also electrically tested in the United States during the TOPAZ International Program discussed in Chapter 3. Finally, a planar thermionic converter similar to the converters used in the START tests, and built by the same manufacturer that constructed the START test thermionic devices, demonstrated 24,000 hours of life in a test completed in 1994 (Thayer 1994).

Previous experience in testing thermionic devices indicates that converters can be made to work in series-connected circuits to develop a usable voltage level. The committee therefore recommends that the future sponsoring agency look closely at the START tests in order to identify and make use of the lessons learned and increase the probability of a successful test in the future. To aid in this effort, the committee identified several areas that should be given attention in any future tests:

  • As with any difficult test program, additional time and financial resources should be included in any future test plan to accommodate problems that will inevitably arise. Technical difficulties, and the time and resources required to deal with them, should be considered as a part of the standard operating procedure for any high risk, experimental scenario.

  • Some members of the thermionic device design and manufacturing team should be involved in the system tests. In this way, their expertise can be used early in the test cycle to minimize errors, help overcome testing obstacles, and avoid previously identified mistakes.

  • The test fixture should be tested and characterized to make certain that performance requirements are met before fitting the thermionic devices into the fixture. For instance, temperature stability characteristics and temperature gradients in the heating elements should be clearly identified.

  • Any future test setup should account for a high electromagnetic interference environment, because the high temperature test fixtures used in previous thermionic experiments generated a large amount of such interference.

The committee believes that the sponsoring agency should conduct an independent test of the original START test converters. First, the sponsoring agency should determine conclusively if the devices still work or if they are no longer functioning, as shown by the results at NEMERI. If the results of the reevaluation of the converters are different from those of NEMERI, an effort should be made to understand why there are differences. Any discrepancies between the two sets of data must be resolved so that the true test issues and device design issues can be identified.

Once the core issues are identified, the sponsoring agency should gather a group of experts to look closely at the START tests. This group should document proper test methods needed to have a successful test in the future.

Recommendation 7. When working on a system-level solar thermionic design, the sponsoring agency should reexamine the string thermionic assembly research testbed (START) tests to record lessons learned. The reexamination should begin with a retest of the original, individual converters to differentiate between problems due to converter design and generator configuration and those due to the test setup. The sponsoring agency should gather an independent group of experts to devise testing methodologies so as not to repeat past mistakes.

REFERENCES

Rouklove, Peter. 1967. “Thermionic Converter and Generator Tests,” Jet Propulsion Laboratory research documents, Pasadena, Calif.


Thayer, Kevin. 1994. “Life Test and Diagnostics of a Planar Out-of-Core Thermionic Converter,” paper presented at the Eleventh Symposium on Space Nuclear Power, Albuquerque, N.Mex., January.



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