TUNING OF THE RADIO FREQUENCY QUADRUPOLE FOR THE FRONT END TEST STAND AT RUTHERFORD APPLETON LABORATORY

The Front End Test Stand (FETS) is a project that aims to accelerate a 60 mA beam of H- ions to 3 MeV at 50Hz with an RF frequency of 324 MHz. It is designed to demonstrate a high beam current with fast chopping. It is also being considered for the ISIS II upgrade. The FETS RFQ is the main accelerating structure, and the 3 MeV final energy is reached at the end of this. It is a four-vane type and consists of four sections, one metre long each. There are sixty-four tuner ports, but only sixty-two are available for tuning. Four of them are for automatic tuners that are applied when the FETS RFQ is in operation. The other ports, fifty-eight, are allocated for the slug tuners, which must be adjusted manually.

The fields along the FETS RFQ have to be uniform (i.e. flat) at 324 MHz as it is a sensitive accelerating structure. The tuning system is used to do this and ensure that the RFQ is operating at the correct frequency. In order to correct the fields from the imperfectly machined RFQ, a bead pull measurement is introduced. Perturbation and tuning techniques have been studied to understand the ideas behind this technique. The inserted length from the tuner affects the overall frequency of the RFQ and local field distribution. If the tuner is inserted into the RFQ cavity leading to the decrease in cavity volume, the frequency will increase. On the contrary, to reduce the RFQ frequency, the tuner has to move out from the RFQ cavity. By using the bead pull measurement data, the new tuners’ lengths can be calculated. After adjusting the tuners, measurements are taken to confirm the new fields. If the fields are not in a sufficiently uniform pattern, the tuning process is repeated.

The system was tested using a one-metre long FETS RFQ bead pull measurement away from the FETS area using section two of the RFQ. This gave experience with the bead pull technique and allowed the measurements to be studied. After that, the system was re-designed for the four-metre FETS RFQ and constructed in the limited space of the FETS area. One section and then all four sections of the FETS RFQ were installed for the first four-metre bead pull measurements. These have been taken to study the fields in the FETS RFQ and the effects of the tuners on the FETS RFQ cavity.

With one section of the RFQ, the measurements were taken for data preparation such as frequency resolution, tuner adjustment effects and direction of the bead pull. With the complete FETS RFQ, because of limitations in time, only the forward direction was chosen for the measurement. The sag effect is also important, especially with the long length pulley setup. The corrected measurement data might not match the programmed position because of the sag from the bead and string weights. It was demonstrated that this sag could be corrected automatically during the measurements. Possible errors in the tuning measurement were studied and the largest determined to be in the measurement and adjustment of the manual tuners.

Using the FETS RFQ tuning algorithm, the field flatness was reduced from a spread of 7.42% to 0.8%, both with and without the RFQ cooling system in operation, at 324 MHz resonant frequency. With acceptable operation fields, the FETS RFQ is now ready for high power commissioning and beam tests.