- Email: [email protected]
IMPELA electron accelerators for industrial radiation processing
Radiat. Phys. Chem. Vol. 35, Nos 4-6, pp. 619~i26, 1990 Int. J. Radiat. Appl. lnstrum., Part C Printed in Great Britain
0146-5724/90 $3.00 + 0.00 Pergamon Press plc
IMPELA Electron Accelerators for Industrial Radiation Processing
Gerald E.
Hare
Atomic Energy of Canada Ltd, Accelerator Business Unit, 436 B Hazeldean Road, Kanata, Ontario, Canada K2L IT9
ABSTRACT IMPELA electron accelerators are derived from a common basic design of rf accelerating structure which is capable of handling beams with powers from 20 to 250 kW at 5 to 18 MeV. A prototype has been built which operates at 50 kW and I0 MeV. The paper describes the major elements of the system with particular reference to features which assist in maintaining irradiation quality, simple operation and high reliability. A cost model based on the prototype is used to demonstrate the economies of scale available and the impact of local prices for utilities.
KEYWORDS Irradiation Equipment; electron accelerators; controls; electron accelerator power.
quality assurance; electron accelerator
INTRODUCTION Electron beam equipment is used in the majority of the industrial applications of radiation and there has been a steady growth in the new applications being found to be technically and economically attractive. Most industrial electron beam machines are low energy units (less than 1 MeV) and are used to treat thin products, but there is growing interest in the processing of thicker and more dense products. Interest also exists in well established applications such as the sterilisation of some medical products which can experience unacceptable material damage if subjected to the slower processing inherent in gam~na sterilisation. Another area of growth comes from the rapidly accelerating demand for new plastic materials as this area outstrips the steel industry as a supplier of bulk and fabricated items to an ever increasing number of users. Since many of these applications require the high volume processing of thick items, machines are needed which can deliver high powers at energies of 7 to 15 MeV. All but a few of the electron beam machines at present being used in industrial processing are DC devices; that is, use constant accelerating voltages and provide continuous output. Their practical upper limit in voltage is 5 MeV and they are large but they can provide high power outputs and have simple controls. Rf accelerators can easily produce energies much higher than 5 MeV and they are much smaller devices but they require the control of more parameters. Industrial rf machines with energies above 5 MeV have not so far demonstrated an ability to provide powers above about 20 kW. By using high power rf accelerator structures developed in nuclear breeding programs and industrial prograramable logic controllers, it is now possible to extend the range of reliable operation of electron beam equipment to powers above i00 kW and energies up to 18 MeV (Ungrin et al 1988). This paper describes how this is achieved in the INPELA family of electron accelerators.
RPC 354/6~K
619
620
GERALD E. HARE
. Lo.os
./. Eoo.,E.T
CIRCULATOR
CONTROL CONSOLE
"CROWBAR" ~'~
"~-
BEAM ENERGY
270= DEFLECTION MAGNET
Fig.
i.
