Focussing Magnets for Drift Tube Linac

Authors:
Vikas Teotia , Sanjay Malhotra , Kumud Singh , U.Mahapatra , S.Bhattacharya , G.P.Srivastava2 Shweta Roy , Piyush Jain , P. Singh and S. Kailas

Abstract:
A linear accelerator comprising of Radio frequency quadrupole (RFQ) and drift tube linac (DTL) is being developed at BARC. The Alvarez type CW DTL accelerates protons from energy of 3 MeV to 20 MeV. The drift tube linac is excited in TM010 mode, wherein the particles are accelerated by longitudinal electric fields at the gap crossings between drift tubes. The particles experience transverse defocusing forces at the gap crossings. The transverse defocusing correction is done by housing magnetic quadrupole focussing lenses inside the drift tubes. In a DTL at low energy, the strong spacecharge defocusing requires high integrated quadrupole gradients, in constrained axial lengths of the drift tubes and in limited transverse volumes. The technical difficulties and costs of developing a high-gradient, high duty cycle electromagnet fitting in the small volume initial drift tubes, led to the choice of Permanent Magnet Quadrupoles (PMQ). The PMQs are placed inside the hermetically sealed drift tubes and provide a constant magnetic field gradient in the beam aperture. The drift tubes are mounted concentrically inside the resonating DTL tank and are connected to the tank body with stems. Rare earth high energy permanent magnets have been used to achieve the required field gradients in the beam aperture. The drift tube body is subjected to RF heating and hence the sealed drift tubes are required to be cooled from inside. The temperature rise of the drift tube assemblies has to be limited to avoid degradation of permanent magnets and also to limit the thermal expansion of the tubes. This paper discusses various aspects of magnetic design, selection of magnetic materials and the engineering development as well as describes the prototypes developed in our laboratory. This paper further presents analysis of a novel approach of spilt-pole type focusing magnets and brings out its advantages compared to earlier designs.

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