Digital Low Level RF Control Techniques and Procedures Towards the International Linear Collider

Abstract

In this work digital Low Level Radio Frequency (LLRF) control techniques and procedures developed and successfully tested in the scope of the International Linear Collider (ILC) are described.One central requirement for the operation of ILC is the high gradient near (5% below) quench limit operation during the whole flattop of multiple cavities driven by a single klystron. In order to achieve such an operation, for every cavity the driving power (Pk) and the loaded quality factor (QL) have to be controlled individually. Thus this operation mode is called PkQL operation. At the superconducting RF test facility (STF) at KEK in the scope of the Quantum Beam (QB) project two superconducting 9-cell cavities were driven by a single klystron. At this setup an automated procedure was developed and tested, resulting in the world's first actual and fully successful PkQL operation. A stable one hour long-time run with a beam of in average 6.4 mA current was achieved. The cavity gradients were 16 and 24 MV/m and with this 5% below the virtual quench limits of 16.8 and 25.2 MV/m. The achieved vector sum gradient and phase stabilities were Delta A/A = 0.009% and Delta phi = 0.009 deg., respectively. Furthermore in a simulation a successful automated PkQL setting procedure and PkQL operation for 39 cavities fulfilling all ILC requirements were demonstrated.Due to the PkQL operation a further requirement in scope of ILC is to operate the cavities at high loaded quality factors. In order to demonstrate the feasibility of such an operation, the loaded quality factors of both superconducting 9-cell cavities at KEK STF in the scope of the QB project were set to QL,cav1 = QL,cav2 = 2*10^7. Under this condition a stable one hour long-time operation with a beam of in average 6.1 mA current was conducted. Both cavity gradients were 20 MV/m. The achieved vector sum gradient and phase stabilities were Delta A/A = 0.011% and Delta phi = 0.015 deg., respectively.At ILC it is planned to operate the klystrons 7% in power below their full saturation. Due to the saturation behavior in amplitude the control gain converts to 0 at this region, which inhibits effective control. In order to keep the control gain constant up to the point of saturation and with this allowing effective control up to this point, predistortion-type FPGA-based klystron linearization algorithms were decided to be used. Four different kinds of algorithms have been developed, successfully tested, and compared. The best algorithm is based on lookup tables combined with a linear interpolation. In a simulation it was demonstrated that an extension of this algorithm with an adaptive grid spacing further improves the linearization performance.For offline tests of the klystron linearization algorithms an FPGA-based klystron and cavity simulator was developed, implemented, and tested