The Prototype IMPELA Installation
I N D U S T R I A L ACCELERATOR DESIGN PRINCIPLES Electron accelerators for industrial use must be reliable, easy to operate, provide direct and accurate control of all key parameters which effect the quality of irradiation and be easily integrated with the users product handling and quality assurance systems. This means that they should be operating well within the limits of performance of their design and provide direct and redundant monitoring of all key parameters. Cortrol response times must be appropriate to the needs of personnel and machine protection and maintaining the quality of irradiation of each item being processed, and have an easily customised interface to product handling and identification systems. Real time records of all key machine operating parameters must be accessible on demand, automatically. A full power prototype of a 50 kW, i0 MeV IMPELA which addresses these requirements is being tested: Fig. i. HIGH POWER RF STRUCTURES An rf accelerator provides an accelerating gradient for the electron beam by using an rf signal to set up a pattern of electric and magnetic fields in a series of conducting cavities. These fields change at the frequency of the rf signal and, provided the beam bunches are synchronised to the rf fields, continuous acceleration is achieved. The power to set up the field increases as the square of the accelerating gradient (MeV per meter of structure). If high gradients are maintained continuously, very high average powers are needed and large amounts of heat will have to be removed from the accelerating structure. Such a machine is called a continuous wave (CW) device and for these reasons, CW machines are usually long. For practical energies, they are often composed of a number of accelerating structures in series and use multiple rf sources which have to be carefully synchronised in their operation. So long as the appropriate peak power is supplied, the structure will behave in the same way as a CW device, so lower average power systems can be achieved by operating a given cavity design in pulses; the accelerator voltage and peak current will be the same but the average current and power will be lower. This in turn makes cooling easier and the system power supply is smaller. Compact rf accelerators used in radiography and cancer therapy typically operate with beam pulses 4 micro seconds long and repeated at a rate of 200 to 300 times per second. The average current and power is therefore 0.001 of the peak. Since a certain amount of energy is lost irreversibly each time the cavities are excited and de-excited, multiple short pulses are less efficient than fewer longer pulses. The IMPELA system combines the best features of CW and pulsed machines (McKeown, 1985). It operates at a long pulse of 200 microseconds which reduces the impact of irreversible pulse-related effects, and at a pulse repetition rate of 200 to 300 pulses per second the duty factor is 4% to 6%. In normal operation a duty factor of 5% will be used but this can be pushed to a maximum of 7%. The long pulse allows the direct, accurate measurement and control of key beam quality parameters during each pulse rather than having to apply controls based on the running average of the information from a number of pulses. This is
7th International Meeting on Radiation Processing
621
particularly important in high power machines where a large portion of the dose is delivered in each pulse. The cavity design for the IMPELA system is derived from extensive research on structures for accelerating high average electron currents which was part of a nuclear fuel breeding research program. Figure 2 shows details of a design which was tested experimentally to 150 kW/m at 2.45GHz (Labrie, Euteneuer, 1986). These experiments confirmed theoretical calculations that indicated that cavities of this design can handle powers of hundreds of kilowatts per meter without encountering major problems with permanent distortion.(McKeown, Labrie, 1983). Wl. W2 = WEB COOLING CIRCUITS Cl, C2 = CIRCUMFERENTIAL COOLING CIRCUITS
/ /
/
,7"
,
:} ]
W2
;,,
k,
,
,5,,
I / W2
~'~
/ /~ i
AN::2L::ANTEI/NG CAVITY/ COUPLING CAVITY CIRCUMFERENTIAL
COUPLING SLOT
Fig.2.
WEE COOLING
COOLING CHANNEL
CHANNELS
High Power Cavities For Industrial RF Electron Accelerators
The IMPELA system is based on this work and consists of four basic components which can be assembled in various configurations to provide systems which are capable of a range of energies and powers: Fig. 3. UNIT
SlCT~O~S
lU~mO
COUmUEn
mJICYOe
___--
W4~ a.lmbm
ACCK~RATOR O ~ O N S lO M.W.
--
is-~ kwl
I
I J ...... : M.w.
I
]
7
I I i
tzc-2sckW)
Fig. 3.
J
I
• ,0..v
I
. ,..v
The IMPELA System Of Structures
622
GERALD E. HARE
MACHINE CONTROLS A machine intended for routine use in an industrial environment must have a control system that ensures the following needs can be met: - protection of personnel, - protection of the machine, - closed loop control of key performance elements, - easy interfacing to a variety of product handling and data collecting systems - operation by people with the skills normally found in processing industries, - reliable operation in an industrial environment. Each of these items has special requirements and in a number of cases they are addressed by specially designed control circuits. There is also a large central monitoring and directing function and on IMPELA this is handled by a proven, high capacity, programmable industrial controller: Fig 4. In addition, this centralisation of data in digital form provides a powerful and flexible system for archiving and transmitting key information and allows interconnection to other digital systems through standard interfaces (Lawrence, et al, 1988).
Fig. 4.
