Design of an 80-MHz RFQ linac for heavy ions

Abstract

At the Tokyo Institute of Technology (TIT) a fourvane RFQ is to be applied for inertial confinement fusion research [l]. The RFQ (TIT RFQ) is designed for acceleration of paIticles with charge to mass ratio (q/A) of 1/16 from 5keV/amu to 21 3keV/amu. The planned maximum injection beam current is lOmA for l6O+. Beam dynamics was calculated using a PIC (Particle-In-Cell) code which can take influence of the multipole components in the intervane potential into account. For input beam current of 1 OmA transmission of 60% was obtained.A half-scaled cold model was fabricated to investigate fundamental rf characteristics. In the cold model experiment, the difference in electric field strength between each quadrant was minimized to fi% by using side tuners and flat field distribution along the beam axis was achieved by adjusting end tuners.In a previous paper[l] a design of the TIT RFQ with the vane-tip curvature radius of 0.75, was presented. The computer code PARMTEQ was used to simulate the beam dynamics in the RFQ and the computer code GENRFQ was used to generate the vane parameters for PARMTEQ calculation. For this old design the beam transmission was expected to be 72% for the injection current of 10mA.In the meantime, one of the authors of this paper developed a new simulation code "QLASSI (Quadrupole Linear Acceleration Simulator with Space and Image charge effect)"[2] which can simulate the beam dynamics including influence of the multipole components in the intervane potential. This code was applied to calculation of the beam dynamics for the old design. Since the result of this calculation showed very poor transmission efficiency of 3496, the TIT RFQ had to be redesigned.In this paper we describe the modifications of vanetip design as well as the cavity geometry, which are necessary to improve the beam dynamical performance. The beam transmission pehormance for the new design is presented. Recent results on a half-scaled model including development of tuning device are also reported. NEW SIMULATION CODE QLASSIIn computer code QLASSI, the equation of motion in the RFQ is expressed as where U$q, U,, and Vi, are the external RFQ potential, the space charge potential and the image charge potential, respectively. In the calculation, eq.(l) is numerically integrated for each particle using fourth-order RungeKutta method. Harmonics up to the dodecapole moment are taken into account in Ugq. U,, is given by the sum of monopole Coulomb potential from all other particles. Uic is determined by solving a 3D Dirichlet's boundary problem defined by the beam space charge and the metallic electrode surface. Figure 1 shows axial transmission profile calculated using QLASSI for the old design. For the injection beam current of lOmA the transmission is only 3496, which is less than half of the one calculated using PARMTEQ. NEW DESIGN OF THE TIT RFQThe TIT RFQ was redesigned since the predicted transmission was limited to 34%. In the new design the curvature radius of vane-tip was increased from 0.75, to ro in order to suppress th...