I. IONIZATION OF CESIUM AT SURFACES. II. THE ENERGY DISTRIBUTION FOR ELECTRONS IN A THERMIONIC DIODE PLASMA CANNOT BE TRULY MAXWELLIAN. Technical Report No. 405

Technical Report · Fri Aug 17 04:00:00 EDT 1962

OSTI ID:4773334

Nottingham, W B

Indications from voltage-current curves of thermionic power converters suggest increased ionization at, or very near, Cs-coated surfaces by low-energy electrons. Experiments were conducted to study this phenomenon quantitatively for a low Cs concentration. The tube contained three indirectly heated Ta surfaces surrounded by a shield. These heated surfaces, E1, E2, and E3, were Ta tubes located at the three corners of an equilateral triangle. The extension of a line from the corner E1 perpendicular to the base of the triangle formed by the line E2 and E3 served to locate the position of a fine-wire ion collector. Electrons were accelerated from surfaces E2 and E3 to bombard E1. 1ons produced at or near E1 were then accelerated toward the ion collector to serve as a measure of the ion production. Results of this experiment indicate that no appreciable ion current was produced in Cs vapor controlled by a T/sub cs/ = 350 deg K, by electrons having an energy less than 3.89 volts. The square root of the lon-to-electron current ratio was a linear function of the surface electron energy in excess of the ionization potential. Although not accurately determined, the ionization coefficient is given by P/sub 1/ = P/sub 1/(V-V/sub 1/ ) with P/sub 1/ = 20 cm/sup -1/ v/sup -1/ for t he standard condition of 3.54 x 10/sup 16/ atoms/cm/sup 3/. For the refractory emitters, such as W, Ta, and Mo, operated in an energyconversion diode, the ratio of the emitter temperature to the Cs temperature strongly affects the operating properties. If this ratio exceeds 3.2, an ion-rich sheath usually forms at the emitter surface. Electrons are accelerated in the plasma space by this injection potential. An analysis made here shows that at the open-circuit condition, the energy distribution of the electrons at the plasma edge of the emitter sheath camot be a true Maxwellian over the entire range in electron energy. Instead, it is thought to be made up of two quasi-Maxwellians. The low-energy electrons are trapped and may have a high average energy close to that of the injection potential. The untrapped electrons will have an electron temperature equal to that of the emitter and their density wi1l be that associated with an apparent or fictitious density many times that of the actual density. A theory was developed to describe quantitatively the relations that must be satisfied for the description above to apply. From typical numerical results that are applicable to specific diode configurations it is concluded that if the emitter-to-Cs temperature ratio exceeds 3.6, volume ionization is not required for a stable sheath. In the range 3.2--3.6 surface ionization is usually insufficient, and it is necessary to have volume ionization to sustain the sheath. Further studies are required to establish the mechanism. It may be direct ionization by high-energy electrons, or ionization from excited states of the cesium atoms. (auth)

• Research Organization: Massachusetts Inst. of Tech., Cambridge. Research Lab. of Electronics
• NSA Number: NSA-16-032482
• OSTI ID: 4773334
• Report Number(s): NP-12058
• Country of Publication: Country unknown/Code not available
• Language: English

Related Subjects

CESIUM
COATING
CONFIGURATION
CURRENTS
DENSITY
DIAGRAMS
DIODES
DISTRIBUTION
ELECTRIC POTENTIAL
ELECTRON TUBES
ELECTRONS
EMISSION
ENERGY
ENERGY RANGE
HEATING
INJECTION
IONIZATION
IONS
LAYERS
MAXWELL DISTRIBUTION
MEASURED VALUES
MOLYBDENUM
OPERATION
PHYSICS
PLASMA
POWER
REFRACTORIES
SURFACES
TANTALUM
TEMPERATURE
THERMIONICS
TUBES
TUNGSTEN
VAPORS
VOLUME
WIRES