A person awarded a fellowship under the provisions of this subchapter shall continue to receive the payments provided in
section 1324(a) of this title only during such periods as the Secretary of Energy finds that he is maintaining satisfactory proficiency in, and devoting essentially full time to, study or research in the field in which such fellowship was awarded, in an institution of higher education, and is not engaging in gainful employment other than part-time employment in teaching, research, or similar activities, approved by the Secretary of Energy.