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The conditions leading to crystal deterioration and burnout in tr tubes, which exhibit apparent satisfactory leakage characteristics, are investigated experimentally. The characteristics of coaxial keep-alive structures are evaluated by monitoring with probes and light intensity measurements the residual electron density in the interaction gap spacing. The prime factor leading to crystal deterioration is random fluctuations in the electron density which are attributed to a wandering of the keep-alive discharge along the cone wall. During the period of fluctuation the spike energy can become excessive. An improved keep-alive design, in which the cone wall is insulated and the cathode is fabricated from stainless steel, eliminates this effect. TR tubes with the improved keep-alive structure have successfully operated in excess of 1000 hours without any apparent crystal deterioration.