History of linacs - page 2

      In 1946 Dr. L. Alvarez and his coworkers at the Lawrence Berkeley Radiation Laboratory developed a linear accelerator based on a somewhat different principle. In their configuration, shown schematically in figure 2, a similar series of cylindrical metal electrodes were used, but mounted along the longitudinal axis of an evacuated metallic cylinder. Radio frequency power from a 200 MHz radar source was introduced into this metallic cavity. The Alvarez structure, now commonly known as the drift tube linac (or DTL), has a standing wave electric field in the whole cavity, that is in the same direction in each gap between the cylinders at any instant in time. The field is accelerating (forward) for one half of the rf cycle and deaccelerating (backward) for the next half cycle. As with the Wideröe structure, the individual metallic electrodes ("drift tubes") shield the ions from the electric fields when it is in the wrong direction.



      In the late 1940's, after the second World War, a program was initiated by E.O. Lawrence at the University of California Radiation Laboratory (now known as the Lawrence Berkeley National Laboratory) with the US Atomic Energy Commission to investigate electronuclear breeding of Pu²³⁹, U²³² and tritium by bombarding depleted uranium with accelerator-produced neutrons. A series of high power rf linacs for protons and deuterons was built and tested starting in 1950 at the site which is now the Lawrence Livermore National Laboratory. When the first linac was built, the only other proton linac which had been operated was the 32 MeV linac built by L. Alvarez, although a 68 MeV p⁺ linac was under construction at the University of Minnesota. The large linacs in the Material Testing Accelerator program (12-48 MHz) were disassembled in the mid 1950's, but the scientists who worked on these systems went on to build many successful proton linacs at research facilities such as Brookhaven National Laboratory, Argonne National Laboratory, Fermi National Accelerator Laboratory and Los Alamos National Laboratory. Most of these modern linacs, which are used for physics research, were based on the original 200 MHz design of L. Alvarez.

      Typical linear gradients achieved in these Alvarez linac structures are 1-2.5 MV/m, with the gradients in the gaps ranging from 6-10 MV/m. Using the development of strong focusing magnets (quadrupole magnets) that occurred in 1938, these structures also employed quadrupole magnets within the cylindrical drift tubes. These magnets are needed to keep the ion beam focused, because it becomes slightly defocused by the radial electric field components present in the accelerating gap. The need to have magnets in the drift tubes, and several other problems such as the defocusing in the gap, require that the relative ion velocity, β ≡ ν /c, must be greater than 0.04 for effective use of such a "drift tube linac". This is particularly true for high beam currents of protons, which will diverge rapidly due to the repulsive charge of the other ions in the beam.