that such particular portion of this surface cannot be reached directly not striking any other anode surface first by any straight line originating external to ion source 70 and aligned parallel to axis 30 if much of the contamination is generated from a specific piece of external hardware it is possible to shadow anode surfaces from contamination arriving from that source described in geometric terms a particular portion of the electron collecting surface would be protected if this surface were contoured so that such particular portion of this surface cannot be reached directly by any straight line originating from a point external to the ion source that approximates the location of the contamination source only in the case where extreme protection is required would it be necessary to shield an anode area from contamination that might arrive from any point external to the ion source assuming that the recesses are at least several times wider than the debye length which was shown in the description of prior art to be an easy requirement to meet the plasma can penetrate into the recesses where substantial uncoated anode area is still available for the collection of electrons after the deposition of contamination on the ion source and exposed surface of the anode because this uncoated area is readily available for electron collection near the coated area the deposition of contamination has a.