ASNT

and the material’s work function W (units matching boltzmann constant, k , product with T ). Higher temperature implies higher electron beam current: this needs to be balanced with the life of the emitter, which decreases as temperature increases due to evaporative loss of tungsten over time.

The electrical circuits necessary for applying current to the filament, maintaining focusing potentials, and biasing the entire cup at cathode potential are isolated from ground through high-voltage isolating transformer circuits. They make up a part of the high-voltage generator used to power an X-ray tube. Some technical solutions for these design goals employ separate vessels, using dielectric oil for insulation and cooling, connected by high-voltage cables. The overall shape of the cathode cup is inf luenced by the effects of high negative voltage. Cathode cup corners and edges are rounded and blended, and final finish is a high-luster polished surface. This reduces the number of asperities that may produce or allow field emission current discharge events. These events can lead to high-voltage vacuum breakdown. Breakdown events can sustain high currents, tens to hundreds of amperes over short time periods that can damage surfaces by roughening. Ions created by the ionization of residual gas in the insert accelerate back toward the cathode; sputtering, ion implantation, and material subsurface damage can compromise cathode lifetime. High-voltage generating equipment can be damaged or, at the very least, produce a preprogrammed hiatus in high-voltage service to the X-ray tube. The seams where metal cathode support meets the main cathode insulator in vacuum are carefully shaped to manage metal-insulator-vacuum triple point breakdown. Various means are employed to lower the electric field at these junctions to inhibit arcing along surfaces. Finally, the internal surfaces of the frame that separate the cathode and anode are polished and treated to avoid surface arcing and breakdowns caused by the high-electric fields. Low electron-impact secondary electron yield properties are important for high-voltage stability. There is no convective means for heat dissipation available in the insert frame since electron beams must be generated and transported in a vacuum; pressure at or below 10 –4 kPa (10 –6 mbar) when the tube is cold, and no higher than 10 –2 kPa (10 –4 mbar) when hot. Conduction and radiative cooling are used to bring heat out from the hottest element (the filament or other emitter) to the casing. The cathode cup is mounted

− W kT

⎞ ⎠ ⎟

⎛ ⎝ ⎜

J = AT 2 e

(Eq. 1)

The filament wire diameter is chosen to maintain electron emission temperature over the intended life of the tube. If tens of milliampere electron beam current is required, several turns of 0.1 to 0.2 mm (0.0039 to 0.0079 in.) diameter tungsten wire is necessary for adequate emitting area if the temperature of the wire is maintained near 2500 º C (4532 º F). Proper focusing requires that the emitter maintain its shape, since no physical modifications can be made to the cathode once it is sealed into the tube at manufacture. Non-sag tungsten (a microalloy of tungsten and potassium) filament wire is one of the early advances in the history of X-ray sources that enabled true control of emission current independent of the accelerating potential. The electron beam focusing is achieved by electrostatic means. The potentials on the cup parts, filaments, and anode affect the beam shape. Gridding is the term applied to shutting off the electron beam at the cup by application of sufficiently high potential between the filament and the cup or between the filament and additional electrodes, separated from the cup body by insulation. The proper design of the filament shape, position, cup shape, gridding electrode, and desired focal spot shape is a goal of electron beam optics. Commercial software packages are available. Most radiographic inspection tubes are equipped with two filaments, one large and one small. The large spot can deliver more X-ray power, thus enabling shorter exposure times and larger, denser object inspection. The small spot can produce images that show finer detail resolution, but is limited in beam output capacity—exposures may be longer, objects for inspection are smaller. Two different heating currents are necessary for the two different filaments.

CHAPTER 3

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Part 1