Model for initiation of quality factor degradation at high accelerating fields in superconducting radio-frequency cavities

A model for the onset of the reduction in SRF cavity quality factor, the so-called Q-drop, at high accelerating electric fields is presented. Breakdown of the surface barrier against magnetic flux penetration at the cavity equator is considered to be the critical event that determines the onset of Q-drop. The worst case of triangular grooves with low field of first flux penetration Hp, as analyzed previously by Buzdin and Daumens, [1998 Physica C 294: 257], was adapted. This approach incorporates both the geometry of the groove and local contamination via the Ginzburg-Landau parameter kappa, so the proposed model allows new comparisons of one effect in relation to the other. The model predicts equivalent reduction of Hp when either roughness or contamination were varied alone, so smooth but dirty surfaces limit cavity performance about as much as rough but clean surfaces do. When in combination, contamination exacerbates the negative effects of roughness and vice-versa. To test the model with actual data, coupons were prepared by buffered chemical polishing and electropolishing, and stylus profilometry was used to obtain distributions of angles. From these data, curves for surface resistance generated by simple flux flow as a function of magnetic field were generated by integrating over the distribution of angles for reasonable values of kappa. This showed that combined effects of roughness and contamination indeed reduce the Q-drop onset field by ~30%, and that that contamination contributes to Q-drop as much as roughness. The latter point may be overlooked by SRF cavity research, since access to the cavity interior by spectroscopy tools is very difficult, whereas optical images have become commonplace. The model was extended to fit cavity test data, which indicated that reduction of the superconducting gap by contaminants may also play a role in Q-drop.Comment: 15 pages with 7 figure

Magnetic Field Enhancement at Pits and Bumps on the Surface of Superconducting Cavities.

One of the dominant causes of the large gradient spread and gradient limits in superconducting cavities is the abrupt quench of superconductivity due to “defects” on the surface. Many types of defects have been identified ([1], pp. 203-204). Some involve the presence of impurities. But there are also pure Nb defects such as pits and protrusions. We present results of calculations to show that the field enhancement factor for pits can reach the value of 4. We also calculate the magnetic field enhancement at the surface of ellipsoidal protrusions. This enhancement has a value of about 1.5 for a semispherical projection

The optimal shape of cells of a superconducting accelerating section

The shape of TESLA accelerating structure can be improved to decrease maximal surface magnetic field by sacrificing and increasing maximal surface electric field. This sacrifice may be necessary because the magnetic field is a hard limit but electric field emission can be decreased by processing. For the case of superconducting cavities RF magnetic strength should be of a more concern

SASE FEL at the TESLA facility, phase 2

The last description of the TESLA Test Facility FEL has been written in 1995 (TESLA- FEL report 95-03). Since then, many changes have developed compared to the design, partially because the design was incomplete, partly because of gained knowledge over the past few years. In addition, what used to be known as phase II of the project has been subdivided into a finer time scale, starting with the first beam through the machine until the complete user facility with its different possible extensions. This report is therefore an upgrade of the above mentioned Conceptual Design Report. It is by no means complete or final, but gives a more complete overview of the present status of knowledge on present and future developments towards the TTF-FEL user facility

Abstract WORLD RECORD ACCELERATING GRADIENT ACHIEVED IN A SUPERCONDUCTING NIOBIUM RF CAVITY

46 MV/m was achieved in a superconducting niobium cav-ity with an unloaded quality (¢¤ £ factor) ¥§¦¨¥�©�� over at a temperature of � 1.9 K. In pulsed mode, 47 MV/m was achieved. This represents a world record gradient in a niobium RF resonator. At a reduced temperature of � 1.5-1.6 K, an ¢ £ enhanced was measured, ranging fro