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Q: What are the current capabilities of a linac?
A: |
There are several features of linacs that are not completely obvious to many
potential users that should be clarified. One of these characteristics is the
output beam requirements for a given linac design. An ion linac is usually designed
for a specific ion q/m ratio and has the velocity profile for these ions fixed along
its length. Hence, if two different ions with the same q/m are accelerated, the
velocity will be the same for each one independent of its mass. The output ion
energy for a particular mass is usually varied in a linac by using multiple stages
that provide discrete energy "steps" that can be powered independently. The ions can
easily pass through a stage that is not powered due to the focusing used in the structures.
The pulsed operation of a linac also has an effect on the rf and AC input power.
The beam pulse width and beam repetition rate do not yield a linear function for
the required power due to the various rf power loss mechanisms. As discussed earlier,
there is a period of time required to bring each linac cavity up in power to achieve
the accelerating fields in it. In addition, the rf power must "fill" the cavity and
them accelerate the ions. Finally, some of the rf power from the rf power supply is
lost in the cables and vacuum windows between it and the linac cavity. Hence, we get
the following relationships for operation of the linac:
- Total RF = Cavity RF + Beam power + RF Losses
- Beam pulse width = RF pulse width - Cavity fill time
From these, we can calculate the average beam current on the neutron target and the AC power.
- Avg. I = Pulsed I * Beam DF
- AC power = Total RF * RF pulsewidth * PRF * Efficiency
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