FRIB Operators

Welcome! If you haven't already please check out What is an Operator for a better idea of what your role as a new Operator here is. I also introduce some of the basics of the machine there and will reference those portions.

Operators come from a wide variety of backgrounds, some of us have Master's in Physics and some have Bachelor's in totally different fields. All this is to say that nobody joined as an expert in accelerators and even those who had some foreknowledge had to become accustomed to our particular lab. These are pretty complicated machines and by their nature are hard to understand in their totality, a lot of concepts intersect, overlap, and interact is some confusing ways.

Accelerators take plasma, and by repeatedly exposing it to sinusoidal electric fields as it progresses down the beamline. accelerate it to higher and higher velocities. If that's confusing, and it should be at least a little confusing, then it can be simplified to ions going through the accelerator, being accelerated by cavities. You'll hear a lot about cavities as they are as vital as they are temperamental, often causing many of MPS Faults mentioned in What is an Operator You don't need to know how to disassemble a cavity blindfolded in under 60 seconds in the middle of warzone, but knowing the name will help you familiarize yourself with them as they pop up in many Control Room conversations.

This page is intended more as a long term project to read through or a reference you can check. I tried to organize the What is an Operator page from most basic to most complex primarily and most common to rarest secondarily, but here I'll summarize in a fair amount of detail each component from the source of the beam to its furthest possible destination. That being said, the intention is still to help along newer Operators, so I'll try to remain approachable.

Sources are where things begin. Source Physicists start by obtaining a sample of an element, this can be a variety of isotopes that are either gaseous or solid. Solid sources are placed into an oven until material sublimates off, this is unnecessary for gaseous samples. From here they enter into the Plasma Chamber.

There are actually a variety of isotopes in the Plasma Chamber, this is referred to as the cocktail. This allows us to change the beam we're sending to another isotope fairly quickly by adjusting devices.

These isotopes fly around the Plasma Chamber and get excited by Radio Frequency Waves. This strips off some of their electrons and imparts them with more energy. You can already imagine that we don't want random air particles in there, so much like the rest of the beamline, the Plasma Chamber is under vacuum.

To ensure that the ions stay in the Plasma Chamber for long enough, there are two solenoids on either end that create two magnetic fields, one by the injection side where the samples enter, and one by the extraction side where they leave. These two fields are not equal in strength, the extraction side is weaker so that some ions may escape out into the rest of the beamline.

A common graph that Operators use will look at the following measurements that will then be explained.

What is ECR Drain Current? Just realized I don't know

What is Average Pressure? The Pressure within the Plasma Chamber This normally hovers around a few nTorr (as of writing, it's 16 nTorr, about 2e-6 Pa or 2e-11 atm.) Good to check to see if the source is being unstable or to compare to the Drain Current.

What is Microwave Power? The power of the RF that's exciting the ions, I don't check this too much as it's a fairly reliable system that isn't dependent on what's going on inside the Plasma Chamber. This can be adjusted if instructed by the Source Physicists to help beam stability or power.

What is Bias Disk Current? The bias disk is weird, To quote the best source document I've read “All agree that it has clear benefits, but similarly agree that the mechanisms of how those benefits are achieved are less clear.” It is a negatively biased disk in the Plasma Chamber that will be stand among the Drain Current and Pressure as things to check when you think the source may be acting strangely.

The last twist I'll throw at you is that for FRIB there are two Sources, and that's just FRIB! They are Artemis B and HPECR (formerly Venus). We only send from one source at a time, but I'll explain that more in the next section.

The front end is the section of the facility where the FRIB sources live. These are called ISRC1 and 2, and the beamlines attached to them are called SCS1 and 2. Hopefully you get the vague gist of how we allow beam to go down the beamline, but first let's focus on how we stop beam from continuing down.

First are the Source Slits, these are are devices that can move to create a smaller or larger area for beam to travel through. There are 4 source slits per source, two vertical and two horizontal. We do not touch the horizontal source slits. If you imagine the fresh beam that exits the sources, you can think of it as being largely homogeneous vertically, that is any ion should look similar to any other ion if you took a vertical slice. Horizontally it is heterogeneous, with the ionization or charge state varying from one side to the other. Accelerator Physicists use the horizontal slits to exclude unwanted charge states from the beam, Operators and others use the vertical slits to make small adjustments to the amount of beam being sent. Note: Opening the vertical slits too far can cause beam loss and lead to trips so this should be done with a fair deal of consideration.

Also in SCS1 and 2 are the Source Cups. Source Cups are Faraday Cups that are very close to both of the sources, the ideal “safe state” the machine can be in without the lengthy process of turning the sources off and on. Anytime there is expected time where the beam will not be sent, it's common to insert the source cups. This also allows us to do a variety of measurements on the source and the beam that's immediately exiting it. A Faraday Cup is a device that will block beam from going further down the beam line. They're useful for making sure a beam doesn't go further than intended or for measuring the strength of the beam. Based off of the current read from a Faraday Cup we can estimate beam power without having to send it or see how much beam is present at a particular point in the beam line. Another use is that only Operators interact with the Chopper, but many physicists can control the Faraday Cups, so it is a common practice to leave beam on Faraday Cup 1102 and allow the physicists to either retract or insert it as they want beam.

