Improvements to the experimental setup for the basic ammonothermal growth of GaN and the introduction of a silver capsule into the autoclave have yielded an ultrahigh purity (UHP) growth environment with reproducible external wall temperature profiles to within the error of the thermocouples (type K with special limits of error (SLE), +/- 2.5 degrees C). 40 basic ammonothermal growth runs were performed on hydride vapor phase epitaxy (HVPE) GaN seed crystals with different crystal orientations to optimize the growth system. Due to the UHP growth environment, transition metal impurities in the GaN crystals were reduced to less than 1 x 10(17) cm(-3) and oxygen impurity concentrations were comparable to those of the polycrystalline HVPE GaN source material (1 x 10(19) cm(-3)). Total growth rates improved to 344 +/- 30 mu m/day for c-plane growth and 46 +/- 2 mu m/day for m-plane growth. The crystal quality, as measured by the full width at half maximum (FWHM) of the omega-rocking curve using X-ray diffraction (omega-XRC), was comparable to that of the seed crystal, except for Ga-face growth on c-plane oriented seeds clue to poor nucleation and cracking in thick growth layers due to strain originating from the HVPE seed crystals. The difference in total system pressure profiles during growth runs using autoclaves with and without a silver capsule was analyzed and the pressure drop due to outward diffusion of hydrogen during growth was modeled. (C) 2014 Elsevier B.V. All rights reserved.