The key technical challenge in the molecular beam epitaxial (MBE) growth of group III nitrides is the lack of a suitable source of incorporatable nitrogen. In contrast with the growth of the other III-V compound semiconductors by MBE, direct reaction of N-2 with excess group III metal is not feasible, because of the high bond strength of dinitrogen. An incorporatable MBE nitrogen source must excite N-2 forming a beam of atomic nitrogen, active nitrogen (N-2(*)), or nitrogen ions. rf and electron cyclotron resonance sources use electron impact excitation to obtain atomic nitrogen and in the process generate a wide variety of excited ions and neutrals. Experiments have shown that ionic species in the beam degrade the morphology of the epitaxial layer and generate electrically active defects. Recent theoretical studies have predicted that ground state atomic nitrogen will successfully incorporate into the growing GaN surface, while atomic nitrogen in either of the excited doublet states will lead to etching. In this article, we report on the development of an ultrahigh vacuum-compatible arcjet source which uses an electric are to thermally dissociate N-2. The thermal excitation mechanism offers selective excitation of nitrogen and control of kinetic energy of the active species. This source has been fabricated from refractory materials and uses two stages of differential pumping to minimize the pressure in the growth chamber. The arcjet has been reliably operated at power levels of 10-300 W, with no visible degradation of the thoriated tungsten cathode after 300 h. No metal contaminant lines can be found in the optical emission spectrum. Using an Ar-seeded beam for calibration of the optical spectrum, we find that the arcjet plasma is far from local thermodynamic equilibrium, and show that the fraction of atomic nitrogen in the beam ranges from 0.3% to 9%. This corresponds to a flux of 0.1-4 monolayers per second at the MBE sample location. With an articulated Langmuir probe sampling the beam at the MBE growth position, we find a positive ion flux of less than 4X10(-9) A/cm(2), a maximum ion kinetic energy of 3.5 eV, a median electron energy of 1 eV, and a maximum electron energy of less than 4 eV. With increasing arcjet power, the ion and electron fluxes increase and the ion energy distribution shifts to lower energies. No change in the electron spectrum is observed. Quadrupole mass spectra of the ion flux measured on the arcjet axis show that the N+/N-2(+) ratio has a maximum at an arcjet power of about 35 W.