A new cryogen, a mixture of solid argon and a small amount of liquid nitrogen, was studied to improve thermal contact between the solid cryogen and hightemperature superconducting tapes. A feasibility study of the reference design codes and the thermal requirements was conducted to optimize the design of the mixed cryogen cooling system. Experimental results showed that thermal and electrical stabilities of YBCO coated conductor tapes were significantly improved when they were impregnated with the mixed cryogen. Dry-out problems were overcome by the mixed cryogen because the small amount of liquid nitrogen improved thermal contact between the solid cryogen and superconducting tape.