1. J.L. Seftor, V.L. Granatstein, K.R. Chu, P. Sprangle and M.E. Read, IEEE J. Quantum Electronics, Vol. QE- 15, pp. 848?853 (1979).
2. L.R. Barnett, K.R. Chu, J.M. Baird, V.L. Granatstein, and A. T. Drobot, International Electron Devices Meeting Technical Digest, pp. 164?167 (Washington, D.C., Dec., 1979).
3. J.M. Baird, IEDM Tech. Digest, pp. 156?163, and references cited therein, (Washington, DC., Dec. 1979).
4. the preceeding paper in this issue;Y.Y. Lau,1980
5. K.R. Chu, P. Sprangle and V.L. Granatstein, Bull. Am. Phys. Soc.23, 748 (1978).