Development of a New Electromagnetic Extraction Channel from the AGOR Cyclotron

The extraction system of the superconducting AGOR cyclotron consists of an electrostatic deflector and three electromagnetic channels. As the electrostatic deflector has only a moderate field strength (<100 kV/cm), the first electromagnetic channel (EMC1) has to generate a rather strong dipole component resulting in current densities up to 169 A/mm2 in water-cooled copper coils. In the original design the coils consist of sections of hollow conductors, parallel to the beam path, vacuum-brazed to machined “bridges” over the beam aperture. Altogether there are over 200 brazed joints made in three subsequent cycles in the three coils (dipole, quadrupole and first harmonic corrector). In 25 years of operation two channels of this type have been “consumed”. The channels developed water leaks due to erosion of the copper by the high speed cooling water flow in the “bridge” regions that ultimately could not be repaired anymore. To remedy this problem the channel has been redesigned using bent conductors. A production technique for small radius bends and a new joining method to avoid vacuum brazing have been developed. The coil support taking up the 10 kN/m Lorentz forces on the windings are now made from isolating material instead of anodized aluminium to prevent grounding errors. The new channel (EMC1-U) has been in operation now for two years without any failure. A detailed comparison of the old and new design will be presented.


Introduction
Cooled high-current density coils in accelerator applications in vacuum are unfortunately consumables.Many laboratories have applications which make use of copper coils operating in highcurrent densities mode which face degradation.This degradation happens due to erosion as well as the wear and tear of disassembling, assembling and repairs.At the AGOR cyclotron accelerator at UMCG-PARTREC [1,2] a new methodology has been developed to construct and build coils sets for electromagnetic extraction channels.The methodology is based on the premise that the cooling takes precedence in the design.Therefore, starting from the perspective of cooling, we aimed for smoother joints, reduce the number of joints and use press fit joints to minimize electrolytic erosion.In this paper the old and new EMC1 designs are compared with respect to magnetic field, electrical coil design, cooling, the coil production, used materials and erosion.

Comparison between old and new EMC1
2.1.Magnetic field design EMC1 originally was designed [3] with a coil configuration of conductors to create a maximum dipole field of 0.2 T with a super-positioned gradient field of 13 T/m and a correction field for the first harmonic of maximally 50mT.This led to a configuration of conductors as shown in Figure 1, which is used as a design constraint for the new EMC1-U channel.Magnetically, the only difference with the old design is the magnetic field shape at the entrance and exit of the channel as the route of the current conductors significantly changed, see Figure 2.These changes were evaluated by using the results of two COMSOL simulations in which the magnetic field as function of position in the old design and the new design were simulated.For both designs the vertical magnetic flux as function of position was simulated and a line integral was calculated to find the effective field boundary points (EFB).From these EFB points the EFB boundary was constructed.Comparing the calculated EFB boundaries of the old EMC1 to the new EMC1-U, it showed us that the EFB field boundary stayed at the same location and was rotated 15 degrees to a perpendicular position with respect to the beam axis.

The electrical design
To give a brief impression of the electrical properties for new as well as the old channel: the current density in the copper conductor is maximally 169 A/mm 2 in the dipole coil and 115 A/mm 2 for the gradient and the correction coils respectively.The number of loops per coils are 6, 4 and 4 for respectively the dipole, quadrupole and the correction coil.The current densities lead to maximal power dissipation of 2.2 and 3 W/mm over a loop length of 2400 mm.Turbulent water flows in channels of 2, 2.5 and 3.2 mm in diameter.A pressure of 24 Bar is present to create flows ranging between 0.6 and 1.6 l/min depending on the conductor.These flows are set by a flow resistors in the exit tubing of the channels.Setting this resistors keeps also the exit pressure artificially high at 6 Bar.The pressure leads to stress in the materials and joints.However, it also prevents cavitation in the channel.In total, a 75 kW is dissipated in the electromagnetic channel in vacuum when the channel is in operation.

Current bridges
In EMC1, the dipole, quadrupole and corrector consist of a top and bottom set of coils.Each set consists of several loops brazed together.No loops could be dismounted separately within a set.The loops were connected by CNC machined bridges equipped with cooling channels.This is in contrast to the new EMC1-U channel as the new channel consists of bridges which can be disconnected as shown in Figure 3.The disadvantage of these bridges is, that they have no direct cooling but are conduction cooled.Therefore, the bridges have a large cross section to reduce the resistance.To determine how much copper was needed for the bridges, the geometry and dissipation of power was simulated with COMSOL, see Figure 3.It turns out that the temperature increase of the bridges could be kept at 5°C above conductor temperature.The calculation was carried out for only a small section of the coil.Electrical Isolation  The original EMC1 aluminium support was equipped with an oxide layer to isolate the conductors from the each other.It turned out over time that this oxide layer was not sufficient.Sharp edges pierced through the layer and shortened the coils to ground.For that reason, all the conductors from the old EMC1 were wrapped with Kapton, which consequently made it difficult to search for leaks.In the new EMC1-U the support and isolation is done by PEEK, as can be seen in Figure 4.

