Fast-switching magnet serving a spallation-driven ultracold neutron source

A fast-switching, high-repetition-rate magnet and power supply have been developed for and operated at TRIUMF, to deliver a proton beam to the new ultracold neutron (UCN) facility. The facility possesses unique operational requirements: a time-averaged beam current of $40\mu \mathrm{A}$ with the ability to switch the beam on or off for several minutes. These requirements are in conflict with the typical operation mode of the TRIUMF cyclotron which delivers nearly continuous beam to multiple users. To enable the creation of the UCN facility, a beam-sharing arrangement with another facility was made. The beam sharing is accomplished by the fast-switching (kicker) magnet which is ramped in $50\mu \mathrm{s}$ to a current of 193 A, held there for approximately 1 ms, then ramped down in the same short period of time. This achieves a 12 mrad deflection which is sufficient to switch the proton beam between the two facilities. The kicker magnet relies on a high-current, low-inductance coil connected to a fast-switching power supply that is based on insulated-gate bipolar transistors (IGBTs). The design and performance of the kicker magnet system and initial beam delivery results are reported.

Figure 1

Sharing of the main beam from the cyclotron (normal TRIUMF beam) between beamline 1A and beamline 1U. Beam is diverted into beamline 1U by switching on the kicker magnet during the $50–100\mu \mathrm{s}$ notch duration, then switching it off during the next notch. The pulser period is 0.888 ms.

Figure 2

A schematic top view of the proton beamline for the UCN facility at TRIUMF (see also [5]). Beam from the TRIUMF cyclotron enters through beamline 1V from the left side of the figure and proceeds to the right. The proton beam is diverted, as required, into beamline 1U by the kicker and septum magnets. Downstream of the septum, beamline 1U contains bender magnets, quadrupole magnets, and diagnostic elements before ending in the target. The principal elements relevant to the kicker magnet design are discussed further in the text.

Figure 3

Geometry of MedAustron kicker magnet, which served as a reference design for the TRIUMF kicker magnet. The overall length of the MedAustron magnet, including end plates, is $\sim 0.4\mathrm{m}$.

Figure 4

Plot of the required magnet current and estimated magnet voltage to achieve a magnetic field integral of $0.0545\mathrm{T}·\mathrm{m}$, as a function of the total number of turns of the magnet coils with a 1.5 m long yoke. End effects are neglected and $h{(\mathrm{eff})}_{\mathrm{ap}}\times v_{\mathrm{ap}}=120\mathrm{mm}\times 100\mathrm{mm}$ is assumed.

Figure 5

Geometry for a 3D simulation of the UCN kicker magnet. The overall length of the magnet, including end plates, is $\sim 1.71\mathrm{m}$.

Figure 6

Cross section of aperture of kicker magnet: rectangular coil conductors with 2 mm wall (red), insulation (brown), ferrite (green), and $100\mathrm{mm}\times 100\mathrm{mm}$ aperture (white).

Figure 7

Initial design of end ferrites, with cut-out to bend the saddle coils toward the aluminium frame (quarter of geometry shown). The color regions associated with the ferrite represent surface contours of predicted flux density, for a coil current of 240 A.

Figure 8

Simplified electrical schematic of the UCN kicker system (described in the text).

Figure 9

3D engineering drawing of the UCN kicker magnet and connection box. The overall length of the magnet is $\sim 1.71\mathrm{m}$.

Figure 10

Measured postpulse current following 250 A flat-top magnet current: without saturating inductors (blue dashed trace) and with saturating inductors (green solid trace).

Figure 11

Measured postpulse current following 250 A flat-top magnet current, without saturating inductors but with SF5408 fast recovery diodes, in a series/parallel arrangement, connected in series with diode D2.

Figure 12

Measured power-supply waveforms for 250 A demanded flat-top current, with 30 m of cable: Positive-side output voltage (green solid trace); negative-side output voltage (red dotted trace); output current (blue dashed trace).

Figure 13

Measured power supply waveforms for 250 A demanded flat-top current, with 30 m of cable and the magnet replaced with a short circuit: Positive-side output voltage (green solid trace); negative-side output voltage (red dotted trace); output current (blue dashed trace).

Figure 14

Measured rising edge of current pulse to an operating current of 193 A, subsequent to installation at TRIUMF. Dashed blue trace: current. Solid green trace: positive-side output voltage.

Figure 15

Measured flat top of current pulse at an operating current of 193 A. Blue dashed trace: current (ac-coupled measurement with an offset of 193 A added to the average value). Green solid trace: output voltage of low-voltage power supply stage to maintain flat-top current.

Figure 16

Measured falling edge of current pulse from 193 A flat-top.

Figure 17

Measurement with 220 A current, pulsed at 350 Hz with a 1 ms-long flat-top. Dashed blue trace: current. Solid green trace: positive-side output voltage, zoomed in to show the low voltage which maintains the flat-top of the current pulse (described in the text).

Figure 18

EPICS control screen for kicker magnet showing controls for the power supply (top right), controls for the sequencer module (bottom right), flat-top magnet current (top left), and timing parameters (bottom left).

Figure 19

Beam current in beamline 1V as measured with the 1VM4.7 capacitive probe. If the notch is correctly aligned (green solid line) with the time when the kicker magnet is ramping (vertical red lines) no beam is spilled on the septum magnet. A misaligned notch (blue dashed line) will cause significant beam spill.

Figure 20

Signal of beam spill monitor downstream of septum magnet, as a function of the kicker delay with respect to the cyclotron injection pulser. The beam spill signal is expressed as a fraction of the value at which the beam spill monitor will trip the cyclotron.

Figure 21

The current measured by the TNIM (circles) and the number of UCN extracted from the UCN source (boxes) increase linearly with the beam current calculated from the kicker parameters and the current in beamline 1V.