First High Power Test Results for 2.1 GHz Superconducting Photonic Band Gap Accelerator Cavities

We report the results of the recent high power testing of superconducting radio frequency photonic band gap (PBG) accelerator cells. Tests of the two single-cell 2.1 GHz cavities were performed at both 4 and 2 K. An accelerating gradient of $15\mathrm{MV}/\mathrm{m}$ and an unloaded quality factor $Q_0$ of $4\times {10}^9$ were achieved. It has been long realized that PBG structures have great potential in reducing long-range wakefields in accelerators. A PBG structure confines the fundamental ${\mathrm{TM}}_{01}$-like accelerating mode, but does not support higher order modes. Employing PBG cavities to filter out higher order modes in superconducting particle accelerators will allow suppression of dangerous beam instabilities caused by wakefields and thus operation at higher frequencies and significantly higher beam luminosities. This may lead towards a completely new generation of colliders for high energy physics and energy recovery linacs for the free-electron lasers.

Figure 1

A conceptual drawing of an SRF accelerator section incorporating a PBG cell that has fundamental mode and higher order mode couplers with elliptical higher-gradient accelerating cells.

Figure 2

(a) Photograph of the components for the 2.1 GHz PBG cell fit prior to the electron beam welding. (b) Photograph of the final fabricated 2.1 GHz SRF PBG cell.

Figure 3

Unloaded Q ($Q_0$) as a function of accelerating gradient $E_{\mathrm{acc}}$ for the 2.1 GHz SRF PBG cavities: (a) Cavity 1 tested on 03/30/12; and (b) Cavity 2 tested on 4/27/12.