The IMPELA Programmable Logic Controller
Safety The personnel safety function is covered by interlocks on high voltage cabinets and there is provision for the connection of external interlocks from monitors and keyed systems associated with the facility. These interlocks are monitored by the central controller and the status of the elements is displayed for the operator on the control console. They act directly through dedicated hardware, however, to shut off or inhibit the operation of the machine.
7th International Meeting on Radiation Processing
623
Machine Protection Certain malfunctions can occur that could cause damage to the machine unless the beam and rf signal can be switched off and/or the rf energy dumped within the time of a single rf pulse (200 microseconds). These critical items form part of a fast response (3~ second) interlock loop which reacts in the appropriate way for each type of malfunction. If external high voltage arcs occur in the modulator or in or on the main klystron, system stored energy is dumped by releasing the energy in the main capacitor bank. This is done by a "crowbar" system which initiates a controlled discharge between electrodes in a closed cabinet using a signal to a triggering electrode. Arcs can also occur within the waveguide which conducts the power to the accelerating structure. In this case it is not necessary to dump the power, but the rf power must be switched off. To achieve this, the power to the input cavity of the klystron is shunted to ground by a PIN diode. Finally, since a single pulse contains 200 Joules of energy the accelerator vacuum envelope and structure can be damaged by misdirected beam pulses. If the various beam current detectors indicate that the beam is b e i n g l o s t in any section of the system, the fast response circuit inhibits the electron gun emission. The fast response circuit is also triggered if the watchdog timer in the controller indicates a failure of the controller system. Beam Quality The key parameters for ensuring the quality of the irradiation are the beam energy, the beam current, the operation of the beam scanning system and the control of the conveyor speed. All of these are maintained at specified operating levels by feedback control loops. The energy spectrum is a fundamental property of the rf field distribution. The most critical parameter is the energy. Two control systems can be used to ensure that this is accurately maintained. In their basic form, IMPELA's inject the beam into the beam scanning system without further deflection, that is, the scan horn is in line with the accelerator axis. An advantage of the long pulse is that a fast response control loop can sample the ~f field in the accelerating cavities during the pulse and adjust the drive amplitude to the klystron input cavity to keep the field constant with respect to a reference voltage supplied by the central controller. If the layout of the irradiator makes it necessary to incorporate a right angle bend in the beam line, a more direct and redundant energy control can be added by using a 270~ bending magnet and adjusting the signal from the controller in response to the current received on an energy defining scraper in the magnet chamber: Fig. 5.
Linac
~Amplltude
ControlLoop
Slow,EnergySetpolntControlLoop Fig. 5.
I EnergyCurrent Measurement
The Energy Control Loop
Correct operation of the accelerator also requires that the frequency of the rf power is correctly matched to the resonant frequency of the structure. Since this can vary during operation due to the structure heating up, the resonant operating frequency of the structure is detected by observing the phase of the rf power in the structure relative to the phase of the signal in the wavequide and any shift in phase generates a signal which is used to adjust the frequency of the output of the voltage controlled oscillator. Beam current is measured by detector coils at the gun, at the exit from the accelerator structure and just before the scanning magnet (Fig. 6). Comparing these signals gives diagnosis of beam transmission and warning of a misaligned beam ( used in the fast response circuit referred to above) and the signal from the last detector is used to
624
GERALD E. HARE
regulate the current emitted from the gun cathode by adjusting its on state voltage. running average of this current signal is also used to regulate the output level of a signal which can be applied for control of the conveyor speed.
Fig. 6.