You'll also find magnets in the Front End, any diagram you see will have bends in the beamline, they're vital for excluding unwanted beam. The beam doesn't naturally bend, but we can make it bend with Dipoles these are magnets that create a magnetic field either pointed up or down, using basic vector cross multiplication (right hand rule!) you'll see why this causes beam to turn. All Dipoles exist to turn beam. Note that due to math I really don't want to have to type out that these magnets do not change the speed of the ions, or the magnitude of their velocity. Think of a dipole as only moving the steering wheel in your car but leaving the pedals alone. By either turning on or off dipoles in the Front End, along with inserting one of the source cups, we can ensure we're only sending beam from one source.

Another kind of magnet is the Quadrupole, which is like a combination of two dipoles. Quadrupoles exist to focus the beam and should not redirect it, though they can if the beam isn't centered. You'll find both horizontal and vertical quadrupoles, these work in tandem to first smush the beam in one direction, then the other. Their field is a bit more complex to imagine, but the configuration is the elementary consequence of having four magnets in each corner of a square, alternating polarity so the bottom left and top right magnets are both North and the top left and bottom right are both South.

After SCS1 and 2 join together we get our ULEBT, or Upper Low Energy Beam Transport. Remember, we've created a plasma but haven't really accelerated it so this is very low energy. The “Upper” refers to the architectural location of this portion, it is literally above the rest of LEBT. The first major thing here is the Chopper, which is probably the most familiar bit of the machine for you at this point. It's crucial that the Chopper is in LEBT as the lower energy beam is much easier to redirect.

The Chopper is great, but has more utility than I've previously let on. This comes from the Repetition Rate and Pulse Width Parameters. Pulse Width tells the Chopper to spend this much time allowing beam to continue down the beamline. So if the Pulse Width is 100us it will stay off for that period of time and then turn on until it repeats the cycle. How long the cycle lasts is determined by the Rep Rate, which tells the Chopper how many cycles there should be every second. You can then find the period of the cycle by finding inverse of the Rep Rate.

So let's return to our example, let's say we have a Pulse Width of 100us and a Rep Rate of 100Hz. We can say the period of each cycle is then 1/100 s just as it would be 1/50 s if the Rep Rate were 50. Let's convert 1/100s into microseconds to match the units of our Pulse Width and we get a cycle period of 10,000us. So the Chopper will allow beam to pass by for 100us, then stop beam from the remaining 9,900us. A little more math reveals that we're only allowing beam to pass by 1% of the time. This is an important calculation that we refer to as the Duty Factor.

By Default, we like to send CW Beam, or Constant Wave. This means a Rep Rate of 100Hz and a Pulse Width of 9950us. You might have realized that those numbers don't give us a 100% Duty Factor, and you'd be right! We use the 50us gap to help with machine timing, so it's our best approximation of allowing 100% of beam to pass through. We don't change these values too often, but they are helpful for Beam Development.

From here we have our most famous Dipoles, those being the E-Bends. E-Bends are very important to our PPS System. With the E-Bends powered off there is no way for beam to go into the LINAC, note that beam should be stopped on the Chopper or ideally a source cup before turning off the E-Bends. There are 4 E-Bends in total I DON'T ACTUALLY KNOW WHY and they first divert the beam into traveling downwards through VLEBT or Vertical Low Energy Beam Transport for obvious reasons, then it redirects them into moving horizontally through the LLEBT or Lower Low Energy Beam Transport.

Getting Bored? That's alright, don't forget to bother the other Operators about what's going on and also we're on to a new kind of device!

That's right! More Beam power. You might remember that we can make slight adjustments to beam power using the Source Slits, but for larger changes a common device we use are the Attenuators. I like to think of them as industrial sieves that you strain beam through. There are 4 different attenuators on their separate drives. Each one of those is actually two smaller drives, so each attenuator can be in a total of four different states. The first attenuator has the following states

The others are different, but we'll use this one as an example. If both drives are not extended, then there is no part of the attenuator that is in the path of the beam. If the first drive is extended then the 2x attenuator enters the path of the beam, blocking about half of it though attenuators are notoriously inaccurate and weird. NEVER trust what the attenuator says. Only extending the second drive would make the 5x attenuator enter the path of the beam, leaving you with about 20% of whatever it was initially. Extending both inserts the 100x attenuator. You'll notice if you try changing the attenuator configuration that it will shoot up sometimes or return odd numbers at others. This is a consequence of this drive system. You can see the Attenuation Factor on one of the TV Displays in teh Control Room, ask one of the other Operators where it is.

Generally because of how they work, the different attenuators will interact in pretty weird ways that normally kills any beam output you were hoping to get through. Best practice is to skip one attenuator when possible, so should you need to use 2 attenuators it is better to use the first and third than the first and second. Attenuation also muddies up the beam in an odd way, which mostly isn't a big deal, but the Accelerator Physics sometimes wish to not use them for this reason.

I call it the RFQ mostly because you'll almost never hear the full name of the Radio Frequency Quadrupole, I like the RFQ because it's the first time we really accelerate beam in the direction of the beamline and also it is normally pretty well behaved.