• 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

• T. E. Plawski, C. Hovater
  JLAB, Newport News, Virginia

A digital low level radio frequency (RF) system typically incorporates either a heterodyne or direct sampling technique, followed by fast ADCs, then an FPGA, and finally a transmitting DAC. This universal platform opens up the possibilities for a variety of control algorithm implementations. The foremost concern for an RF control system is cavity field stability, and to meet the required quality of regulation, the chosen control system needs to have sufficient feedback gain. In this paper we will investigate the effectiveness of the regulation for three basic control system algorithms: I&Q (In-phase and Quadrature), Amplitude & Phase and digital SEL (Self Exciting Loop) along with the example of the Jefferson Lab 12 GeV cavity field control system.

• B. Mustapha, A. Kolomiets, P.N. Ostroumov
  ANL, Argonne

An integral part of the ongoing ATLAS efficiency and intensity upgrade is an RFQ to replace the first section of the existing injector. The proposed RFQ is 3.8 m long made of 10⁶ cells with 30 keV/u input energy and 260 keV/u output energy. The RFQ was designed using the DesRFQ code which produces a file consisting of the length, modulation and the 8 coefficients of the 8-term potential for every cell. To independently check the design we created full 3D models of the RFQ including cell modulation in both Micro-Wave Studio (MWS) and Electro-Magnetic Studio (EMS). The MWS model was used to verify the phasing and energy gain along the RFQ using particle tracking and the EMS model was used to extract the electric field cell by cell assuming the electrostatic approximation. A very good agreement was obtained between the full 3D model and the 8-term potential description in TRACK. In addition to the standard sinusoidal vane profile we studied the option of converting the cells with maximum modulation (~ 40 cells) into trapezoidal cells. The output energy was increased from 260 keV/u to ~ 300 keV/u with minimal change to beam dynamics. This option is the final RFQ design.