Advanced Thermometry Studies of Superconducting RF Cavities
Jens Knobloch, Ph.D.
Cornell University 1997
Abstract
Superconducting niobium radiofrequency rf cavities for
e + e -accelerators presently are limited to accelerating gradients of 25 MV/m --- far less than their theoretical capability of 50 MV/m. Power dissipation by field emission electrons presents the main impediment to higher gradients. Other mechanisms, including thermal breakdown and multipacting also contribute to anomalous losses.
To improve our understanding of cavity losses, we constructed a new system to map the temperature distribution of 1.5 GHz cavities during operation in superfluid helium. Based on existing devices, our system represents significant improvements in both resolution and acquisition speed. Hence, previously undetected losses and transient effects could be studied. Furthermore, a procedure was developed to examine the cavity interior in an electron microscope and an x-ray analysis (EDX) system, thereby permitting the correlation of thermometry data with the physical appearance of defects, as well as the identification of foreign elements.
The powerful combination of thermometry and microscopy was used for extensive field emission studies. Our results show that emission occurs predominantly from conducting particles. Their emission strength, however, is influenced by the adsorption of gases released during otherwise unrelated cavity events, such as thermal breakdown.
Of particular interest for improved cavity performance are emitters that explode ("process") when they are heated by the emission current. Thermometry data suggests that processing occurs when both the current density and the total current exceed thresholds. Microscopy demonstrates that the ionization of gases from the emitter is crucial to the initiation of the explosion. This fact is underscored by results obtained from the examination of emitters processed with intentionally administered helium gas.
To obtain more quantitative results, we performed numerical simulations of rf processing, including the ionization of gases by the field emission current. These simulations illustrate the conditions required for emitter explosion, and they confirm the importance of a plasma during such events.
Numerous other performance degrading mechanisms were studied as well, leading to our discovery of flux trapping during cavity breakdown and the detection of two point multipacting. Other observed losses, arising from hydride precipitation and titanium in grain boundaries, were a direct result of standard cavity preparation procedures and are avoidable with appropriate precautions.
For a printed version send e-mail to:
jk30@cornell.edu