Beam Position And Current Detection Elements
Alignment of the beam as it enters the scanning system is detected by a system of four "finger" probes and the scan amplitude is controlled by signals from beam detectors at each end of the scan. Data Access On the prototype, the central controller monitors over 200 signals and can provide an output of as many as the user requires at an RS 232 port or via a proprietary software package called "FIX" to IBM PC compatible computers. An almost "real time" picture of the machine can be obtained if the data requirement is limited to I0 to 15 key items and parametric release of products can be supported by a box by box or package by package set of records. If more data is required the recording process will be slower. Operator Interface Machine operation for normal processing is through a keyboard in response to prompts and status messages on a video screen. For safety reasons, full automatic power-up is not employed. The machine requires operator authorisation to proceed at various stages of the powering up to allow for critical safety and process checks (such as the loading of the conveyor and status of the quality assurance system). For trouble shooting, information is available at the video screen ranging from a simple indication of interlock status to more advanced displays of the performance of various control loops. Access to the more advanced levels of information may be limited to qualified maintenance and service staff. RELATIVE CONTRIBUTION TO COSTS A meaningful absolute calculation of full operating costs for any facility can only be carried out if a detailed design is available and full account is taken of local construction requirements and rates, utility charges and tax benefits. Each type of radiation equipment has a particular cost profile however and it may be useful to examine two aspects of processing costs as they relate to the use of rf accelerators. In Fig. 7, the relative contribution to the cost per megarad tonne is shown for a series of facilities using rf accelerators of different powers; all capital being paid back over i0 years at 12% interest and utility charges being 5 cents (Canadian) per kWh for power and 30 cents per i000 L for water. No allowance has been made for tax remissions on capital costs and it is assumed that the plant operates for 6000 hours per year. It is evident that there are major economies of scale in going from 20 kW systems to i00 kW systems.
7th International Meeting on Radiation Processing
625
Utilities Labour _..._...,.--
Maintenance, Service and Parts
Accelerator
Space Shielding
20
50
100 150 kW of Beam Power
200
250
Fig. 7. Relative Contribution To Cost Per Megarad Tonne of Major Cost Components (Electricity at 5C/kWh) It is important tD be aware of the impact of utility costs on these figures. The cost of 5 cents (Canadian) per kWh is among the lowest available and electricity charges can be much higher than this. In Fig. 8, the relative contribution of electric power charges to the processing cost is shown at 30 cents per kWh. The relative contribution can be high and may reach over 40% of the total for high power systems.
50
20
100
150
200
250
kW of Beam Power
Fig. 8. Relative Contribution To Cost Per Megarad Tonne of Major Cost Components (Electricity at 30C/kWh.)
CONCLUSION Rf accelerators using a combination of long pulse operation and cavity designs which ensure efficiency and stable field distributions at high power dissipation can deliver high power and high energy beams for industrial users. Coupling these to proven up-todate industrial controls provides a system which is easy to operate, reliable and can easily be interfaced to any customers specific control and monitoring needs. As a result, industrial radiation processing can be extended to a wide variety of applications where
626
GERALD E. HARE
Fig. 9.
The Prototype
50kW 10MeV IMPELA
relatively thick products must be treated in high volumes and at high speed and electron beam processing can be considered as an alternative for many existing applications where the benefits of using electrons are required but the product is too thick or dense for existing lower energy machines. A full 50 kW i0 MeV device embodying these principles River Laboratories and is undergoing tests: Fig. 9.
has been built at
AECL's Chalk
REFERENCES Labrie, J-P., H. Euteneuer (1986). Power Handling capability of water cooled CW Linac structures. Nucl. Inst. & Methods A 247 (1986), 281 Lawrence, C.B., S.T. Craig, J-P. Labrie, S. Lord, and B.F. White (1988). Control System. Proceedings of European Particle Accelerator Conference published by World Scientific in May 1989.
The Impela (EPAC 88). To be
McKeown, J. (1985). A new generation of intense radiation sources (1985). Sixth Annual Conference of the Canadian Nuclear Society. June 2-4 1985.
Invited paper~
McKeown, J., J-P. Labrie (1983). Heat transfer, thermal stress analysis and the dynamic behaviour of high power rf structures. IEEE Trans~ Nucl. Sci. NS-30m No.4, 1983. Ungrin, J., N.H. Drewell, N.A. Ebrahim, J-P. Labrie, C.B. Lawrence, V.A. Mason, and B.F. White (1988). Impela: an industrial accelerator family. Proceedings of European Particle Accelerator conference (EPAC 88). To be published by World Scientific in May 1989.
