# SNS Extraction Kicker System and First Article bPFN Test\*

J. Mi, C. Pai, D. Davino, H. Hahn, R. Lambiase, Y.Y. Lee, W. Meng,

J. Sandberg, N. Tsoupas, W. Zhang, D. Warburton.

Brookhaven National Laboratory, Upton, NY 11973, U.S.A.

Abstract

The Spallation Neutron Source (SNS) extraction kicker system brings the
proton beam from the accumulator ring through a beam transfer line into
the target area. The 14 kicker magnets are located in one straight
section. The kicker magnets are energized by 14 Blumlein type Pulse
Forming Networks (BPFN). The first article of the SNS extraction kicker
BPFN was assembled and tested at this laboratory. This paper describes
the kicker BPFN system arrangement and parameters. The first article
BPFN design and its main components used are explained. High voltage
BPFN test results and the load current waveform are illustrated in this
paper. Temperature measurements of the kicker ferrite blocks at full
power showed only small or no heating. This paper discusses the
modifications to the BPFN design, such as a saturating inductor and 25 Ω
termination, to minimize the transverse coupling impedance.

## 1 Extraction kicker system introduction

The Spallation Neutron Source (SNS) extraction kicker system consists of
14 ferrite magnets, 14 high voltage pulse modulators (HVPM) and
auxiliary power supplies. 14 ferrite magnets are separated into two
groups. There are 7 kicker magnets in each group. The two magnet groups
will be installed in one of accumulate ring straight section. (See
Fig. 1) 14 HVPM are installed in the PFN building.

![](media/image1.jpeg){width="3.05in" height="2.2090277777777776in"}

Figure 1: Extraction kicker magnets location \[1\].

Each of the 14 kicker magnets is energized by one HVPM, which is built
as a Blumlein type Pulse Forming Networks (BPFN). Each magnet is
energized by one HVPM through two 50 Ω coaxial cables connected in
parallel, which transfers the pulse current from the BPFN to magnet
situated in accumulator ring tunnel. The coaxial cable length is about
375 feet long.

The service building is next to the PFN building. 14 auxiliary power
supplies are positioned in the service building. Each auxiliary power
supply consists of a high voltage charging power supply, thyratron
filament and reservoir power supplies and others. The high voltage
charging power supply charges the PFN capacitor bank. The filament and
reservoir power supply heats the thyratron in the PFN tank. Several
power supply control units are positioned in the service building.

## 2 kicker magnet parameter and PFN main design data

According to the beam dynamic requirements and ring beam line layout, 14
ferrite kicker magnets can kick the beam from the accumulate ring
through a beam transfer line into the target area. The 14 kicker magnet
parameters are listed in table 1. The BPFN parameters are listed in
table 2.

![](media/image2.pct){width="3.25in" height="2.160416666666667in"}

Table 1: Kicker magnet parameters.

![](media/image3.pct){width="3.25in" height="1.4076388888888889in"}

Table 2: BPFN design parameter requirement

![](media/image4.pct)

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## 3 first article Bpfn design

### 3.1 BPFN Design

In order to reduce maintenance radiation risk and to maintain the kicker
system easily, the PFN is moved from the accumulator ring tunnel to the
PFN building. A Blumlein type Pulse Forming Networks (BPFN) was chosen
as the High Voltage Pulse Modulator. Two 50 Ω coaxial cables connected
in parallel transfer current pulse to the magnet from BPFN. Figure 2
shows the Blumlein type Pulse Forming Network circuit principle. Two
PFNs with 6.25 Ω impedance are connected in series as BFPN. A high power
fast switch thyratron is picked to discharge the energy stored in the
BPFN. A 25 Ω high power resistor, Rm is connected across the BPFN output
to the two 50 Ω cables. This resistor has two functions. One is to match
the two 50 Ω coaxial cables in order to present a 25 Ω resistor as
required for the damping of the coupling impedance resonance in the
kicker magnet. The second function is to absorb the whole BPFN energy. A
saturating inductor, Ls is inserted between the matching resistor Rm and
BPFN output. This saturating inductor has two functions. One function is
to shorten the magnet pulse current rise time \[3\], and the other is to
isolate the coupling between the BPFN and the 25 matching resistor when
the beam is accumulating in the ring, in order to minimize the beam
transverse coupling impedance.

