STUDY OF A HIGH-CURRENT 176 MHZ RFQ AS A DEUTERON INJECTOR FOR THE SPES PROJECT

Since external forces scale as (q and internal forces according to / A) V ⋅ (q / A) I ⋅ , the beam dynamics is exactly the same for deuterons with a beam current of 6 mA and a inter vane voltage of 2 / of the nominal case. Moreover, to improve the deuteron transmission, there is the possibility to increase the voltage and the surface field to 1.8 Kilpatrick. 3 The SPES project[1], aimed at the construction of a RIB facility at LNL, is based on the use of a primary beam of protons, and foresees a further development based on the acceleration of deuterons and light ions. In this article we report about the preliminary study of a 176 MHz RFQ to be used as an injector for such kind of beams. The structure explored foresees a “four ladder” symmetric resonator, built in brazed copper. In particular beam dynamics, electrodynamics design and preliminary thermo-structural analysis of the cavity is presented. Simulations show that high transmission, more than 90%, required the focusing force, represented by the B focusing parameter, takes more than 40% of the overall electric field; this is important above all in the shaper and bunching sections. We know that the higher is B the lower is the accelerating force, represented by A accelerating parameter, which influences the RFQ length. INTRODUCTION In the ISOL production method chosen for the LNL facility, the number of fission reactions producing exotic species can be increased of about an order of magnitude by using deuterium ions instead of protons. Using LANL codes (PARI and PARMTEQM) we could reach a 98.9% simulated transmission for ions with mass equal to 3. Transmission reaches 99.8% for deuteron beam with the maximum voltage; this means that inside the RFQ structure we lose 12 μA of deuterons, which is an acceptable value. On the other hand, the transmission is a very important parameter for a deuteron RFQ. Indeed the D-D fusion reaction d(d,He)n, that has no threshold, happens when an accelerated particle hits a deuteron condensed on the electrode surface. As a consequence losses are dangerous at any energy. The RFQ length was contained in 7.5 m; this result was possible by optimizing the accelerating section of the RFQ. In fact in this section less focusing force is necessary to maintain stable beam. So we could reduce B parameter by increasing A one (figure 1). Our primary goal was to design a CW mode normal conductive RFQ able to accelerate more than 99% of the deuterons at the entrance of the structure in order to minimize activation problems. Besides we were interested to conceive a structure whose length does not exceed 8 m, with the aim of containing manufacturing and maintenance costs. Indeed such structure were to keep relatively low global power consumption (less than 500 kW), and a realistic local power dissipation (maximum value less than 20 W/cm).