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This paper describes the suppression of thermoacoustic instabilities in a swirl-stabilized test rig. To that end, a high-bandwidth secondary fuel injector is installed and commanded with robust H infin and simpler gain-delay controllers. Depending on the operating condition, several distinct flame types occur, which are investigated with OH chemiluminescence. A model of the plant is obtained by means of closed-loop identification experiments. The model-based H infin approach confers more design freedom to the engineer and gives better results. In particular, the sensitivity can be distributed arbitrarily to target specific and separate frequency ranges. The pressure spectrum reductions achieved are compared with the predictions of the design process and are found to agree well. The influence of the injected natural gas on the flame is studied through OH chemiluminescence. Experiments show that the controlled flame assumes a position similar to the one of a flame being in a stable operating condition.