A source of epithermal neutrons based on a vacuum-insulated tandem accelerator and a lithium target is developed for the technique of boron neutron capture therapy. A 2 MeV stationary proton beam with a current of up to 5 mA was obtained in the accelerator. With a view of increasing the current, the transport of a beam of negative hydrogen ions from the ion source to the accelerator is studied using a wire scanner OWS-30 (D-Pace, Canada) and a movable diaphragm. Dependences of the ion beam profile and the current on the residual gas pressure are measured and the influence of the space charge is detected. We measured the phase portrait of the beam in the radial and azimuth directions and discovered the effect produced by the aberrations of the focusing magnetic lens. We also gaged the value of the normalized beam emittance. The change in beam focusing mode and the introduction of optical diagnostics of the beam position in the diaphragm of the first accelerating electrode made it possible to significantly improve the stability of the accelerator operation at high current of the proton beam, up to 6.7 mA.