One answer is to ensure that the Exposure of the Constant Light on the Subject is far greater than the Exposure of the Constant Light on the Background. (aka the Lighting Ratio of the Subject to the Background Curtain). A suggested starting point to test would be to get an Exposure Difference of 4 Stops, but experience tells me that I think you’ll need closer to 6 Stops to get rid of those curtain folds.
The (constant) light falls off according to the Inverse Square Law. In simple terms to get a big Exposure difference, you’ll need to get the Subject close to the light and the Background Curtain as far away from it as possible.
So the Lighting Set would be: Dancer Extreme Stage Front; Constant Light as close as possible to Dancer. Use the farthest Background Stage Curtain.
One immediate issue with that Lighting Set is the problem of keeping the Constant Light even across the path of the dancer, obviously as the Constant Light is moved closer to the Dancer the distance of travel of the dancer will be limited because if the Dancer travels too far you run the risk of the Constant Light falling off enough on the Dancer to render an uneven exposure.
This link has an introduction to The Inverse Square Law as it applies to some Photography Scenarios and has pictorials which I think you will find useful.
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Setting the Constant Light at an angle is an idea to try, that will increase the distance of the Constant Light to the Background, but it will also increase the range of distances of the Constant Light to the Subject – at one end of the Dancer’s travel the Constant Light will be closer to the dancer than at the other end – and this might render an appreciable exposure difference.
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Another option is to use a flat and also matt black backdrop: this could be either painted board of stretched material/canvas, but it is essential that matt black paint is used. Obviously even with using a flat (physically flat) matt (no gloss) black (painted) background, you would strive to achieve as much Light Fall off on the Background as possible
WW