novel approach for a relatively simple low-cost freqnency-scanning mifliiter-wave antenna are described. The antenna consists of a silicon dielectric redangular rod with periodic metalfic stripe perturbations on one side. The feasibility of electronically scanning through a range of angles by varying the freqnency fed into the silicon rod is shown. CafcuIations were made to determine the allowable physical size of the sificon rod in order to maintain a single fundamental mode of operation and the effect which Sk variations and perturbation spacing have on the angle of radiation and the range of angular scan for a given frequency shift. Efforts covered the freqnency range 55-100 GHs with specific points of intereat at 60,70, and 94 GHz. The reanhs of the experiments conducted are compared with the theoretical cafcrdations. I.

AbsH-lhe design and experkmtaf fin-for a low-cost easfly fabricated mflffmeter-wave fine scmmer is described. 'HI& antenna wnsfsts of a l-mm X l-mm sificon dieketrie rod with a metal grating @siodic shcture) on the upper surface and pi-rA dfodea mounted on the sidewalf. Anarrow 8°beamis radiated from the grated @rturb@surface atan angle dependent on the guide and pwturbalion spacing. Ihe beam angle Ls switckd over a 10°angle by application of a de forward current thmogh the p-in diode nmdufatra9.

Experiments have been conducted with magnesium oxide surfaces in which high dc fields were applied to the surface, while at the same time secondary emission measurements were being made. Very high secondary emission ratios were obtained reproducibly (100 to 1 and greater). Under static conditions it was found that the ratios would increase exponentially with increasing fields. In addition, for constant fields, the ratio of liberated electrons to bombarding electrons was constant, over a wide range of bombardment energy. Measurements were made of secondary emission with a square wave field applied to the surface and time lag effects were noted. Decay time in the order of 30 microseconds were measured. The rise time was found to be dependent upon field and bombarding currents. The mechanism of field dependent secondary emission was shown to be related to an avalanche effect, triggered by the bombarding electrons. beam currents he used were 3 to 4 microamperes, under which conditions he sometimes observed a blue fluorescence of his target upon electron bombardment.Malter found experimentally that the yield from his target increased as a power of his collector voltage:where I c = collected current, V c -collector voltage, and B,m = constants. His collector current was also found to rise as a power of the beam current. In addition he found that the surface exhibited long time lag characteristics. That is, after the primary beam was cut off, the secondary emission first decayed rapidly and then more slowly, persisting for many hours in some cases.The theory that Malter and later Koller, 7 developed was that the high secondary emission ratios observed (i.e., 1000 to 1) were due to positive ions formed at the surface of the secondary emission material by electron bombardment. These positive charges were thought to reside on the surface for long lengths of time, due to the high resistivity of the film. During this surface ionization period, high fields could exist in the material and electrons could be liberated in greater quantities due to the field.Pensak 5 did some work too, which may be pertinent, although he studied conductivity rather than secondary emission. In his experiments, steady bombarding currents, penetrating a thickness of amorphous silica film, induced a conductivity producing a current of one hundred times the bombarding beam. He used a doublebeam technique, to measure the potential at the surface.The beam used for inducing conductivity was kept at about 10,000 volts and 0.02 jua. He concluded that the induced currents were proportional to the energy absorbed by the beam, and that the beam in question must have sufficient energy to penetrate the film for induced conductivity. It was found too, that an increase in induced currents could be obtained with an increase in field across the insulator. No time lag measurements were specifically referred to in this article. 7 L. R. Koller and R. P. Johnson, Phys. Rev. 52, 519-523 (1937). 877