Near T_c=846 K barium sodium niobate exhibits a strong thermal focusing of laser light polarized along its optic c axis (focal length F=+2 to +3 cm for 1 W in a 0.45 mm beam waist). The beam convergence theta (T) is proportional to K^-1(T)dn_c(T)/dT, where K is the thermal conductivity and n_c is the c-axis index of refraction. For zero applied field, theta (T) far from T_c is dominated by the isotropic dn/dT term, which varies as t^{2 beta -1}, where t is the reduced temperature and beta is the coexistence curve exponent (approximately ¹/₄ in this tricritical system). However, within a degree or two of T_c, theta (T) is dominated by K(T), which becomes highly anisotropic and has a cusp-like dip for thermal diffusion along the polar c axis. In the present experiment the authors apply a DC electric field along c and show that a few kV cm^-1 is sufficient to suppress the cusp.