The power supplies giving Diamond a boost
The electrons that produce Diamond’s ultra-bright light whizz round the storage ring fast enough to travel around the entire world 7.5 times in a single second. But they don’t start out life super speedy, and they need a huge energy boost to get them ready for work!
Diamond’s electrons are generated in the injection system, where they are produced by a glowing filament (just like a dim light bulb) and accelerated to ninety thousand electron volts (90 keV). From there, a linear accelerator (linac) takes over, accelerating the electrons to a hundred million electron volts (100 MeV, or 0.1 GeV).
That’s not fast enough though, so the electrons from the linac are fed into the booster ring, where they’re are accelerated to 3 GeV by passing through an RF cavity millions of times. It’s like microwaving the electrons to get them to accelerate, which is not an easy task. The electrons want to travel in a straight line, and have to be forced to bend around the ring by dipole bending magnets. As the energy of the electrons increases, it gets harder to keep them moving around the booster ring, and the bending magnets need more power.
FIgure 1: One compartment of the Booster Power Supply.
The complete process of ramp up and ramp down must take place in 0.2 seconds (200 milliseconds, or 5 Hz). The electrons are very sensitive to the magnetic field, so the current must be accurate to 0.05 A, even when the power supply is delivering 830 A (an accuracy of 0.006%). Diamond’s Booster Dipole Magnet Power Supply has the job of supplying the electrical power to the magnets, and ensuring they ramp up and down as required.
Figure 2: A capacitor switch for the Booster Power Supply.
The magnets have a very large inductance, and need a very large power supply - 2000 V at 1000 A. Diamond’s booster dipole magnet power supply takes an average of 200 kW of power from the mains, enough to supply around 450 houses. It has to be extremely reliable, and easy to maintain, and it needs its own air conditioning unit to keep it cool.
And, unlike a standard power supply, it has to be able to deal with power being drained from the magnets as they drop back to 30 A, as Colin Abraham, Diamond’s Senior Power Supply Engineer, explains:
“To do this, the power supply has a large bank of capacitors, and keeps the power ‘sloshing’ backwards and forward between the magnets and the capacitor bank, like water sloshing between two buckets.”
So, to recap... the booster power supply has to:
- provide a high voltage at a high current
- be extremely reliable and stable, so that each cycle is exactly the same (any differences will affect the science experiments being carried out)
- deal with large temperature fluctuations
- deal with power sloshing backwards and forwards,
Figure 3: Members of the Power Supply team working in the Booster Suuply Hall.
The power supply also has to be able to ‘smooth out’ that 5 Hz cycle, so that it doesn’t impact Diamond’s other electrical systems. It wouldn’t be good if the lights flickered fives times a second, for example.
To control all of this, the power supply has its own ‘brain’, a Digital Power Supply Controller that is linked into Diamond’s EPICS control system. In the event that there’s a problem, the control system can shut down the power supply safely, and tell the engineer where to look for the fault.
In order to keep the power supply running smoothly, it’s built from plug-in modules that can be replaced quickly. Spares are kept in storage, so that the power supply can be up and running again while the damaged module is being repaired. It’s not quite like changing a light bulb (the modules weigh over 100 kg) but it’s not that different.
The power supply is 2 metres tall, 10 metres long and 3 metres wide. It’s the largest and most complicated power supply within Diamond, but the power supply team has to look after about 1300 others (most of which are far smaller, with the smallest designed to deliver a maximum current of 5 A). All power supplies, even those delivering less than 5 Amps, are essential for Diamond’s day-to-day operation.
Figure 4: The Booster Power Supply in the Diamond synchrotron building.