The Control System for a 2 MeV Tandem Accelerator used forContraband Detection
D. Dale, D. Bishop, H. Hui, B. Milton, B. Roberts
TRIUMF, 4004 Wesbrook Mall, Vancouver, B. C., V6T 2A3, Canada
Abstract
The Contraband Detection System (CDS) is a systemdesigned for detection of explosives and other contrabandutilizing Gamma Resonance Absorption (GRA). In theCDS 9.17 MeV gammas are produced by 1.75 MeV
protons interacting with a Carbon 13 target. The 2 MeV
Tandem is capable of producing upto 10mA of C.W.
proton beam. The Control System consists of two AllenBradley SLC500 series Programmable Logic Controllers(PLC). One resides at ground potential and the other islocated inside the Tandem pressure vessel at 1 MegaVolt.
Supervisory control and communication between the two
PLCs is accomplished using the Allen-Bradley Data
Highway Plus and fiber optics. Operation of the system isdescribed.
1 Introduction
TRIUMF and Northrop Grumman have developed a new
system for the detection of concealed explosives and drugs.
The Contraband Detection System (CDS) consists of an HCUSP ion source at 40kV potential, a low energy beamtransport (LEBT), the Tandem accelerator, a high-energy
beam transport (HEBT), a target and detectors. The
Tandem accelerator consists of two subsystems: one at
ground potential, the second biased at the terminal
potential. At ground potential is a variable high voltage
power supply system, a SF6 gas system, and two electron
skimmers located at either end of the Tandem. At highpotential is the terminal, which consists of a gas stripper
system, two sets of triplet quad magnets for beamfocussing, two sets of steering magnets, diagnosticcollimators and the associated vacuum system. TheTandem Control System must also interface to the
Operational Control and Support System, which handles
the LEBT, and HEBT.
The High Voltage Power Supply (HVPS) [6] consists of avariable DC power supply, a resonant mode inverter and atransformer. The DC power supply is a standard 700 VDC
60 A regulated supply. The inverter converts a DC level toa sine wave using a LC resonant circuit with the primary of
the transformer forming part of the inductance. A stack of
80 printed circuit boards on an insulated gap ferrite coreforms the transformer. Each board contains 32 secondaries,
each with a rectifier and doubler. The doubled secondaries
are connected in series to obtain 12.5 kV per board. The 80
boards are then connected in series to achieve 1 MegaVolt.
The high voltage output is adjustable by both the DC
supply voltage as well as the operating frequency of theinverter. At present the prototype Proof of Principle (seeFigure 1.) 2 MeV tandem is in operation on the Northrop
Grumman site in Bethpage New York.
2 Requirements
As for any accelerator, the main requirement for theTandem Control System is to provide the means to produce
and maintain a stable high quality beam. The stability
required for GRA is closely related to target thickness [2].
As shown in Table 1 the energy spread allowed is <12keV
for the Chlorine reaction and < 25keV for the Nitrogen
reaction.
The control system must also provide operator access foropen loop control and diagnostics for all devices. The
architects of the Operational Control and Support System
specified National Instruments LabWindows/CVI control
software. LabWindows/CVI takes LabView a step closer toFigure 1: Elevation View

a full-blown control system by allowing the user to write
application specific “C” code. Since space inside theterminal is extremely limited, duplicating this system wasnot practical.
1435
N(γ,P)13C
Cl(γ,P)34S
Proton Energy (MeV)
1.75
1.89
Energy Spread (keV)<25<12
Spot Size (cm)0.6 x 2.4
0.6 x 2.4
Divergence (mrad)
1212Current (mA)
1012Table 1: CDS requirements for the selected reactions
3 Design and implementation
The control system uses “the standard model” two-layerarchitecture. The supervisory layer consists of a PC
running LabWindows/CVI [4] with a third party driver
HighwayView [5]. The device control layer isaccomplished with two Allen-Bradley SLC5/04
Programmable Logic Controllers (PLC) [3]. One PLC
resides inside the terminal biased at the High Voltage(HVPLC). The second PLC resides at ground potential(GNDPLC). The layers are networked with the AllenBradley Data Highway Plus (DH+). The PLC in theterminal is electrically isolated via Allen-Bradley fiber
optic DH+ modems. Communication between the twoPLCs is accomplished with the Global Status word. A 16bit status word is transmitted over the highway each time
the PLC receives the token. A watchdog system has been
implemented to ensure communication is continuous. Aread block and a write block have been set aside withineach PLC for supervisory control and readback. The SLC
5/04, Allen-Bradley’s top of the line for the SLC500
family, was selected since HighwayView only supports
DH+.
3.1 Supervisory interface
Several techniques have been used to minimize traffic on
the DH+. Status information has been mapped into a blockof contiguous registers. N7:0 – N7:99 for the HVPLC andN7:100 – N7:130 for the GNDPLC. This allows thesupervisory system to obtain all the status informationfrom the PLCs in four highway packets. Similarly all
setpoints and commands are sent to the N10:0 – N10:99(HVPLC) , N10:100 – N10:130 (GNDPLC) registers.
Setpoints and Commands are only sent when changed. The
PLC rungs have been programmed to allow all interlocks
to be bypassed by setting a BYP_{devicename}INT bit to
1. As well each limit switch may be “FORCED” to eitherof it’s states by setting FRC_{devicename}OPEN to 1 or
FRC_{devicename}CLOSED to 1. The supervisory
software must provide the appropriate protection against
unauthorized use.
3.2 HVPLCThe HV PLC is responsible for all the devices in theterminal. These include:•four Quadrapole magnets•
two horizontal steering magnets

