April 20, 2017

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By J. Knobloch

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The two are designed to complement each other. The chapter also covers the measurement techniques we used to analyze cavity behavior, and it explains the procedure we adopted to examine cavities in the electron microscope. The remainder of the dissertation covers the results obtained with the thermometry/quality factor measurement system and from our microscopic examination of cavities. Chapters 5 and 6 are devoted entirely to our studies of field emitters. First we discuss the data that yields information on the mechanisms responsible for enhanced field emission.

The higher the Q0 the lower the cavity’s power dissipation. 8 See Reference [2] for more details. 9 Cavities are the main source of impedance in an accelerator. 10 A fundamental electric field limit is neither predicted by the theory of superconductivity nor has one been observed experimentally up to 210 MV/m. 2. Studying loss mechanisms 5 results. Thermal breakdown can also be initiated by the heat from bombarding field emission electrons. Multipacting is a resonant process, in which a large number of electrons build up an avalanche by repeated impact with the cavity walls.

Attainable Epk . Not only does the power dissipated by a single emitter rise with Epk , but also the total number of active emitters increases with field. Field emission therefore is the chief limitation of present day cavities. 2. 2(a)). In the presence of an electric field, however, the originally infinitely thick barrier is deformed into a triangular, finite-thickness barrier. 2(b)). The triangular barrier is further lowered and rounded at its tip due to the attractive force generated between emitted electrons and the conducting surface (image charge effect).

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