A Procedure to Set Phase and Amplitude of the Rf in the Sns Linacs Superconducting Cavities*

This paper describes a procedure to set the phase and amplitude of the RF fields in the Spallation Neutron Source (SNS) linac’s superconducting cavities. The linac uses superconducting cavities to accelerate the H ion beam from the normal conducting linac at 185 MeV to a final energy of ~1 GeV. There are two types of cavities in the linac, 33 cavities with a geometric beta of 0.61 and 48 cavities with a geometric beta of 0.81. The correct phase setting of any single superconducting cavity depends on the RF phase and amplitude of all the preceding superconducting cavities. For the beam to be properly accelerated it must arrive at each cavity with a relative phase (φs), called the synchronous phase, of about −20 degrees. That is, it must arrive early with respect to the phase at which it would gain the maximum energy by 20 degrees. This timing provides the longitudinal focusing. Beam particles arriving slightly later gain more energy and move faster relative to the synchronous beam particle. The problem is to set the phase and amplitude of each cavity in the linac so that the synchronous particle arrives at each cavity with the correct phase. The amplitude of each superconducting cavity will be adjusted as high as possible constrained only by the available RF power and the breakdown field of the cavity. 1 COMMISIONING THE SNS SUPERCONDUCTING H PROTON LINAC The superconducting linac [1] uses 6 cell cavities operating in the π mode to accelerate the H beam. At 185 MeV, the energy at the beginning of the superconducting linac, the β (v/c) of the beam is 0.55. The first section of the linac uses 6-cell cavities with a geometric β of 0.61. Thus, the beam must arrive at the first cell early and leave the last cell late with respect to design synchronous phase. The 33 (β = 0.61) cavities accelerate the beam to ~387 MeV. At this energy the beam β is 0.706 and the beam must arrive late at the first cell and leave the last cell early. At 247 MeV the beam β matches the cavity β and the coupling of the beam to the accelerating field in the cavity is the strongest. To make the most efficient use of each superconducting cavity the field amplitude will be set as high a possible. This amplitude will not be known until the accelerator is ready for operation and the accelerating field is calibrated. Using standard RF measurement techniques the field level can be calibrated to about ± 5%. This level of accuracy is not good enough to set the phase and amplitude of the RF fields in each of the superconducting cavities before the beam is turned on even if there was a method to determine the absolute phase of each cavity. Therefore, I propose to use the beam to calibrate the field amplitude and phase of each superconducting cavity. 1.1 Procedure to Commission SC Linac • Calibrate RF pickup probes of first cavity. 1 Tune the first cavity to 805 MHz by exciting cavity with klystron, switching off the RF drive, and watching the phase shift of the RF in the cavity with the RF reference. When the phase shift during ~ 100 μ seconds is less than ~ 2 degree the cavity is tuned to 805 MHz ± 56 Hz. 2 Use beam to excite cavity. Compare excitation of cavity with calculations (RF off.) With the cavity tuned as in step (1) the phase of accelerating fields in the cavity will be excited with the phase that extracts the maximum energy from the beam. (Exactly 180° from the phase of maximum acceleration.) 3 Use date from step 2 to calculate coefficients used by low level RF (LLRF) system to display the amplitude in MV/m. The cavity phase could be displayed relative to the beam phase. 4 Amplitude and phase of cavity are set. Phase of cavity excited with beam is ± 180° + φs of required phase. (Add ± 180° + φs). • Set amplitude and phase setpoints of first cavity and turn on RF. • Calibrate RF pickup probes of next cavity. • Set amplitude and phase setpoints of next cavity. Turn on RF. • Repeat previous steps until all cavities are set. • Periodically check beam energy with time-of-flight measurements. 2 PREREQUISITES FOR COMMISIONING THE LINAC Before the first superconducting cavity can be commissioned with the above procedure, the beam from the room temperature linac must be transported all the way through all the superconducting cavities to the beam dump. This will require adjusting the quadrupole doublets ___________________________________________ *Work supported by the Office of Basic Energy Science, Office of Science of the US Department of Energy, and by Oak Ridge National Laboratory. [email protected] 0-7803-7191-7/01/$10.00 ©2001 IEEE. Proceedings of the 2001 Particle Accelerator Conference, Chicago