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The paper describes the design and construction of a short experimental length of linear accelerator using a helical waveguide as the slow-wave structure to accelerate protons from 2.5 MeV to 4 MeV. Factors influencing the design of the helix structure are considered; these include the maximum voltage between turns which can be tolerated and the available power and frequency of the r.f. supply. The theory of a helix supported on a dielectric tube is given, and from the results the variations of pitch are determined to give the required acceleration. The accelerator was driven by a push-pull triode oscillator operating under 6Â¿s pulsed conditions at 300Mc/s with a peak output of 600 kW. The oscillator and r.f. components are described in detail. The beam-energy spectrum at the output of the accelerator was measured for a variety of working conditions, changes being made in beam-energy input and power and frequency of the r.f. supply. The results obtained confirm the theory and show that with certain restrictions a helix slow-wave structure of the type described provides a convenient me hod of proton acceleration.