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This paper deals with a method for the alignment of a Gaussian beam. In the fusion experiment W7-X (a stellarator) under construction, a quasioptical transmission system is used to deliver high-power millimeter waves to heat the plasma. In order to be directed properly into the stellarator, each beam has to be well aligned. A low-power image of the main beam which can be used to measure the quality of the alignment and, if necessary, lead to a signal to correct the alignment, is produced by a grating coupler. In this paper, the principle of an alignment system using such a coupler is described and the design for a prototype is presented. Low-power tests are reported. Results of the measurements of the characteristics of the prototype system are given and compared with theory.