Research On The Beam Loading Effects And RF System Of HLS Storage Ring

The admittance method is developed to analyze the beam-cavity interaction. The RF current induced in the cavity by the beam is equivalent to admittance. The relational functions are expressed in microwave transmission-line equation, which independent variables are the RF parameters set by the ALC loop (cavity voltage) and by the tuning loop (pre-detuning angle). Compared to the former processing, thenew one is convenient to analyze and simple to calculate the RF parameters.Due to the low cavity voltage (50kV), the beam loading is very heavy during the HLS storage ring injection. The pre-detuning angle is set to 45°in order to suppress the steady beam loading and leave enough space for the injecting beam. The response of the ALC loop to the transient effect under heavy beam loading is calculated and measured. By adjusting the gain and bandwidth of ALC loop, the injecting is improved significantly. The beam lost is decreased, and the beam currents accumulate stably and rapid up to 300mA as long as the injecting rate keeps up 1.5mA. Even in a lower injecting rate, the beam currents can exceed 200mA.The model of high impedance current source is applied to simulate the output character of tetrode amplifier. The function of tetrode output power to arbitrary load is derived. Utilizing the function, in the case of the tetrode connecting with the RF cavity directly, the relation of Robinson instability to the length of transmission line is obtained. In the case of the circulator inserted, the arc phenomenon of the circulator following the high current beam lost is analyzed. A fast interlock circuit has been inserted into the RF system in order to protect the circulator.A new RFKO (RF Knock Out) system has been commissioned for partial filling operation. The single bunch is achieved, and the beam currents reach 18mA at present. Some bunch-train patterns such as 6 X 3 or non-uniform bunch train are also tested. The oscillation of single bunch and multi-bunch is measured.The experiment of RF modulation is introduced. The AM (Amplitude Modulation) with the modulated frequency closed to the longitudinal synchrotron oscillation frequency increases the beam lifetime. The experiment also finds out the intensity of the beam spectrum reduced after applying the RF modulation, which shows that the modulation is useful for suppressing the coupled-bunch instability.A new technique for beam loading compensation during injection is proposed. The idea is using a DC-couple PLL to lock the pre-detuning angle (by adjusting the RF frequency) for fast compensating the injecting beam, and the cavity tuning loop recovers the frequency slowly under the programmable control to response the beam current change. The basic PLL circuit is tested successfully.