Variation of injected neutral beam power at constant particle energy by changing the beam target aperture of the textor neutral beam injectors

The neutral beam injectors of the tokamak experiment TEXTOR produce neutral particle beams in the megawatt range at 55 keV and up to a 10-s pulse leng th of the light atoms hydrogen, deuterium, He-3, and He-4 for heating the f usion plasma of TEXTOR. The two injectors are equipped with one 5-MW ion so urce (plug-in neutral injector) each. The injected power of similar to 1.5 MW of each injector can be varied from 0 to 100% by opening the main beam t arget vertical aperture in steps of similar to 2 cm to the full opening of 50 cm. The symmetric truncation of the neutral beam profile at a target pos ition 4.5 m from the ion source leads to no major deformation of the profil e downstream at the entrance into the torus plasma at a 6-m distance from t he ion source. Whereas usually the particle energy, i.e., acceleration volt age, and beam current or, alternatively, the gas pressure in the neutralize r at fixed energy must be varied to change the injected power, these beam p arameters can be kept constant with the reported method to study the effect of different injected neutral beam powers on the fusion plasma. The transm itted power to the torus is detected by the calorimetrically measured remai ning power on the beam target. The resulting transmitted neutral beam power as a function of the target aperture is in good agreement with the expecte d integral of the thus-truncated Gaussian-like beam profile, i.e., the erro r function. The scaling of the resulting injected neutral beam power, beam profiles, vertical full-width-at-half-maximum, and central power density wi th variation of the beam target aperture are in good agreement with the bea mline simulation code PADET.