A unique black "burn" pattern was observed at the outer perimeter of the cyclotron accelerator vacuum chamber. The cause of this black "burn" pattern is attributed to Lorentz stripping of the accelerated H - beam, which then bombards the vacuum chamber walls. This finding provides an explanation for the failure of the CP-42 rf dee insulators. These dee insulators are bombarded by the neutral beam current. This understanding led to corrective action that resulted in reduced cyclotron operating downtime.