0146-5724/90 $3.00 + 0.00 Pergamon Press plc
IMPELA Electron Accelerators for Industrial Radiation Processing
Gerald E.
Hare
Atomic Energy of Canada Ltd, Accelerator Business Unit, 436 B Hazeldean Road, Kanata, Ontario, Canada K2L IT9
ABSTRACT IMPELA electron accelerators are derived from a common basic design of rf accelerating structure which is capable of handling beams with powers from 20 to 250 kW at 5 to 18 MeV. A prototype has been built which operates at 50 kW and I0 MeV. The paper describes the major elements of the system with particular reference to features which assist in maintaining irradiation quality, simple operation and high reliability. A cost model based on the prototype is used to demonstrate the economies of scale available and the impact of local prices for utilities.
KEYWORDS Irradiation Equipment; electron accelerators; controls; electron accelerator power.
quality assurance; electron accelerator
INTRODUCTION Electron beam equipment is used in the majority of the industrial applications of radiation and there has been a steady growth in the new applications being found to be technically and economically attractive. Most industrial electron beam machines are low energy units (less than 1 MeV) and are used to treat thin products, but there is growing interest in the processing of thicker and more dense products. Interest also exists in well established applications such as the sterilisation of some medical products which can experience unacceptable material damage if subjected to the slower processing inherent in gam~na sterilisation. Another area of growth comes from the rapidly accelerating demand for new plastic materials as this area outstrips the steel industry as a supplier of bulk and fabricated items to an ever increasing number of users. Since many of these applications require the high volume processing of thick items, machines are needed which can deliver high powers at energies of 7 to 15 MeV. All but a few of the electron beam machines at present being used in industrial processing are DC devices; that is, use constant accelerating voltages and provide continuous output. Their practical upper limit in voltage is 5 MeV and they are large but they can provide high power outputs and have simple controls. Rf accelerators can easily produce energies much higher than 5 MeV and they are much smaller devices but they require the control of more parameters. Industrial rf machines with energies above 5 MeV have not so far demonstrated an ability to provide powers above about 20 kW. By using high power rf accelerator structures developed in nuclear breeding programs and industrial prograramable logic controllers, it is now possible to extend the range of reliable operation of electron beam equipment to powers above i00 kW and energies up to 18 MeV (Ungrin et al 1988). This paper describes how this is achieved in the INPELA family of electron accelerators.
RPC 354/6~K
619
620
GERALD E. HARE
. Lo.os
./. Eoo.,E.T
CIRCULATOR
CONTROL CONSOLE
"CROWBAR" ~'~
"~-
BEAM ENERGY
270= DEFLECTION MAGNET
Fig.
i.
