Operational Parameters for the Superconducting Cavity Maser*

Tests of the superconducting cavity maser (SCM) ultra-stable frequency source have been made for the first time using a hydrogen maser for a frequency reference. In addition to characterizing the frequency stability, the sensitivity of the output frequency to several crucial parameters was determined for various operating conditions. Based on this determination, the refrigeration and thermal control systems of the SCM were modified. Subsequent tests showed substantially improved performance, especially a t the longest averaging times. In frequency stability tests, characterization of the short term performance of the SCM was not possible due to hydrogen maser fluctuations, but for the longest measuring time the low SCM instabilities could be characterized. This was expected, since cryogenic cavity oscillators show unsurpassed performance for short measuring times. In initial tests, our measurements showed a frequency stability of about 2 . 10-l4 for times between 30 and 3000 seconds. The long term stability (3000 a) is appraocimately 5 times better than we were previously able to measure. In order to better understand the limits to SCM stability we performed a detailed study of the dependence of the operating microwave frequency ( ~ 3 2.69GHz) on: operational temperature, pump frequency, pump power, coupling strength and the bias field applied to the ruby maser. Of these, sensitivity to changes in temperature and pump power are crucial because of the great difficulty in stabilizing these two parameters. We discovered operational parameters for which both of these sensitivity coefficients approach zero, and have identified a sixth parameter, the temperature gradient across the oscillator, the effects of which severely compromised the effectiveness of our thermal regulation system. After the flrst tests the low temperature cryogenic system was rebuilt with two changes, Conversion to a continuous-flow cooling system appropriate to the new 1.57K operating temperature allows long term operation without the refill cycles required by the previous 1.OK closed cryostat. Secondly, cooling and thermal regulation were arranged in such a way that neither the heater power required for temperature regulation, nor thermal leakage due to mechanical supports, flow through the body of the oscillator itself. This prevents unavoidable fluctuations in heat flow from generating corresponding frequency fluctuations due to thermal gradients across the oscillator. Frequency stability measurements on the rebuilt oracillator showed impraved results, giving fractional frequency stability in the mid-lo-'' range for times between 100 and 1000 seconds,