Limiting Primary Beam Power

The task page for these devices is here.

Overview

There are three primary ways in which operators can limit the primary beam power: the chopper, the attenuators, and the source slits. Each have their own specific use cases and inefficiencies.

The Chopper

The chopper is the first device in the beamline that we use to control beam intensity. It is arguably the device that operators use the most to control beam intensity because of how many time MPS trips on a shift. Operators use the chopper to limit beam power by manipulating the reptation rate and pulse width to achieve a desired duty factor. At 100Hz and 9950us we have an expected duty factor of 99.5% which is the highest we can go, we call this configuration of settings Continuous Wave (CW). Duty factor is basically a measure of how much beam the chopper lets through, so with a duty factor of 99.5% the chopper is letting that much beam through. We use the following calculation to determine duty factors:

(Rep. Rate) * (Pulse width) * 0.000001 = Expected duty

Example: 5Hz * 5000us * 0.000001 = 0.025 = 2.5% Expected Duty Factor

Using Attenuators

Attenuators are fickle and not reproducible at best; at worst they can give you more beam by inserting them. There are four attenuator drives in the front end with attenuators ranging from 2x to 100x. Attenuators are used to lower beam intensity by pre-determined amounts, we usually use attenuators to provide the FS group with appropriate amounts of beam to see it on their viewers. Attenuators lower the beam in proportion to the attenuation factor they have been assigned; for example, a x2 attenuator will half the intensity of the beam while a x100 attenuator will give you 100 times less beam. Be careful about combining attenuators, many of them don't play nice and their actual attenuation factor often strays from the calculated one. Just pay extra attention and always double check your math and beam intensity before sending.

Using the Source Slits

We use the source slits to limit beam intensity almost only when we are running full power, with no attenuators in, and running CW. The slits are hard to predict, there will be times when you are cutting with them and you are not losing any beam and vice versa. Another important note about the slits is that operators only change the vertical slits at full power to avoid any charge state buffoonery. We do mess with the horizontal slits but only when we are tuning for FSEE and need to select the appropriate charge state, we rarely change the vertical slits in this scenario.

Links

Task page for limiting beam power