The Prototype IMPELA Installation
I N D U S T R I A L ACCELERATOR DESIGN PRINCIPLES Electron accelerators for industrial use must be reliable, easy to operate, provide direct and accurate control of all key parameters which effect the quality of irradiation and be easily integrated with the users product handling and quality assurance systems. This means that they should be operating well within the limits of performance of their design and provide direct and redundant monitoring of all key parameters. Cortrol response times must be appropriate to the needs of personnel and machine protection and maintaining the quality of irradiation of each item being processed, and have an easily customised interface to product handling and identification systems. Real time records of all key machine operating parameters must be accessible on demand, automatically. A full power prototype of a 50 kW, i0 MeV IMPELA which addresses these requirements is being tested: Fig. i. HIGH POWER RF STRUCTURES An rf accelerator provides an accelerating gradient for the electron beam by using an rf signal to set up a pattern of electric and magnetic fields in a series of conducting cavities. These fields change at the frequency of the rf signal and, provided the beam bunches are synchronised to the rf fields, continuous acceleration is achieved. The power to set up the field increases as the square of the accelerating gradient (MeV per meter of structure). If high gradients are maintained continuously, very high average powers are needed and large amounts of heat will have to be removed from the accelerating structure. Such a machine is called a continuous wave (CW) device and for these reasons, CW machines are usually long. For practical energies, they are often composed of a number of accelerating structures in series and use multiple rf sources which have to be carefully synchronised in their operation. So long as the appropriate peak power is supplied, the structure will behave in the same way as a CW device, so lower average power systems can be achieved by operating a given cavity design in pulses; the accelerator voltage and peak current will be the same but the average current and power will be lower. This in turn makes cooling easier and the system power supply is smaller. Compact rf accelerators used in radiography and cancer therapy typically operate with beam pulses 4 micro seconds long and repeated at a rate of 200 to 300 times per second. The average current and power is therefore 0.001 of the peak. Since a certain amount of energy is lost irreversibly each time the cavities are excited and de-excited, multiple short pulses are less efficient than fewer longer pulses. The IMPELA system combines the best features of CW and pulsed machines (McKeown, 1985). It operates at a long pulse of 200 microseconds which reduces the impact of irreversible pulse-related effects, and at a pulse repetition rate of 200 to 300 pulses per second the duty factor is 4% to 6%. In normal operation a duty factor of 5% will be used but this can be pushed to a maximum of 7%. The long pulse allows the direct, accurate measurement and control of key beam quality parameters during each pulse rather than having to apply controls based on the running average of the information from a number of pulses. This is
7th International Meeting on Radiation Processing
621
particularly important in high power machines where a large portion of the dose is delivered in each pulse. The cavity design for the IMPELA system is derived from extensive research on structures for accelerating high average electron currents which was part of a nuclear fuel breeding research program. Figure 2 shows details of a design which was tested experimentally to 150 kW/m at 2.45GHz (Labrie, Euteneuer, 1986). These experiments confirmed theoretical calculations that indicated that cavities of this design can handle powers of hundreds of kilowatts per meter without encountering major problems with permanent distortion.(McKeown, Labrie, 1983). Wl. W2 = WEB COOLING CIRCUITS Cl, C2 = CIRCUMFERENTIAL COOLING CIRCUITS
/ /
/
,7"
,
:} ]
W2
;,,
k,
,
,5,,
I / W2
~'~
/ /~ i
AN::2L::ANTEI/NG CAVITY/ COUPLING CAVITY CIRCUMFERENTIAL
COUPLING SLOT
Fig.2.
WEE COOLING
COOLING CHANNEL
CHANNELS
High Power Cavities For Industrial RF Electron Accelerators
The IMPELA system is based on this work and consists of four basic components which can be assembled in various configurations to provide systems which are capable of a range of energies and powers: Fig. 3. UNIT
SlCT~O~S
lU~mO
COUmUEn
mJICYOe
___--
W4~ a.lmbm
ACCK~RATOR O ~ O N S lO M.W.
--
is-~ kwl
I
I J ...... : M.w.
I
]
7
I I i
tzc-2sckW)
Fig. 3.
J
I
• ,0..v
I
. ,..v
The IMPELA System Of Structures
622
GERALD E. HARE
MACHINE CONTROLS A machine intended for routine use in an industrial environment must have a control system that ensures the following needs can be met: - protection of personnel, - protection of the machine, - closed loop control of key performance elements, - easy interfacing to a variety of product handling and data collecting systems - operation by people with the skills normally found in processing industries, - reliable operation in an industrial environment. Each of these items has special requirements and in a number of cases they are addressed by specially designed control circuits. There is also a large central monitoring and directing function and on IMPELA this is handled by a proven, high capacity, programmable industrial controller: Fig 4. In addition, this centralisation of data in digital form provides a powerful and flexible system for archiving and transmitting key information and allows interconnection to other digital systems through standard interfaces (Lawrence, et al, 1988).
Fig. 4.
