The foil thickness (black) was estimated from the measured H^- (blue) and H⁰ (red) fractions, which was consistent with the results obtained from the multi-wire profile monitor (MWPM). Early foil thinning was due to sublimation caused by beam irradiation, but later foil thinning and the excess of H^- occurred due to a deformation and pinhole formation. The proposed method allowed us to monitor foil degradation during operation to determine a realistic foil lifetime and prevent foil failure.