The assembly consists of a copper disc with slightly convex faces and a toroidal rim, with an axial hole for the beam. Two copper pipes extend radially, at right angles to each other. At the distal end of each is a copper bellows. From the bellows of the thicker pipe extends an internal coaxial steel pipe with an end fitting, from which emerge two insulated wires in a common braided sheath. These no doubt supplied current to a focussing quadrupole magnet embedded in the drift tube (disc).
The bellows of the thinner copper pipe has a black rubber O-ring and internal threads at the end; otherwise it is empty.
Basic Principles of Linac Drift Tubes
Object 1997.0392.01 is a single drift tube, one of a series of electrodes of various lengths, each with an axial cylindrical passage for the particle beam, that are positioned inside a vacuum chamber along the beamline of a linear particle accelerator (linac). This vacuum chamber functions as a resonant cavity for the standing-wave electromagnetic field in which the electric field is parallel to the cavity axis in order to accelerate the beam of electrically-charged particles. Protons or ions are accelerated through the cavity in the gaps between the ends of successive drift tubes by an electric field that alternates at radiofrequency, but inside the tubes the particles are shielded from the electric field when it reverses direction, so that they “drift” through each tube with constant velocity. In electron linacs, the corresponding electrode elements are copper cavities into which radiofrequency waves are fed to accelerate the particle bunches. (See object EM.N-09538, SLAC cavity cutaway.)
The length of the drift tubes increases progressively with the distance from the linac particle source, and is determined by the frequency and power of the electric field and the mass and charge of the particle to be accelerated, so that the particle passes through each electrode in exactly one-half cycle of the accelerating voltage. To counteract dispersive radial forces also experienced by protons or ions, magnetic or electrostatic focusing elements are included in the drift tubes to ensure that the particle beam remains focused in the center of the beamline. For a discussion of “strong focusing” using electrostatic quadrupole lenses in a proton linac, see object 1978.1073.01.1, Alvarez Proton Linear Accelerator.
The Brookhaven 50 MeV linac apparently had one focusing quadrupole magnet embedded in each drift tube. (The pole pieces of the quadrupole magnet in object 1997.0392.01 are enclosed and not visible.)
The drift tubes decrease the volume available to the electric field at a region where it is strongest and at a region where the magnetic field is the weakest. Thus, the drift tubes detune the frequencies of the cells (regions between centers of successive drift tube gaps). Tuning devices are often inserted into the cells to achieve a ‘flat’ field in the cavity of the linac. For a cavity tuning device used in the Brookhaven 50 MeV linac, see object 1997.0392.02, tuning ball.
For more on the basic principles of linacs and drift tubes, see the following web-based references:
The drift tube parameters for the Brookhaven 50 Mev proton linac are included in the latter reference.