The IMPELA Programmable Logic Controller
Safety The personnel safety function is covered by interlocks on high voltage cabinets and there is provision for the connection of external interlocks from monitors and keyed systems associated with the facility. These interlocks are monitored by the central controller and the status of the elements is displayed for the operator on the control console. They act directly through dedicated hardware, however, to shut off or inhibit the operation of the machine.
7th International Meeting on Radiation Processing
623
Machine Protection Certain malfunctions can occur that could cause damage to the machine unless the beam and rf signal can be switched off and/or the rf energy dumped within the time of a single rf pulse (200 microseconds). These critical items form part of a fast response (3~ second) interlock loop which reacts in the appropriate way for each type of malfunction. If external high voltage arcs occur in the modulator or in or on the main klystron, system stored energy is dumped by releasing the energy in the main capacitor bank. This is done by a "crowbar" system which initiates a controlled discharge between electrodes in a closed cabinet using a signal to a triggering electrode. Arcs can also occur within the waveguide which conducts the power to the accelerating structure. In this case it is not necessary to dump the power, but the rf power must be switched off. To achieve this, the power to the input cavity of the klystron is shunted to ground by a PIN diode. Finally, since a single pulse contains 200 Joules of energy the accelerator vacuum envelope and structure can be damaged by misdirected beam pulses. If the various beam current detectors indicate that the beam is b e i n g l o s t in any section of the system, the fast response circuit inhibits the electron gun emission. The fast response circuit is also triggered if the watchdog timer in the controller indicates a failure of the controller system. Beam Quality The key parameters for ensuring the quality of the irradiation are the beam energy, the beam current, the operation of the beam scanning system and the control of the conveyor speed. All of these are maintained at specified operating levels by feedback control loops. The energy spectrum is a fundamental property of the rf field distribution. The most critical parameter is the energy. Two control systems can be used to ensure that this is accurately maintained. In their basic form, IMPELA's inject the beam into the beam scanning system without further deflection, that is, the scan horn is in line with the accelerator axis. An advantage of the long pulse is that a fast response control loop can sample the ~f field in the accelerating cavities during the pulse and adjust the drive amplitude to the klystron input cavity to keep the field constant with respect to a reference voltage supplied by the central controller. If the layout of the irradiator makes it necessary to incorporate a right angle bend in the beam line, a more direct and redundant energy control can be added by using a 270~ bending magnet and adjusting the signal from the controller in response to the current received on an energy defining scraper in the magnet chamber: Fig. 5.
Linac
~Amplltude
ControlLoop
Slow,EnergySetpolntControlLoop Fig. 5.
I EnergyCurrent Measurement
The Energy Control Loop
Correct operation of the accelerator also requires that the frequency of the rf power is correctly matched to the resonant frequency of the structure. Since this can vary during operation due to the structure heating up, the resonant operating frequency of the structure is detected by observing the phase of the rf power in the structure relative to the phase of the signal in the wavequide and any shift in phase generates a signal which is used to adjust the frequency of the output of the voltage controlled oscillator. Beam current is measured by detector coils at the gun, at the exit from the accelerator structure and just before the scanning magnet (Fig. 6). Comparing these signals gives diagnosis of beam transmission and warning of a misaligned beam ( used in the fast response circuit referred to above) and the signal from the last detector is used to
624
GERALD E. HARE
regulate the current emitted from the gun cathode by adjusting its on state voltage. running average of this current signal is also used to regulate the output level of a signal which can be applied for control of the conveyor speed.
Fig. 6.
