SMOKE IN COCKPIT IN C650 CREATES NEED FOR EMER DSCNT; DIVERSION TO ALTERNATE.

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WE WERE IN CRUISE FLT AT FL390 FROM STUART; FL; TO MLI. APPROX 85 MI ESE OF BNA BOTH MYSELF AND THE FO SMELLED SMOKE. WE CHKED ALL CIRCUIT BREAKERS AND SYS. ALL APPEARED NORMAL. NO SYS ANOMALIES WERE OBSERVED. WITHIN A MIN OR TWO; SMOKE WAS OBSERVED COMING FROM UNDER CAPT'S INST PANEL. WE IMMEDIATELY NOTIFIED ZTL AND REQUESTED A LOWER ALT. THEY ISSUED US LOWER IMMEDIATELY. AS WE STARTED DOWN; THE COCKPIT AND CABIN FILLED WITH SMOKE. WE HAD DONNED OUR MASKS/GOGGLES AND INITIATED EMER DSCNT PROCS. ARTCC GAVE US AN UNINTERRUPTED DSCNT TO 12000 FT AND ALLOWED US TO DIVERT TO OUR REQUESTED ARPT OF BNA. NOTE: DURING DSCNT; ALL SMOKE CLRED FROM THE COCKPIT AND CABIN. THE SMOKE NEVER RETURNED ONCE CLRED. WE LANDED AT BNA WITHOUT ANY FURTHER PROB. THE ACFT REMAINED IN BNA OVERNIGHT TO ALLOW OUR COMPANY MAINT TECHNICIANS TO COME TO BNA. THE ACFT WAS FULLY INSPECTED. THE PROB WAS DISCOVERED TO BE THE WEMAC BOOST FAN WHICH HAD BURNED ITSELF OUT; CAUSING THE SMOKE. ALL OTHER SYS WERE CHKED AND FOUND NORMAL. THE WEMAC BOOST FAN WAS THEN SECURED AND THE ACFT WAS RELEASED TO RETURN TO MLI. WE HAD NO FURTHER PROBS. NOTE: THIS FLT WAS AN EMPTY (NO PAX) FLT AT TIME OF OCCURRENCE.