3.2 BPFN Main parts Chosen

3.2.1 BPFN Capacitor

Two PFNs with 6.25 Ω impedance are connected as Blumlein type pulse
forming network. Each PFN consists of a 15 sections of LC network. A
high voltage capacitor is chosen as the energy storage capacitor. The
PFN capacitors are made by General Atomics, with a very low inductance,
around 15nH. The cap capacitance is 5nF. The voltage rating is 50KV with
a 50% reversal voltage.

3.2.2 High Power Fast Switch

A Marconi thyratron, model CX1925X, (Figure 3) was chosen as the high
power fast switch to discharge energy from the BPFN into the matching
resistor Rm and two coaxial cables.

Figure 2: BPFN diagram

![](media/image5.jpeg){width="3.0in" height="2.25in"}

Figure 3: CX1925X thyratron picture.

3.2.3 High Power Resistor

The 25 Ω matching resistor, Rm consists of twelve ceramic resistor
discs. Each disc is 125 mm in diameter and 25.4 mm thick. The resistor
stack is shown in figure 4.

![](media/image6.jpeg){width="3.0in" height="3.329861111111111in"}

Figure 4: High power resistor stack picture

3.2.4 Saturating Inductor

An assembly of ten-ferrite toroids is positioned at the BPFN output.
Each ferrite toroid has a 3.9 in. OD, a 2.1 in. ID, and one half inch
thick. The ferrite Initial Permeability, μ ~i~ is 2300. The Flux
Density, B is 4800G at 5 oersted. The ferrite toroids stack is shown at
figure 5.

![](media/image7.jpeg){width="2.75in" height="2.1770833333333335in"}

Figure 5: Saturating inductor ferrite toroids stack picture

3.2.5 High Voltage Charging Power Supply

A capacitor charging power supply, model ALE 802L is used to charge the
BPFN. This charging power supply rated 50kV can charge the BPFN to 40kV
in a 60Hz pulse repetition frequency.

3.3 First Article BPFN Assembly

The BPFN is sitting in a rectangular tank. A Dow Corning 561 silicone
Transformer Liquid is used for the BPFN cooling. It provides a
combination of dependable safety features and high-performance
characteristics. Dow Corning 561 silicone transformer liquid is
heat-stable dielectric coolant featuring much greater fire and oxidation
resistance than mineral oils, without the environmental hazards of PCB-
based askaers \[2\].

The rectangular BPFN tank is cooled by pumping silicon fluid through a
water-cooled heater exchanger. The BPFN assembly and tank picture is
shown in figure 6.

![](media/image8.jpeg){width="3.2in" height="2.40625in"}

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Figure 6: BPFN tank assembly picture.

## 4 test result 

4.1 BPFN Tank High Voltage Test

The BPFN assembly has been running for more than 200 hours at an
operating condition of 35kV, 2.5kA and 60Hz repetition. A continuous 100
hour running without stop has been reached. The assembly has been tested
at 40kV for 8 hours. There were no any broken parts found after these
test when the tank was opened.

4.2 Magnet Load Current Waveform

A load current waveform is shown in figure 7. The test voltage is 35kV
and the load peak current is 2.5kA. The current pulse flat top time is
about 780nS. By using the saturating inductor for steeping the current
pulse rise, the load current pulse rise time is reduced by about 20 nS.
The measured pulse current rise time from 1% to 95 % is less than 200nS.
The pulse flat top ripple is about ±2.5 %.

![](media/image9.png){width="2.5in" height="1.8694444444444445in"}

Figure 7: Magnet current pulse waveform,

1kA/div and 0.2us/div

*4.3 Kicker Magnet Ferrite Temperature Test*

The type of ferrite used in the prototype kicker magnet is CMD5005. In
order to understand the heat loss in the ferrite blocks, a ferrite heat
loss measurement test was performed. The test was performed at full
power, 35kV 2.5kA and 60Hz repetition. The test result shows only small
or non-measurable heat created at the CMD5005 ferrite blocks.
Consequently, no water cooling structure is need for the kicker magnet.

## 5 conclusion and thanks

The SNS Extraction Kicker first article has been assembled and tested.
The test results meet the requirement. The authors would like to thank
K. Hartmann, S. Perlstein, J. Addessi, R. Zapasek and others who helped
with the PFN assembly and test.

## 6 reference 

\[1\] J. Wei, " Preliminary Change Request for SNS Ring Hybrid Lattice",
BNL/SNS TECHNICAL NOTE, NO 066, October 12, 1999.

\[2\] "Information About 561 Transformer Fluid", Dow Coring Product
Information

\[3\] G. Wait etc, "The Application of Saturating Inductors for
Improving the Performance of the CERN PS Kicker System". CERN