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Anode heating by plasma heat flux in a vacuum arc was modeled. The model included the nonlinear 2-D heat conduction equation with temperature-dependent thermophysical material properties and heat losses by Stefan-Boltzmann radiation from the anode surfaces. The heat conduction equation was solved numerically for 32-mm-diameter tungsten cylindrical anodes with different anode thicknesses and arc currents. The time-dependent heat flux to the tungsten anode was determined using measured anode temperatures. The time-dependent effective anode voltage Uef was found to decrease from an initial to a steady-state value, e.g., from 11.4 to 7.05 V, for 175 A, anode thickness d = 30 mm, and electrode gap h = 10 mm. It was shown that Uef increases with d at t = 0 and in the transient period, while at steady state, Uef slightly decreases with d and h.