••
•
••
•
•
••
•
•
•

two vertical steering magnets
gas leak valve for the stripper+/- 1kV bias power supplies in the stripper
1kV electron suppressor power supply
2 cryo cold heads
convectron vacuum gauge
2 cold cathode vacuum gauges
Turbo pump
2- 4 segment collimators
2 ring collimatorswater circulator
water temperature monitor
The PLC configuration consists of a SLC 10 slot rackwith the SLC5/04 CPU, a scanner module, 4 - 4 channel
thermocouple modules, a 32 bit 24VDC input module anda 16bit high current 24VDC output module. Connected tothe scanner module is an Allen-Bradley Flex I/O terminal
block I/O system which contains a Remote I/O adapter, 3 8 channel 12bit analogue to digital converters and 3 - 4channel 12bit digital to analogue converters. The scannerand the Flex I/O system was selected to conform to spacelimitations. Custom manufactured hardware includes interface printed circuit boards for measuring the cryopump
temperatures, terminal AC voltage and an independentfiber optic PLC power controller.
3.3 Gndplc

The Ground PLC monitors and controls all devices atground potential and the hardwired connections to theOperational and Support Control System of the CDS.
These include:•SF6 pressure, flow and temperature•compressor for the cryopumps•
LEBT vacuum•
HEBT vacuum•
beam kicker enable•
2- 5kV electron skimmer power supplies•High Voltage power supply system•Variac and AC contactor
The PLC also ensures power to the terminal is removed
when the Tandem is evacuated. The ground PLC configuration consists of a SLC 10 slot rack with the SLC5/04
CPU, 3 - 4 channel 16bit analogue to digital converters, 2 4 channel 14 bit digital to analogue converters, a 32 bit
24VDC input module, 2 16 bit high current 24VDC output
modules. Custom hardware consists of an interface/fastloop printed circuit board for the HVPS.
3.4 Control loops
There are three control loops utilized in the Tandem
control system.
1. Stripper Gas PID
A PID loop in the PLC is used to stabilize the stripping gas
pressure. The convectron vacuum gauge readback is monitored and the stripping gas leak valve is adjusted tomaintain a fixed pressure.
2. High Voltage Fast Loop
A proportional loop has been implemented with analogue

circuitry to provide fast regulation of the high voltage
power supply. The circuit monitors the high voltage power
supply via a resistor divider chain along the acceleration
column. This value is compared to the setpoint of theHVPS with the error being limited to +/- 20kV. This error
voltage is then fed to the inverter to adjust the operatingfrequency. The error is limited to keep the inverter
frequency within the operating range for the transformer.
3. High Voltage PID
A PID loop is used to adjust the DC power supply based on
the inverter’s operating frequency. The PID monitors theerror voltage fed to the inverter and adjusts the DC power
supply’s setpoint. This slow control eliminates running forlong periods at frequencies significantly different from thetransformer’s optimum frequency.

The authors would like to thank the CDS accelerator
commissioning team particularly Steve Melnychuk andRoman Ruegg. The authors would also like to thank Mike
LeRoss for his help with the interface boards. This work
was conducted under USAF contract #F0865094C0097.
References
[1]
[2]

[3]

4 Status
The Tandem was installed at the Northrop Grumman
facility in Bethpage N.Y. in February this year. Duringinitial commissioning 9.17 MeV gammas were producedwith a target current of 50µA.
Acknowledgments

[4]
[5]
[6]

J. Sredniawski, “A New Proof-of-PrincipleContraband Detection System”, ONDP Conferencein Nashua N.H., October 1995
B. Milton, et. al., “A High Current Tandem
Accelerator for Gamma-Resonance ContrabandDetection”, SNEAP Conference in Aachen,Germany, October 1997
Automation Products catalog, Allen-BradleyCompany Inc. , Publication B112, Milwaukee,
Wisconsin, USA (http://www.ab.com)
Instrumentation Reference and Catalogue, National
Instruments,Austin,Texas,
USA(http://www.natinst.com)
HighwayView users manual,SEG, Watertown,
Massachusetts USA
Dr. J. Cross, ERDL, Nova Scotia, Canada, private
communication