NOVELTY - The reactor (100) has an elongated/reactor chamber (102) including a gas expansion region connecting a delivery region and a reaction region. The gas expansion region comprises an inner peripheral surface defining an inner space (106) including length extending along process direction (P) and a cross-sectional area that uniformly increases along the process direction toward the reaction region, where the gas expansion uniformly heats and expands process gas flow before entering the reaction region within uniformly expanding inner space.    USE - Roll-to-roll/continuous thermal process reactor e.g. atmospheric-pressure (AP) thermal process reactor and Sub-Atmospheric (SA) thermal process reactor, for reacting a precursor material placed on a top surface of a flexible continuous work piece e.g. sheet-shaped work piece and web, to form a Group IBIIIAVIA solar cell absorber of a photovoltaic device utilized during thermal processing, thin-film reaction and thin-film deposition.    ADVANTAGE - The reactor controls volume of an expansion section and a contraction section, so that heating and cooling gas flows through the regions with reduced turbulence, thus developing roll-to-roll or in-line growth copper indium gallium diselenide (CIGS) growth techniques, and hence controlling CIGS material compositions. The reactor allows axial and transverse uniformity of gas flow with reduced unwanted gas turbulence within the process space to be greatly improved by tailoring the internal volume of the elongated chamber at an axial position to an internal temperature profile. The reactor provides gradual temperature change in the elongated chamber depicted by a segment to occur in the expansion section, so that the resulting gas flow expansion is compensated with the gradually expanding volume or space of the expansion section, thus improving the axial and transverse uniformity of the gas flow with reduced unwanted gas turbulence that enters the reaction section from the expansion section and entirely throughout the reaction section. The reactor allows the cross-sectional area of the gas expansion region to be dimensioned to increase along process direction in proportional relationship to expansion of the gas along the process direction so as to reduce turbulence of the gas in the gas expansion chamber. The reactor allows floor pieces to effectively prevent process gas flow disturbances caused by abrupt changes in bottom wall elevation. The reactor allows axial gas velocity to be held essentially constant through changing reactor temperatures between about 0.01 to 0.1 meter/second and about 0.02 meter/second when the elongated chamber's internal volume is scaled with reactor temperature, thus avoiding the gas disturbances caused by increasing gas volumes in isothermal regions and increasing gas temperatures in constant-volume regions.    DETAILED DESCRIPTION - The elongated chamber has length selected from a group consisting of axial length (L) or transverse length. The elongated/reactor chamber is selected from a group consisting of a non-contact reaction chamber and a partial contact reaction chamber.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a roll-to-roll/continuous thermal process reactor.    Axial length (L)    Process direction (P)    Roll-to-roll/continuous thermal process reactor (100)    Elongated/reactor chamber (102)    Inner space (106)    Continuous work piece (110)    Heating element (115)