First High Gradient Tests Towards TRAVELING WAVE SUPERCONDUCTING CAVITY
The principal goal of this project is the development and experimental demonstration of a Superconducting Traveling Wave Accelerating (STWA) structure. The proposed STWA structure has crucial advantages compared to standing wave (SW) designs like the recently developed re-entrant cavity (that in turn has significant advantages over the original 9-cell TESLA cavity). The primary STWA advantage is an increased accelerating gradient, up to a factor 1.24 while maintaining the same cavity magnetic and electrical surface field ratios Epeak/Eacc and Bpeak/Eacc as the re-entrant SW structure.
Furthermore, the proposed SC TW acceleration method will provide accelerating parameters that allow much longer accelerating structures to be built, also critical for the effective gradient enhancement. The length of the SW accelerating structure is limited by the high sensitivity of the field flatness along the structure to dimension errors. The proposed TW structure does not have this restriction. If manufacturing and surface processing technology allow, the STWA structure in principle can be produced in 10 m long sections, limited only by the length of the cryomodule. This means that the effective accelerating gradient if a TW structure is employed can be increased by a further 22%, giving an overall 46% gain over the SW ILC structure. The proposed modification will result in a total accelerating structure length reduction by a factor of 1.46.
In general, this proposed scheme based on a high gradient STWA cavity will allow higher accelerating gradients that are the main goal of the superconducting accelerating community.
In this project, a Superconducting Traveling Wave Accelerating structure will be manufactured and demonstrated experimentally. We plan to optimize the proposed STWA cavity geometry to achieve the maximum available accelerating gradient for a given cavity surface field strength. The goal of the project is to design and manufacture a prototype single-cell superconducting cavity with a feedback waveguide required for operation of the traveling wave device.
The first step in the development of a traveling wave superconducting cavity has been made: a single-cell prototype cavity with the return waveguide (see picture above) has been fabricated and tested. The multi-cell traveling wave cavity will be fabricated using the same technology.
|Quality factor vs. peak surface magnetic and electric fields for Traveling Wave and Standing Wave cavities fabricated by AES.|
The goals of the tests were the following:
- To demonstrate that the waveguide section of the TW cavity design does not limit the acceleration gradient
- To demonstrate that multipactoring in the waveguide is not an issue.
Test results (measurements made at 2K):
- Optical investigations of the surface were made for two cavities fabricated by AES. The welding of the cavity N1 (the first cavity made to test the technology) was found to contain a significant number of pits. The welding of the cavity N2 did appear much better.
- The tests on the cavity N1 after BCP showed that quench occurred at a field of ~60 mT as expected, taking into account the quality of the welding.
- The test of the cavity N2 after BCP showed a quench at a surface field of ~93-95 mT, corresponding to an acceleration gradient of ~31 MeV/m for the TW cavity. The SC cavities made by AES previously showed higher surface fields (~120 mT) but the high field Q-slope appears earlier, at ~80 mT. However, the TW cavity N2 sustains higher surface electric fields than other SW AES cavities. It is not clear where the quench takes place (in the waveguide or in the cavity) and there is no consensus about it at the moment.
- No strong field emission was observed.
- Multipactoring in the waveguide does not limit the gradient.
- The waveguide mode has a quench at ~120 mT that is significantly higher than those for the operating mode. The multipactoring regime was easily passed.
- High field Q slope effects have not been observed up to 95 mT and 58 MV/m surface fields in the N2 tests.
- Q-factors were found rather low with these tests, an effect that can be easily explained by the use of short beam pipes. New NbTi flanges have been ordered, and the new tests for Q measurements have been scheduled for end of April and May 2010.
- The presence of the additional waveguide that is the main difference from the SW design does not limit the acceleration gradient;
- The acceleration gradient in the TW cavity is about the same as that achieved in the standard SW cavities made by AES.
- Low Q-factors are caused by the short beam pipes.
Further tests are scheduled in April 2010 that include:
- tests of the cavity N2 with the NbTi flanges;
- additional BCP at lower temperature;
- EP as an option (it is not clear at the moment because there is still no evidence of field emission at the gradients achieved so far).