THE STUDY ON THE DESIGN OF AN INTENSE-BEAM RFQ WITH STABILITY

THE COUPLING GAP Detailed studies and discussions on the determination of coupling gap and designing of the dipole mode stabilizer rods in an RFQ with intense beams have been done. By making use of the existent cold model of one section RFQ, measuring of the frequencies of the eigen modes, tuning and measuring of the RF field distributions and experiments related to the dipole mode stabilizer rods are carried out. By making comparison between the results from the experiments and from the computer simulations, the reliability and creditability of the code used in designing the RFQ cavity is proved. In the same time, a beneficial physical discussion on the design of RFQ with stability is given. The cut-off frequency of RFQ cavity is chosen as 351MHz. The balance between the cut-off frequency and the operating one will compensate by the tuners. The inter-vane voltage is kept constant along the structure in our RFQ design. So, as concerns the operating mode (TE210) mode, the coupling gap does not affect the operating mode frequency and its RF field distribution since no capacitance exists in the gap for this mode. But it is not the case for other modes, the frequency difference between the operating mode and the neighboring quadrupole modes at its left and right side can be chosen to be equal by adjusting the gap width. For our two-segments RFQ, the left and right side neighboring modes are π mode of TE210 (π TE210) and zero mode of TE211 (TE211), respectively. Simulations show that, when the gap width mm g 8 . 1 = , the frequency interval of π TE210 , TE210 and the interval of TE210, TE211 are nearly the same. Table 1 shows the simulation results. INTRODUCTION The four-vane type RFQ used for our accelerator driven sub-critical nuclear power system (ADS) accelerates the proton beams with the initial energy of 75KeV and pulsed beam currents of 50mA (the average beam currents are 3mA) from the ion source to the final energy of 3.5MeV. The operating frequency of RFQ is 352.2MHz, and the length of RFQ is about 4.75m, which is about 5.57 times long as the RF wavelength. For such a long accelerating structure, small perturbations would distort the field distribution intolerably [1]. To overcome the disadvantage brought by the long length of RFQ, resonantly coupled structure is adopted [2,3]. As concerns our case, the RFQ consists of two resonantly coupled segments, and each segment consists of two sections connected by flange. The resonant coupling is implemented by separating the two segments by coupling plate. At the center of the coupling plate, an opened hole allows the vane tips of the neighbor segments to nearly touch. The capacitance between the vane tips of one segment and the other one provides the RF coupling between the two segments. The gap between the vane tips at the coupling joint is carefully chosen to make the frequency difference between the operating mode (TE210) and its left and right side neighboring quadruple modes (TE211) equally. To minimize the effect of the gap on the beam, its position is chosen to correspond to a zero crossing RF field when the bunch passes the gap. On the basis of perturbation theory [4], the accelerator structure stability is then best guaranteed. Table 1: The frequency and Q value of the first five eigen modes in the case without rods Quad. Modes Freq. (MHz) Q Dipole Modes Freq. (MHz) Q π TE210 349.01 8013 π TE110 343.39 8787 TE210 352.11 9362 TE110 344.49 9019 TE211 355.25 7790 TE111 350.94 7169 π TE211 357.78 7588 π TE111 353.46 7281 π TE212 371.30 5376 π TE112 367.43 5260 The frequency of TE210 shown in the table is less than the objective operating mode frequency 352.2MHz because of the insufficient simulation accuracy, but its effect on the frequency difference between different eigen modes is omissible. As shown in table 1, the frequency interval of π TE210 and TE210 is about 3.102MHz, and the interval of TE210 and TE211 is about 3.135MHz, which are almost equal. However, the neighboring dipole modes of the operating mode are very close to the operating mode, the frequency intervals between TE210 and TE111, π TE111 are only 1.176MHz, 1.343MHz, respectively. In addition, the Q value of the dipole modes is comparable to that of the operating mode. It is why the dipole mode stabilizer rods are used. However, the effect from the neighboring dipole modes (TE11n) of the operating mode is also an important factor for the stability of RFQ. To avoid a possible detrimental effect brought by the nearby dipole modes, 4 dipole mode stabilizer rods [3] are mounted on the end plates and two sides of the coupling plate, respectively. In this paper, we show the determination of the coupling gap and the dipole mode stabilizer rods in our RFQ designing process. In figure 1, the normalized RF field distribution of TE210 along the beam axis is given. Except the sharp down-drop at both entrance and exit of RFQ (the gaps at the ends of RFQ are about 1cm), the filed distribution flatness is about 1.2%. The flatness is defined as max min max / ) ( E E E − . Because of the comparable big simulation grid size, the sharp down-drop of field at the coupling gap is invisible. Proceedings of APAC 2004, Gyeongju, Korea