TO DEFIBRILLATE or cardiovert, passage of sufficient electrical current through the heart must occur. 1-3 However, the optimal currents for the termination of various arrhythmias have not previously been defined. In traditional clinical practice, the operator selects an energy arbitrarily; the American Heart Association at present recommends an initial shock energy of 200 J to terminate ventricular fibrillation.4 However, the actual current flow is determined not only by the selected energy but also by the transthoracic impedance. If the transthoracic impedance is high, low 1038energy may generate inadequate current to achieve defibrillation. For example, we previously found that defibrillation was achieved by only 20% of 100 J shocks given to patients with very high ( > 97 Q) transthoracic impedance, as opposed to a 70% success rate of 100 J shocks given to patients with low or average impedance.5 Although this problem could be overcome by selecting higher shock energies, excessive energy and current may cause morphologic and functional damage.6-9 Thus, low energies and current, when appropriate, are preferable. In view of these considerations, a reasonable first approach would be to base energy selection on transthoracic impedance: a relatively low energy, 100 J, should achieve defibrillation or cardioversion in most patients with low impedance, whereas higher energies, 200 J or more, are needed for high-impedance patients. We therefore devised an impedance-based energy

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A determination of the surface tension and surface migration constants of metals in the solid phase, based on the use of pulsed Geld emission microscopy, is discussed. The experimental technique is described. An application to 6eld emission cathodes of Herring's theory of transport phenomena in solids is given, which yields the necessary relations required for the determination of the basic physical constants from the experimental data. Results are presented and discussed for the case of tungsten. The method is applicable to a number of other metals, several of which are currently under investigation. I. INTRODUCTIOÑ'RANSPORT phenomena are known to occur in heated solids. Several physical processes may be involved, such as volume diGusion, evaporation and condensation, or plastic Row; the dominant process is determined by the experimental conditions, particularly the temperature and the size of the object under study. The experimental determination of the physical constants associated with transport phenomena provides , basic information concerning the solid state; studies concerning refractory metals have a practical signiGcance related to their use as cathode materials.The evaporation of tungsten was studied some time ago by Zwikker. ' More recently, tracer techniques have been used to investigate the volume self-diffusion of various metals, e.g. , tantalum. ' The surface migration of numerous combinations of impurities on base metals has been investigated with Geld emission microscopy. 'Recently, an intermediate temperature has been used to maintain the electrical stability and longevity of pulsed field emitters4; under such conditions, the tungsten emitter may change its shape by a transport mechanism which has been identiGed as surface migration. 'The choice of operating conditions requires a knowledge of the diffusion constants and surface tension. Each of these constants may be determined with good accuracy through the measurement of the rate of change of cathode geometry. In an early experiment Muller' obtained an indirect determination of the geometric change from the corresponding change in the field emission current-voltage relationship, and deduced values for the activation energy and the diff'usivity constant; uncertainties were introduced in these measurements by the lack of a sensitive method for the direct measurement of small geometrical changes, and by the inability to determine the emitter geometry accurately by direct observation.Pulsed T-F emission microscopy' provides a considerably more sensitive method by which the rate of surface migration can be directly measured during the transport of a sufficiently small volume of material that the gross cathode geometry remains essentially unchanged. Other advantages of the method include a controlled and favorable cathode geometry, excellent depth resolution and high magni6cation, and continuous monitoring through the emission pattern of the surface under study. The present paper reports the use of this method to obtain improved values of the activa...

The energy exchange attending field electron emission (Nottingham effect) is shown to be localized to the emitting area of the cathode. It is further shown that the magnitude and direction (i.e., heating or cooling) depend strongly on cathode temperature, work function, and applied electric field. The temperature boundary separating emission cooling and heating is considerably below theoretical expectations for clean and for zirconium-oxygen-coated tungsten. The existing theory of the Nottingham effect, examined in the light of these and other results, must be modified to include the variation of average energy of the conducting carriers with temperature and field.

There is growing interest in high current field emitter arrays (FEAs) capable of delivering high current density and high conductance electron beams, particularly for microwave applications. Large, high packing density molybdenum and silicon FEAs have been placed in ultrahigh vacuum chambers, carefully conditioned, and tested for maximum performance and have yielded total FEA currents of 20–200 mA and beam current densities of 5–2000 A/cm2 at gate voltages of 80–150 V. However similar Mo and Si FEAs, and GaAs edge arrays, when placed in a prototype 10 GHz klystrode amplifier, have failed at 1–4 mA, even for pulsed operation at a low duty factor. Hence, the current must be increased by 25–50 in order to meet klystrode design objectives. We compare the intrinsic FE stable current limits of various materials: Mo, Si, GaAs, ZrC, and ZrC films on Mo emitters. We conclude that Mo FEAs have a high FE current limit but demand an extremely clean environment, Si or GaAs FEAs are more tolerant of a poorer environment but have a relatively low FE current limit, while ZrC/Mo FEAs have at least as equally high a FE current limit as Mo FEAs but are much more robust and tolerate a much poorer environment. We hypothesize that failures of high density Mo FEAs at relatively low current levels (below 1 μA per tip) in a microwave tube are due to ion bombardment of the FEA creating sharp nanoprotrusions on the sides of the emitters, which emit intense, focused electron beams directed at the gate, leading to a vacuum arc. Binh’s recent study presents strong evidence for the ongoing creation and destruction of nanoprotrusions on Mo FEAs. A ZrC thin film coating protects and passivates the Mo FEA surface, thus minimizing nanoprotrusion formation and allowing more stable operation at higher current levels and/or in more degraded environments. Studies of carbide (ZrC, HfC) film coatings on W, Mo, and Si single tips and on Mo and Si FEAs show that indeed the carbide film reduces the work function and operating voltage (by about 25% and 40%, respectively) and increases stability. Other studies still in progress show that blunt ZrC or ZrC/Mo single tips can operate for reasonable periods (∼1 h) at currents averaging 400 μA in ultrahigh vacuum or 200 μA in 10−5 Torr air. More extensive studies are planned to verify and to quantify the advantages of carbide film coatings for producing lower voltage, higher current, environment tolerant, and long life FEAs for various applications.