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Radio frequency Focused Interdigital (RFI) Linac

RFI Linac

The RFI linac structure was invented by Dr. Don Swenson and has been under development since 2003.  It was pursued initially as a linac structure for heavy ion acceleration, where the size advantages of the interdigital format and the focusing advantages of the rf electric focusing would be a big advantage.  As it turned out, the outstanding efficiency of the resulting RFI structure makes it also attractive for proton acceleration.

The Rf-Focused Interdigital (RFI) linac structure represents an effective combination of the interdigital (Wideröe) linac structure and the rf electric quadrupole focusing used in the Radio Frequency Quadrupole (RFQ) and Rf-Focused Drift tube (RFD) linac structures.  The interdigital structure comprises a cylindrical cavity with a series of drift tubes along the axis supported from alternate sides of the cavity.  When excited in the IH electromagnetic cavity mode, the drift tubes alternate in polarity along the axis giving rise to electric fields between the drift tubes that alternate in direction from gap to gap.  Every other gap is suitable for acceleration of charged particles, and by the time the particles reach the next gap, the direction of all the fields have reversed making those gaps also suitable for acceleration.

The rf focusing is introduced into the RFI linac structure by configuring the drift tubes as two independent pieces operating at different electrical potentials as determined by the rf fields of the linac structure.  Each piece supports two fingers pointed inwards towards the opposite end of the drift tube forming a four-finger geometry, which produces an rf quadrupole field along the axis of the linac for focusing the beam.

When the accelerated particles enter the drift tubes, the electric fields are near their maximum.  When the accelerated particles are two thirds of the way through the drift tube, the electric fields are zero and changing sign.  As a result, the focusing action must be pushed upstream to lie as close to the leading edge of the drift tube as possible, leaving the latter portion of the drift tube solely as a drift action (no focusing, no acceleration).  Hence, the drift tubes of the RFI linac structure are asymmetrical, consisting of a minor part and a major part.

The RFI linac structure is four times more efficient and three times smaller in diameter than the conventional Drift Tube Linac (DTL) structure in the energy range from 0.75 to 12 MeV.  It is six times more efficient than the RFQ linac structure in the 0.75 to 6 MeV range.  This high efficiency reduces the rf power dissipation in the rf structures and the problems associated with cooling them, thereby promoting the prospect for cw operation, which in turn, allows large increases in the average beam currents.

The outstanding efficiency of the RFI linac structure is a product of its two principal features:   1) the interdigital configuration, which has demonstrated high efficiency in some heavy ion linac configurations, and 2) the relatively small rf quadrupole focusing lenses incorporated into the relatively small drift tubes.  Without the small size of the latter, larger drift tubes would add additional capacitance to the interdigital resonator circuit and degrade its efficiency. 

This linac structure promises to have significant size, efficiency, performance, and cost advantages over existing linac structures for the acceleration of low energy ion beams of all masses (light to heavy).

The RFI was designed to handle high duty factors: all the way up to 100% (CW) and relatively high currents (up to 30 mA). These capabilities, along with the possibility of neutron production when paired with Linac Systems' Lithium Target, make the RFI well suited to a large number of applications.

RFI Specifications

  • Maximum beam energy: 12 MeV
  • Maximum beam current: 30 mA
  • Maximum duty factor: 100%
  • Accelerated particles: any charged ions

For more RFI technical background, click here.

For a downloadable RFI Tutorial, please click here.

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