THERMIONIC EMITTERS

ELECTRON GUNSTANTALUM DISC CATHODESFEATURES / OPTIONSION GUNSREFRACTORY-METAL PLANAR CATHODE
NOT HARMED BY REPEATED EXPOSURE TOATMOSPHERE WHEN COLD
EXCEPTIONAL STABILITY
LOW ENERGY SPREAD
ACCURATELY PRE-ALIGNEDINTERNATIONAL STANDARD AEI BASE
NON-STANDARD MOUNTING AVAILABLE

ES-042, ES-044 and ES-046
Tantalum Disc Cathodes

There are several factors to consider in choosing a cathode size. Alarger disc has a larger emission area, and thus more total current fora given current density. The current density of course is amonotonically increasing function of temperature. However, a larger
disc loses more power to radiation, and thus more heating power isrequired. A smaller disc tends to have a more uniform temperature
distribution, which results in a more uniform emission. Larger discs
with larger legs tend to have a longer lifetime.
Kimball Physics has determined several combinations of cathodedimensions that have proved to work well in the Kimball Physics
electron guns. The model ES-042 consists of a 0.033 inch (0.84 mm)diameter tantalum disc, attached to a 0.003 inch (0.08 mm) diameter
tungsten 3% rhenium heater wire. The higher current models, ES-044
and ES-046 have larger discs and thicker heater wires (seespecifications table). Custom cathodes are available with different
refractory metals, optional emitter styles (disc, hairpin, or ribbon),various disc sizes, and various filament lengths and diameters.

Tantalum Disc Cathodemounted on a CB-104 ceramic base (4X)

Toll Free: 1 888 KIM-PHYS (1 888 546-7497) e-mail: info@kimphys.com
Tel. 1 603 878-1616 Fax: 1 603 878-3700

All models of the tantalum disc cathode structure are available
mounted on an industry-standard ceramic AEI base, or on a Kimball
Physics ceramic base (CB-104 or CB-105), as well as on custom or
nonstandard bases. Base options include two or four pins, pins madeof molybdenum or Kovar, various pin lengths and positions, as well asdifferent mounting heights of the cathode surface from the ceramicbase. The edge of an AEI base can be ground to a specified custom
diameter. Some combinations of various options may not be possible
due to design considerations. All cathodes are shipped vacuum clean
and ready to install.

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DETECTORSThe cathode is not harmed by repeated exposure to atmosphere whencold; however, at temperatures above 700 K, oxidation of both thetantalum and tungsten takes place in the presence of water vapor, airor oxygen with a resulting decrease in cathode lifetime.
Attemperatures above 1200 K, tantalum nitrides form in the presence ofnitrogen; these compounds degrade the emitting characteristics of thetantalum disc.

Actual emission currents will vary depending on the applied DC voltage
and the geometry of the gun structure. The higher tungsten hairpin leg
temperatures make the decrease in leg diameter due to evaporation
the determining factor in cathode lifetime, assuming lifetime is notforeshortened due to other factors such as contamination, poor
vacuum or damage. As the tungsten legs evaporate, the resistance ofthe tungsten wire increases; thus small changes in a cathode's V-I
characteristic over its lifetime can be expected. The longest possible
lifetime is achieved by running the cathode at the lowest possibletemperature.

COMPONENTS

The tantalum disc is welded to the tungsten hairpin at a single point,
which results in a unipotential and planar emission surface. Since noheating current passes through the tantalum disc, the energy spread is
kept to a minimum (<0.7 eV). Tantalum has a very low vapor pressure
at high temperatures, a high melting point (3188 K) and a work functionof 4.1 eV.

Some power is lost from the tantalum disc due to radiation. Therefore,
in order to obtain the desired disc temperature, the tungsten hairpin
heater must reach an even higher temperature (see temperaturegraphs). The temperature of the tantalum disc determines its electron
current density; this relationship can be described by theRichardson-Dushman equation (see emission graphs).

INDEX / INFORMATIONThe Kimball Physics Tantalum Disc Cathode is a general purpose
thermionic emitter. This refractory metal cathode is quite sturdy andprovides stable and uniform electron emission for a wide variety ofelectron source applications. The cathode structure of the ES-042,ES-044, and ES-046 consists of a tantalum disc that is heated byconduction from a tungsten 3% rhenium hairpin and mounted on anindustry-standard ceramic AEI base or on a compact Kimball Physics
ceramic base. Other bases are available on a custom basis. Whenordering a spare cathode or firing unit for use in a Kimball Physics
electron gun, simply specify the gun model and options (for example,EMG-12, high current) as this will determine the particular cathodeconfiguration.

EMITTERS

Tantalum Disc Cathodemounted on a standard AEI ceramic base

TANTALUM DISC CATHODES

ES-042 Ta disc on CB-104 baseES-044 Ta disc on CB-105 baseES-042 Ta disc on AEI ceramic baseES-042CATHODE MATERIALDISC SIZEHEATER WIRE SIZELEG LENGTHHEIGHT ABOVECERAMIC BASE
EMISSION AREA
EMISSION CURRENTHEATING CURRENTPOWER SUPPLYCAPABILITYCATHODE LOADINGWORK FUNCTION
OPERATING TEMPENERGY SPREAD
LIFETIMEVACUUM LEVEL

ES-044ES-046

Tantalum0.033 in dia. x .004 in thick
0.048 in dia. x .004 in thick
0.062 in dia. x .004 in thick(0.84 mm dia. x 0.1 mm thick)(1.22 mm dia.x 0.1 mm thick)(1.57 mm dia. x 0.1 mm thick)
0.003 in dia. (0.08 mm dia.)0.004 in dia. (0.1 mm dia.)
0.007 in dia. (0.18 mm dia.)approx. 0.11 in (2.8 mm)
approx. 0.12 in (2.8 mm)
approx. 0.15 in (3.8 mm)
on CB-104 base: 0.14 in (3.5 mm)
on CB-105 base: 0.25 in (6.4 mm)
on CB-105 base: 0.27 in (6.9 mm)
on AEI base: 0.256 in (6.5 mm)
on AEI base: 0.256 in (6.5 mm)
on AEI base: 0.256 in (6.5 mm)
5.52x10-3 cm2
11.7x10-3 cm2
19.5x10-3 cm21 mA typical
3 mA typical
5 mA typical
1.4 A to 1.8 A
2.4 A to 2.8 A
5.7 A to 6.6 A
Voltage regulated power supply
Voltage regulated power supply
Voltage regulated power supply
recommended, 2 V, 2 A
recommended, 2 V, 3 A
recommended, 2 V, 7 A
0.25 A/cm2 recommended, typical; High loadings result in reduced lifetime
4.1 eV
2200 K typicalapprox. 0.6 eV
Hundreds of hours with medium currents-510 torr or better, recommended

2.0 X

2.0 X

ES-042 Tantalum Disc Cathodemounted on a CB-104 ceramic baseES-044 Tantalum Disc Cathodemounted on a high current CB-105 ceramic base

2.0 X

2.0 X

ES-046 Tantalum Disc Cathodemounted on a high current CB-105 ceramic baseES-042 Tantalum Disc Cathode mounted on an AEI ceramic baseES-044 and ES-046 also available on AEI bases
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ALL DIMENSIONS ARE IN INCHES(MILLIMETERS IN PARENTHESES)

It is not necessarily possible to achieve all maximum specifications
simultaneously. Specifications Subject to Change Without Notice.