Measurement of ion flow in a negative ion source using a Mach probe

Measurements of ion flow in an ion source made for negative ion extraction are reported in this letter. The ion flow has been measured using the Mach probe as two single probes compared with the results as a double probe. The measured values of the Mach number lie between 0.2 and 0.4. The maximum value of the Mach number is observed near the radio-frequency excitation coil. The flow shows a dominant direction toward the extraction grid; however, the flow pattern, away from the central axis of the source, shows a direction reversal and, therefore, convection. The presence of an ion flow has a strong influence on negative ion motion toward the extraction grid and on the sheath potential. The ion flow will play a dominant role in the ion dynamics and negative ion beam overall efficiency