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If the LOCAL mode is selected, a target value for beam current can be set in the BEAM module.
When the BEAM button is pressed, the power supply will regulate the discharge current by adjusting
the cathode heater to extract the target beam current.
Step 5)
Adjusting the beam conditions
.
The beam current and voltage can be adjusted while the beam is on. However, for some conditions
the beam may need to be turned off while keeping the source on. Also, switching between
MANUAL and LOCAL power supply modes may be necessary to achieve desired beam conditions.
Some beam currents may not be achievable at various beam voltages (e.g. high beam current at low
beam voltage). Please consult run data on our website for the nominal range of beam currents and
voltages.
Additional recommendations for setting the beam current include start the source with a lower
cathode heater current (i.e. lower discharge current) and a higher accelerator voltage. If there are
issues with the beam current and voltage please consult Chapter 6 -
Troubleshooting
.
Step 6)
Optimizing the accelerator voltage
For some applications it can be useful to optimize the accelerator voltage. If the application requires
low beam voltage (i.e. low ion energy) the accelerator voltage is usually required to be high and the
beam spreads out as it leaves the source. On the other hand, if the application requires higher beam
voltage (i.e. high ion energy) the accelerator voltage can be optimized to improve the accelerator grid
life.
After a beam current and voltage are selected, start with a high accelerator voltage. Put the power
supply in MANUAL mode. Decrease the accelerator voltage and examine the accelerator current
and discharge current. When the discharge current begins to increase, electrons will begin to back-
stream. At this condition, the accelerator voltage is too low. The accelerator voltage is optimized by
increasing it above this setting by 50 to 75 V. Illustrated in Table 4.7 is an example of optimizing
the accelerator voltage at a given beam condition.