The invention relates to the encodingdecoding of digital signals, especially using permutation codes involving a calculation of combinatorial expressions. The diffraction grating is spaced apart from the cavity collimator and the grating cooperates with the optical output coupler to define an external cavity. , R134a, CO2, is circulated in a loop between a first location and a second location, the second location being at a higher elevation than the first location. A plurality of expanded microspheres is dispersed through the thermoplastic. The source gases may be simultaneously injected into the process chamber. With an arc established the feed direction is changed to forward and the spraying process continues, if start up should fail and wires again are touching, the reversing function will repeat until a steady arc is maintained.