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  3. FAQ About Thermal Field Emissions Sources

FAQ About Thermal Field Emissions Sources

Frequently Asked Questions (FAQ) about Thermal Field Emission (TFE)sources

Q. Are TFE sources less reliable than thermionic sources (e.g. Tungsten or LaB6)?
A. No – TFE sources can be very reliable, working over many months continuously. All YPS tips are guaranteed for one year of normal operation.
Q. What are the advantages of Schottky thermal field emission compared to cold field emitters e.g. W(310) or W(111)?
A. The major advantages are better beam current stability, higher beam currents, less stringent vacuum requirements and the fact that there is no need for periodic emitter flashing to restore the emission current.
Q. What are the vacuum requirements for operation of a Schottky thermal field emission source?
A. Normally, 10-9mbar or better at the source region.
Q. What brightness can be achieved with Schottky TFE sources compared to thermionic sources?
A. Schottky TFE emitters have a brightness of about 10xE8 Acm-2sr-1 against 10xE5 Acm-2sr-1 for thermionic sources.
Q. Do Schottky emitters need replacing very frequently?
A. Not really. The lifetime of a Schottky emitter is determined by the life of the Zr reservoir in the tip structure. This is normally about 8000 hours.
Q. Are there special steps to be taken in turning on a Schottky source?
A. Check the vacuum level be 10-9mbar at the emitter. Care is needed to avoid thermal shock to the emitter and to avoid electrical breakdown transients as the extraction voltage is applied. Further, the operating temperature should not exceed 1850K; see data sheet for current/temperature graph.
Q. How do I operate the YPS Schottky TFE emitter?
A. In the same way as other TFE sources. The YPS source is supplied with a Schottky emitter data sheet, unique to each emitter.
Q. How stable is the beam current of a Schottky field emitter compared to a cold field emitter?
A. Beam current stability for the thermal Schottky is <0.5%/h compared to 5-30% drift and random jumps for a cold field emitter.
Q. Can any thermionic source be replaced by a Schottky TFE source?
A. Yes, this is the YPS C>More column upgrade. There will be the need for additional differential pumping at the emitter region and a dedicated HT power supply unit for the TFE source. Contact us for details.