A method for supplying plural different process gases to a processing location of a particle beam system, using a system comprising: a pipe having a first end located close to the processing location; a first gas reservoir (63 1); a first gas conduit (65 1) having a first end connected to the first gas reservoir (63 1); a first valve (67 1) provided between a second end of the first gas conduit (65 1) and a second end of the pipe, wherein the first valve is configured to selectively allow and prevent a gas flow from the first gas reservoir (63 1) to the processing location via the first gas conduit and the pipe; a second gas reservoir (63 2); a second gas conduit (65 2) having a first end connected to the second gas reservoir (63 2); a second valve (67 2) provided between a second end of the second gas conduit (65 2) and the second end of the pipe, wherein the second valve is configured to selectively allow and prevent a gas flow from the second gas reservoir (63 2) to the processing location via the second gas conduit and the pipe; a controller configured to alternately open and close the first and second valves (67 1, 67 2) in plural cycles, wherein each cycle includes a first duration in which the first valve is open and the second valve is closed, and a second duration in which the first valve is closed and the second valve is open, and wherein the controller is further configured to change a ratio between the first duration and the second duration in order to adjust a ratio between an amount of gas supplied from the first gas reservoir (63 1) to the processing location and an amount of gas supplied from the second gas reservoir (63 2) to the processing location, wherein the method comprises adjusting the ratio between the amounts of gases supplied from the reservoirs and maintaining a substantially constant flow of the gases to the processing location, wherein the gases are capable of reacting with one another and of forming depositions of material within the pipe. a pipe having a first end located close to the processing location; a first gas reservoir (63 1); a first gas conduit (65 1) having a first end connected to the first gas reservoir (63 1); a first valve (67 1) provided between a second end of the first gas conduit (65 1) and a second end of the pipe, wherein the first valve is configured to selectively allow and prevent a gas flow from the first gas reservoir (63 1) to the processing location via the first gas conduit and the pipe; a second gas reservoir (63 2); a second gas conduit (65 2) having a first end connected to the second gas reservoir (63 2); a second valve (67 2) provided between a second end of the second gas conduit (65 2) and the second end of the pipe, wherein the second valve is configured to selectively allow and prevent a gas flow from the second gas reservoir (63 2) to the processing location via the second gas conduit and the pipe; a controller configured to alternately open and close the first and second valves (67 1, 67 2) in plural cycles, wherein each cycle includes a first duration in which the first valve is open and the second valve is closed, and a second duration in which the first valve is closed and the second valve is open, and wherein the controller is further configured to change a ratio between the first duration and the second duration in order to adjust a ratio between an amount of gas supplied from the first gas reservoir (63 1) to the processing location and an amount of gas supplied from the second gas reservoir (63 2) to the processing location, wherein the method comprises adjusting the ratio between the amounts of gases supplied from the reservoirs and maintaining a substantially constant flow of the gases to the processing location, wherein the gases are capable of reacting with one another and of forming depositions of material within the pipe.