Brazing Technique
The old EMC1 coils were brazed in three vacuum-brazing cycles.Each cycle involved a different brazing material and melting temperature.The brazing took place outside the laboratory and had to be prepared far in advance.EMC1-U, in contrast to EMC1, uses only one type of brazing material i.e.Castolin 1666, 45% Ag alloy, free of cadmium, with a melting temperature of 680°C in combination with the GreenFlux 1802PF.The brazing is carried out manually and performed at a brazing station.Furthermore, we have learned that the brazing had to succeed vertically and that the silver alloy had to be fed from the top, see Figure 5.While brazing, the liquid silver alloy flows through the space in the joint out of exit holes.In this way the flux is flushed out.In cases where this did not fully succeed, "flux bubbles" were seen in the brazed joints which created virtual space or cavities in the joint.Furthermore, the joint is made such that the top of the joint is a press-fit so that no flux can enter the cooling channel so as to avoid future electrolytic erosion.

Coil Production
For EMC1-U, a bending machine was developed to bend the conductors.To prevent flattening, kinking or wrinkling of a bend, the conductors are pulled over a thorn at the moment of bending, see Figure 6.As the thorns have the exact shape of the bend, the thorns keep the tube from deforming while bending.Careful adjustment of the machine makes 90° bending possible with a position accuracy of 0.15 mm and bending radii of 2.5 mm.The 2.5 mm is the distance between the bending circle centre and the conductor surface.An aluminium cast was made to check the dimensions.To precisely position the bridge on the conductor, the bridges were positioned while the coils were mounted on the PEEK support.The brazing was done after dismounting the coils.

Copper
All the conductors from the original EMC1 were made from copper with quality "CuC2".In the new EMC1-U the dipole coils are made from CuC2 copper, and the quadrupole and corrector coils are made of the quality OF-OK.OF-OK material supersedes CuC2 quality and meets the American and European standard for Grade 1. Furthermore, the OF-OK contains a factor of 5 more Pb impurities (50ppm) with respect to CuC2 (10ppm) and C10100 (5ppm), however this does not affect the conductive properties.In contrast, CuC2 contains (80ppm) i.e. 30,18,10,10,10 ppm of the P, S, Se, Te, Zn respectively, whereas OF-OK only has 40ppm impurities left [4].

PEEK
The copper conductors of the new channel are embedded in a PEEK support.PEEK was chosen because of its high Young modulus of 3.7 GPa with respect to other plastics.Stress simulations showed that the stress directly from the coils on the support is maximally 2.4 MPa leading to peak stress at pivotal points up to 60 MPa.Maximum vertical displacement is seen in the simulation of 0.2-0.8mm.Further more, PEEK was chosen because of its maximal tensile strength of 95 MPa, temperature range up to 250°C, low temp expansion coefficients and good vacuum properties.PEEK shows the best outgassing performance over time with respect to Vespel, PBI and Meldi [5].

Pitting erosion
Pitting erosion is identified in EMC1 as small point-like holes generated by an electrolytic process.This process is a reaction between the metal non-copper left-overs of the brazing materials and the copper, thereby dissolving the copper.This type or erosion occurs in all cases in still standing water systems [6].The actual repair is fast, but finding the point-like holes is a time consuming effort, especially when the conductors are wrapped in Kapton.To remedy this type of erosion, a small water flow is created and is present at all times.

Flow erosion
This type of erosion occurs in sharp corners of the old EMC1 design and is identified on x-ray images.Small particles, seen in the residue of the cooling water, erode copper material away over time.In the new EMC1-U design, all corners have been made smooth due to the chosen production methodology.

Results and discussion
The initial choice of pursuing a bent coil system led to a few challenging consequences.One consequence is having four bending machines for 4 different sized conductors.Some conductors needed to be bent over two directions of freedom.This increased the amount of tools needed for the project.Furthermore, the entrance EFB boundary rotated by 15 degrees.Copper bridges warm up to 5°C with respect to the conductor.The quality OF-OK is sufficient, and there is no need for OFE-OK.Erosion-wise, there are only smooth turns and erosion by friction is expected to be small.Even more, the erosion due to electrolytic process is reduced as the joints are made by pressed fit connections.

Conclusion
EMC1 is successfully replaced by a redesign.The new production methodology leads to 75% less brazed joints in the coil sets.The coil sets are supported and isolated by a PEEK structure.Furthermore, less erosion is expected due to the methodology used.The methods allow in-house repair and servicing of the device.As of March 2023, the new EMC1-U has been in operation for almost two years without issues.

Figure 1 .
Figure 1.Schematic view of the three coils of EMC1: (B) dipole, (G) quadrupole and (C) first harmonic corrector.Magnetically, the only difference with the old design is the magnetic field shape at the entrance and exit of the channel as the route of the current conductors significantly changed, see Figure2.These changes were evaluated by using the results of two COMSOL simulations in which the magnetic field as function of position in the old design and the new design were simulated.For both designs the vertical magnetic flux as function of position was simulated and a line integral was calculated to find the effective field boundary points (EFB).

Figure 3 .
Figure 3. COMSOL calculation of the temperature of a current bridge conducting 1500A while the conductors are water cooled.

Figure 4 .
Figure 4. Photo of the bended quadrupole coil supported by a PEEK structure.

Figure 5 .
Figure 5. Joint design: a) through-cut of a brazed joint for inspection, b) schematic drawing of the joint design.