Permanent magnets for the return loop of the Cornell-Brookhaven energy recovery linac test accelerator

214 neodymium permanent magnets have been manufactured for the fixed-field, multiturn return loop of the Cornell-Brookhaven Energy recovery linac Test Accelerator at Cornell University. There are 5 types of quadrupole and combined-function gradient magnets using a variant of the circular Halbach design. These are made out of NdFeB material and glued into an aluminium housing with water channels for temperature stabilization. The NdFeB wedges and magnet construction were done by outside companies, while the final “tuning” using inserts containing 64 iron rods per magnet was done at BNL over a period of about 6 months. Average relative field errors of $2.2\times {10}^{-4}$ were achieved on the beam region. The magnet strengths vary by type but are of order $10\mathrm{T}/\mathrm{m}$ for quadrupole component and up to 0.3 T for the dipole. This paper reports on the field quality and timeline achieved in this production process.

Figure 1

Permanent magnet cross sections with 1 cm grid.

Figure 2

The fixed-field return loop for the CBETA ERL.

Figure 3

Second arc of the installed CBETA return loop.

Figure 4

Cross section of the quadrupole ‘QF’ magnet. Blue arrows show magnetization direction of the PM blocks. The orange line graphs the midplane field $B_y(x,0)$, with green highlighting the good field region and red showing the beam position range in the FFA. The grid has 1 cm spacing.

Figure 5

Cross section of the BD magnet.

Figure 6

Left: Halbach quadrupole nested inside a dipole. Right: vector sum of magnetization vectors in a single layer.

Figure 7

Measured strength of a QD quadrupole as its temperature is stabilized from room temperature to $29.4°\mathrm{C}$.

Figure 8

Production and testing flow.

Figure 9

(a) QD magnet being measured with a rotating coil at BNL magnet division. (b) BD magnet with tuning rod pack inserted.

Figure 10

Multipole FOM of magnets before and after tuning.

Figure 11

CBETA FOM of magnet types before and after tuning. Lines for $1\times$ and $10\times$ improvement are shown.

Figure 12

Relative field errors before and after tuning.

Figure 13

R&D permanent magnet quadrupole “5A”.

Figure 14

Current sheet model of permanent magnet quadrupole “5A” with correction rod locations and 1 cm radius circle shown at center.

Figure 15

Fraction of magnets completed over time.

Figure 16

Fraction of each magnet type completed over time.