The IFMIF-EVEDA RFQ: Beam Dynamics Design

The IFMIF-EVEDA (Engineering Validation and Engineering Design Activities) project foresees the construction of a high intensity deuteron accelerator up to 9 MeV, with the characteristics required for the actual IFMIF facility. The linac will be installed in Rokkasho, and INFN is in charge of the construction of a 5 MeV, 125 mA, deuteron RFQ operating at 175 MHz. In this article the beam dynamics design of this challenging RFQ is described, namely the design, the main outcomes in terms of beam particles physics, and finally the study of mechanical and rf field error tolerances. The RFQ design method has been aimed to the optimization of the voltage and R0 law along the RFQ, the accurate tuning of the maximum surface field and the enlargement of the acceptance in the final part of the structure. As a result this RFQ is characterized by a length shorter than in all previous design, very low losses (especially at higher energy) and small RF power dissipation[1][2][3]. RFQ PARAMETERS AND DESIGN The IFMIF EVEDA RFQ specification and the main design parameters are listed in Table 1. Table 1: IFMIF RFQ Parameters Particles D+ Frequency 175 MHz Input Current 130 mA RMS Input tr. emittance 0.25 N.mmmrad Input/Output Energy 0.1/5 MeV Max Surface Field (1.82 Kp) 25.6 MV/m Length L 9.78 m (5.7 λ) Voltage Min/Max 79/132 kV Total Power with margins 1.6 MW Mean aperture R0(min/max) 4.1 / 7.1 mm Trans. (WaterBag÷Gauss)* 99.1÷95.7 % Power Loss(WaterBag÷Gauss)* 253÷1007 Watts RMS Output Long. Emittance* 0.197 MeVdeg RMS Output Tr. Emittance* 0.26 N.mmmrad *Average of 10 Toutatis runs with 10 particles. The design of such high current RFQ is very challenging due to the necessity to limit deuteron losses, keeping in particular extremely low the losses in the high energy part. The aim is to minimize the neutron production and the consequent activation of the RFQ structure. Due to the high space charge effect at low energy. The value of input energy has been chosen of 0.1 MeV to maintain the value of maximum transverse current up to 200 mA until the end of Gentle Buncher;, this large margin guarantees small losses along the buncher formation process. All the RFQ simulations here reported has been made by using the CEA code "Toutatis" [4]. Transverse Parameters Characterization The IFMIF EVEDA RFQ beam dynamics study was aimed at minimizing beam losses at high energy, reducing the RFQ length and power consumption. The main parameters are plotted in fig. 1, the focusing force "B" at the beginning of RFQ is about 7, i.e. a very high value to compensate the high space charge force and keep the beam in a linear force fields. It is not possible to ramp the value of "B" in the shaper because we get a larger beam size and due to that same losses in the gentle buncher process. The normalized transverse acceptance of the RFQ in smooth approximation is defined by: 3 2 0 2 2 1 / 1 2 4 4 T a B B Acc σ λ π π ⎛ ⎞ ⎛ ⎞ = − + ⎜ ⎟ ⎜ ⎟ ⎝ ⎠ ⎝ ⎠ with λ the wavelength a small aperture,σT0 phase advance at zero current and "B" the focusing factor expressed by: 2 2 2 0 2 2 2 0 8 T RF qV B B and mc R λ σ π = = Δ + The variation of acceptance and "B" is therefore strictly connect to voltage and average aperture R0 along the RFQ. For that reason a closed-form and continuous up to the 2 derivative voltage law V(z) and R0(z) was used. ( ) ( ) ( ) ( ) ( ) ( ) 1 2 1 12 12 12