• M. Aicheler
  CERN, Geneva

The accelerating structures of CLIC will be submitted to 2 x 10¹⁰ thermal-mechanical fatigue cycles, arising from Radio Frequency (RF) induced eddy currents, causing local superficial cyclic heating. In order to assess the effects of superficial fatigue, high temperature annealed OFE Copper samples were thermally fatigued with the help of pulsed laser irradiation. They underwent postmortem Electron Backscattered Diffraction (EBSD) measurements andμhardness observations. Previous work has confirmed that surface roughening depends on the orientation of near-surface grains*,**. It is clearly observed that, through thermal cycling, the increase of hardness of a crystallographic direction is related to the amount of surface roughening induced by fatigue. Near-surface grains, oriented [1 0 0] with respect to the surface, exhibiting very low surface roughening, show limited hardening whereas grains oriented in [1 1 0], exhibiting severe surface roughening, show the most severe hardening. Consistently, surface roughening and hardening measured on [1 1 1] direction lie between the values measured for the other directions mentioned.

* Aicheler M et al.; 2010; Submitted to Int. Journal of Fatigue
** Aicheler M; 2009, Journal of Physics: Conference Series, Proceedings of ICSMA15

• L. Laurent, V.A. Dolgashev, C.D. Nantista, S.G. Tantawi
  SLAC, Menlo Park, California
• M. Aicheler, S.T. Heikkinen, W. Wuensch
  CERN, Geneva
• Y. Higashi
  KEK, Ibaraki

Cavity pulsed heating experiments have been conducted at SLAC National Accelerator Laboratory in collaboration with CERN and KEK. These experiments were designed to gain a better understanding on the impact of high power pulsed magnetic fields on copper and copper alloys. The cavity is a one port hemispherical cavity that operates in the TE013-like mode at 11.424 GHz. The test samples are mounted onto the endcap of the cavity. By using the TE013 mode, pulsed heating information can be analyzed that is based only on the impact of the peak magnetic field which is much bigger in value on the test sample than on any other place in the cavity. This work has shown that pulsed heating surface damage on copper and copper alloys is dependent on processing time, pulsed heating temperature, material hardness, and crystallographic orientation and that initial stresses occur along grain boundaries which can be followed by pitting or by transgranular microfractures that propagate and terminate on grain boundaries. The level of pulsed heating surface damage was found to be less on the smaller grain samples. This is likely due to grain boundaries limiting the propagation of fatigue cracks.