Computer simulation aids researchers at Triumf

Computer simulation has allowed research scientists at Triumf to achieve the tightest specification to date for a magnetic field generated by a pulse forming network.

Opera computer simulation from leading design software house Vector Fields has allowed research scientists at Triumf, Canada's National Laboratory for particle and nuclear physics, to achieve the tightest specification to date for a magnetic field generated by a pulse forming network (PFN).

The 66kV PFN, has a measured ripple in the flat top of only +/-0.3% compared with the +/-1% for state-of-the-art devices previously.

The Triumf team attribute their ability to achieve this to the precision computer simulation from Vector Fields which enabled them to determine a single frequency for the ideal circuit parameters needed, rather than traditional calculations followed by trial and error tuning.

The electromagnetic simulation was also used to evaluate the geometry of the PFN coils enabling precise manufacturing tolerances to be specified.

Triumf coordinate Canada's design contribution to the Large Hadron Collider (LHC) at CERN, which will consist of two superconducting magnetic channels installed in the existing 27km LEP tunnel.

Protons delivered by the injector synchrotrons will be accelerated in two counter-rotating LHC rings to 7TeV each, and brought to collide at the four experiment locations.

First collisions are scheduled for this year.

One aspect of Triumf's contribution involves designing the resonant power supplies and PFNs for the LHC's injection kicker systems which are responsible for switching particle beams from one ring to another with great precision.

"The particle beams must switch cleanly from one ring to another or they will crash into the sides of the channel and cause not only radioactivity, but also severe damage due to the very high beam power", said Michael Barnes, a research scientist at Triumf.

"That requires that each kicker system must produce a field of 1.3mT with a flattop duration adjustable between 4.25 s and 7.8s, with rise and fall times of less than 900ns and 3s respectively and flattop ripple less than +/-0.5%".

"To our knowledge, no one has ever achieved better than a +/-1% ripple".

The primary electrical parameters of the PFN coil, which shape the pulse, are frequency-dependent.

A sensitivity analysis of the field to the value of both individual and groups of circuit components gave an understanding of which were critical to the performance, allowing Triumf to define a PFN with very precise coil component values that required no adjustments.

Barnes and his colleagues used the 2D finite element-based electromagnetic analysis software, Opera from Vector Fields, to gain this detailed understanding of the coil behaviour.

The first step was modelling the coil configuration in Opera.

This was done using the software's CAD facilities.

Each coil is surrounded by a 3mm thick Omega shaped aluminium screen with an inner radius of 140mm.

Both lines were mounted in a rectangular tank with mild steel walls.

After creating the 2D model, the researchers specified the material properties of the components using Opera's material library.

The researchers then ran the Opera electromagnetic analysis to predict self-inductance, mutual inductance, and resistive losses at each frequency.

They repeated the analysis 60 times to evaluate coil behaviour at 60 different frequencies, ranging from 0 to 10MHz.

The detail was empiric in designing a circuit that would deliver the desired coil performance.

Once the theoretical optimum coil configuration and a circuit that would deliver the correct pulse shape had been established, additional electromagnetic simulations investigated the effect on performance of geometric imperfections from manufacturing.

For example, the coil would be wound very precisely within grooves on a fibreglass epoxy former.

"We wanted to be able to specify to the manufacturer the precise tolerances for those grooves", explained Barnes.

Also when the coil is wound into the groove, rather than a perfect circle, its shape is slightly distorted.

"In Opera, we modified the coil model to be two semicircular tubes joined by two straight sections".

The simulation then showed that reducing the outside diameter of the tube by 0.2mm, with negligible extension of the straight section, increased the predicted inductance +0.7 %, however extending the straight sections 0.4mm reduced the inductance back to original value.

Opera was also used to assess the effect of production imperfections in the radius of the Omega shield, which indicated an increase in predicted inductance of 0.12% from radius +0.71 %.

Using the information gained from these analyses, Triumf and CERN produced a prototype pulse forming network that exceeded the original requirement for a +/-0.5% ripple in the field flattop.

Their network, which has now been tested by CERN for more than one million pulses, delivers a ripple in the flattop of +/-0.3%.

CERN has asked Triumf to contribute nine more pulse forming networks to the LHC project.

"We believe this is the first time anyone has used computer simulation the way we did in the design of a pulse forming network", said Barnes.

"When you have to work within a certain window of inductance and resistance, using simulation enables you to try out enough iterations to achieve your design goals".

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