Commissioning of an 81.25 MHz Four-vane Radio Frequency Quadrupole Accelerator with a Ramped Field Profile

Abstract

The four-vane RFQ for CW and low frequency operation has been developed to accelerate from proton to uranium beams in considering the lower specific thermal load and the larger aperture for the lower beam loss. The brazed four-vane RFQ for the low frequency (< 200 MHz) was not ever existed due to the huge transverse dimension and difficulties of fabricating the cavity under the precise requirement. Another unique feature of the RFQ in this study is that the RFQ cavity was designed to have a ramped field profile to reduce the cavity length for the cost reduction and the simple design. Moreover, there is no preceding experience to tune a short and low-frequency RFQ with a ramped field profile until now. A short RFQ with a low operational frequency is insensitive for the perturbation. In other words, it is very difficult to change the field profile with only slug tuners. Therefore, it is very important to fabricate the cavity in precise condition. The inter-vane gap has been controlled within ± 50㎛ during the machining and the brazing process. Nevertheless, the machining and alignment error should be compensated by the tuning procedure. Especially, the low energy end of the cavity is harsh to tune due to the relatively low field strength. A basic idea to resolve the problem of insensitivity for the frequency perturbation in the low energy end region is to modify the endplate to tune the end region field with slug tuners simultaneously. The frequency perturbation can be controlled by changing the geometry of the endplate which contribute to inductance and capacitance components. It was proposed to overcome the limitation of the slug tuner without the modification of the RFQ cavity. Consequently, the ramped inter-vane voltage profile has been tuned as the error is less than ±2 % compared to the designed profile and the difference of the dipole field is less than ±5 % compared to the quadrupole field. The mode stability, which is separating the quadrupole mode frequency and the dipole mode frequency, was achieved without the dipole mode stabilizer rods. The frequency gap between the operational frequency and the nearest dipole mode frequency is about 1.4 MHz over than the generally recommended 1 MHz. The RF system consisted of two coaxial couplers operated by an 80 kW SSPA (Solid State Power Amplifier) respectively. For the initial beam acceleration, a RF coupler has been installed which coupling coefficient was 0.54. After the RFQ cavity is pumped down below 1X10-7 torr, the RF conditioning was performed for the RFQ cavity and the RF power coupler in the pulse mode operation. A pulsed RF power of 20 kW could be stably transmitted with 100 ㎲ duration and 1 Hz repetition. To confirm design, fabrication and field tuning of the RFQ, the initial beam acceleration experiment has been performed with the O+7 beam. With transmitted RF power of 10.4 kW to the cavity, the accelerated beam current of 3 μA was obtained at the Faraday cup located behind the RFQ cavity. The accelerated beam energy was also measured as the designed 500 keV/u by the RBS (Rutherford Back Scattering) method. On the basis of experimental results, it is confirmed that the four-vane type RFQ with a ramped field profile was successfully developed by using a brazing technology and operational after commissioning in a low frequency.