Beam Position And Current Detection Elements
Alignment of the beam as it enters the scanning system is detected by a system of four "finger" probes and the scan amplitude is controlled by signals from beam detectors at each end of the scan. Data Access On the prototype, the central controller monitors over 200 signals and can provide an output of as many as the user requires at an RS 232 port or via a proprietary software package called "FIX" to IBM PC compatible computers. An almost "real time" picture of the machine can be obtained if the data requirement is limited to I0 to 15 key items and parametric release of products can be supported by a box by box or package by package set of records. If more data is required the recording process will be slower. Operator Interface Machine operation for normal processing is through a keyboard in response to prompts and status messages on a video screen. For safety reasons, full automatic power-up is not employed. The machine requires operator authorisation to proceed at various stages of the powering up to allow for critical safety and process checks (such as the loading of the conveyor and status of the quality assurance system). For trouble shooting, information is available at the video screen ranging from a simple indication of interlock status to more advanced displays of the performance of various control loops. Access to the more advanced levels of information may be limited to qualified maintenance and service staff. RELATIVE CONTRIBUTION TO COSTS A meaningful absolute calculation of full operating costs for any facility can only be carried out if a detailed design is available and full account is taken of local construction requirements and rates, utility charges and tax benefits. Each type of radiation equipment has a particular cost profile however and it may be useful to examine two aspects of processing costs as they relate to the use of rf accelerators. In Fig. 7, the relative contribution to the cost per megarad tonne is shown for a series of facilities using rf accelerators of different powers; all capital being paid back over i0 years at 12% interest and utility charges being 5 cents (Canadian) per kWh for power and 30 cents per i000 L for water. No allowance has been made for tax remissions on capital costs and it is assumed that the plant operates for 6000 hours per year. It is evident that there are major economies of scale in going from 20 kW systems to i00 kW systems.
7th International Meeting on Radiation Processing
625
Utilities Labour _..._...,.--
Maintenance, Service and Parts
Accelerator
Space Shielding
20
50
100 150 kW of Beam Power
200
250
Fig. 7. Relative Contribution To Cost Per Megarad Tonne of Major Cost Components (Electricity at 5C/kWh) It is important tD be aware of the impact of utility costs on these figures. The cost of 5 cents (Canadian) per kWh is among the lowest available and electricity charges can be much higher than this. In Fig. 8, the relative contribution of electric power charges to the processing cost is shown at 30 cents per kWh. The relative contribution can be high and may reach over 40% of the total for high power systems.
50
20
100
150
200
250
kW of Beam Power
Fig. 8. Relative Contribution To Cost Per Megarad Tonne of Major Cost Components (Electricity at 30C/kWh.)
CONCLUSION Rf accelerators using a combination of long pulse operation and cavity designs which ensure efficiency and stable field distributions at high power dissipation can deliver high power and high energy beams for industrial users. Coupling these to proven up-todate industrial controls provides a system which is easy to operate, reliable and can easily be interfaced to any customers specific control and monitoring needs. As a result, industrial radiation processing can be extended to a wide variety of applications where
626
GERALD E. HARE
Fig. 9.
The Prototype
50kW 10MeV IMPELA
relatively thick products must be treated in high volumes and at high speed and electron beam processing can be considered as an alternative for many existing applications where the benefits of using electrons are required but the product is too thick or dense for existing lower energy machines. A full 50 kW i0 MeV device embodying these principles River Laboratories and is undergoing tests: Fig. 9.
has been built at
AECL's Chalk
REFERENCES Labrie, J-P., H. Euteneuer (1986). Power Handling capability of water cooled CW Linac structures. Nucl. Inst. & Methods A 247 (1986), 281 Lawrence, C.B., S.T. Craig, J-P. Labrie, S. Lord, and B.F. White (1988). Control System. Proceedings of European Particle Accelerator Conference published by World Scientific in May 1989.
The Impela (EPAC 88). To be
McKeown, J. (1985). A new generation of intense radiation sources (1985). Sixth Annual Conference of the Canadian Nuclear Society. June 2-4 1985.
Invited paper~
McKeown, J., J-P. Labrie (1983). Heat transfer, thermal stress analysis and the dynamic behaviour of high power rf structures. IEEE Trans~ Nucl. Sci. NS-30m No.4, 1983. Ungrin, J., N.H. Drewell, N.A. Ebrahim, J-P. Labrie, C.B. Lawrence, V.A. Mason, and B.F. White (1988). Impela: an industrial accelerator family. Proceedings of European Particle Accelerator conference (EPAC 88). To be published by World Scientific in May 1989.