RIKEN performed first tests:
First test of graphenic carbon vacuum windows with heavy ions within the context of hadron beams.
https://www.nishina.riken.jp/researcher/APR/APR053/pdf/134.pdf
We developed these windows together with KETEK a while ago:
https://www.researchgate.net/publication/273471966_High_Performance_X-Ray_Transmission_Windows_Based_on_Graphenic_Carbon
The activities are part of the EU-project I.FAST: