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Resistor assemblies for NEC accelerators
Nuclear Instruments and Methods in Physics Research A287 (1990) 113-116 North-Holland
RESISTOR ASSEMBLIES * FOR NEC ACCELERATORS D.C. WEISSER
Nuclear Physics Department, Australian National University, Canberra, Australia
Resistor assemblies have been under test in the 14UD Pelletron at the Australian National University in Canberra, Australia since February 1988, with no failures. A full set of resistors was installed in March 1989 and their performances has been satisfactory. The design incorporates coaxial shielding and individual mounting resulting in excellent flexibility in installation and maintenance . A pair of resistors is used to span each insulated gap . The design can be easily adapted to other types of accelerators .
1 . Introduction A new resistor protection assembly, which incorporates the features of several existing designs is described . It is adapted to both the 25 mm pitch of the column and to the 12 .7 mm pitch of the NEC accelerator tube. The individual mounting of resistors for each gap avoids high electric fields between units . Installation, inspection and replacement are also convenient . The resistors are mounted as adjacent pairs which minimises their effective aerial size . Each resistor experiences a cylindrical electric field and is protected from overvoltage by an electrode which combines the function of a decoupling capacitor and a spark gap . Thirty-four resistors were tested for one year in one un:L of the 14UD Pelletron at Canberra and performed excellently . The entire machine was subsequently equipped with production versions of the assemblies .
2. The case for resistors The 14UD Pelletron at the Australian National University ANU, Canberra has operated well over its voltage specification and for more than 5000 h a year, for many years, using a corona point system for voltage distribution . The replacement of such a successful system has only been consid ~-d because of several intrin-
sic problems with corona in SF. : 1) Corona systems, operating in SF6 create ,gaseous n corrosive breakdown products which attack nylon
pellets in Pelletron chains 2) Solids are formed from the corrosion of metals by the breakdown products on corona point tips and on the facing planes . These solids are liberated as free particulates which can detonate sparks . An Australian Provisional Patent with world wide coverage has been applied for . 0168-9002/90/$03 .50 0 Elsevier Science Publishers B .V . (North-Holland)
3) Corona point systems exhibit large variations in voltages across nominally identical gaps due to the sensitivity of the corona current to the sub-millimeter geometry of the point to plane region and to the buildup of solid breakdown products on the corona point tips [2]. 4) Corona point assemblies have a two to three year lifetime in the AN U Pelletron and, although relatively inexpensive to purchase, require substantial
shutdown time to replace . 5) Corona systems in fixed SF, pressure, operate satisfactorily only over ù limited range of gradients . In principle, resistors have none of these disadvantages . However, tLey should be adopted only if they can be protected from spark damage, so that their operational life is of the order of ten years .
3. Spark protection assembly design Resistor protection schemes have now developed to the extent that long lifetimes have been demonstrated in several different installations. The goal of the present effort is to adopt the best features of the successful devices and combine them into assemblies tailored to the geometry of the 14U D Pelletron accelerator . The devices must be cheap tc manufacture, mechanically and electrically reliable and easy to install and inspect . Two field configurations were considered, the planar field, as exemplified in the Daresbury [3] and Hahnn W fS _ 1 Meitner Institute [4] designs, and the coaxial fac ad used at Rochester [5] and Brookhaven [6] . Both field-configurations have demonstrated success in protecting -esistors for long periods . The Daresbury [3,4] installation has been effective for more than four years in a machine with 20 MV sparks and large stored energy . The cylindrical protection devices at Brookhaven have a comparable record . Coaxial protection at Brookhaven replaced a planar system, which was deficient in that it Ill . NEW DEVELOPMENTS
D. C. Weisser / Resistor assemblies for NEC accelerators COLUMN RESISTOR ASSEMBLY WELWYN RESISTOR, 40KV, 982:2%Mi2 RESISTIVE ELEMENT
TITANIUM CAPACITIVE ELECTRODE SPARK GAP
MOUNTING CLAMP
TUBE RESISTOR ASGEMBLY
WELWYN RESISTOR, 20KV, 300'_2%Mt2
Fig. l . Resistor assemblies for both 25 and 12 .7 mm pitches.
allowed resistance values to change. This might be taken to imply that the coaxial geometry afforded better protection. It should be pointed out, however, that the planar version at Brookhaven did not employ fully enclosing spark gaps as does the Daresbury design . The present decision to opt for coaxial geometry was based in pay t on the Brookhaven experience and, in at least equal measure, on the other advantages of the coaxial option, that is, ease of manufacture, installation and inspection . Fig . 1 shows ANU resistor assemblies for the 25 rnrn pitch of the column and the 12 .7 mm pitch of the tube .
The electric field at the ends of the resistors have been regions of high stress leading to failure [4]. Such failures have been eliminated by enclosing the last couple of centimeters of the resistor in a metal electrode. The electrode reduces the electric field at the junction between the resistive element and the brass cap. As well, during transient overvoltage situations, the electrode can couple current into the first few convolutions or turns off the resistive element, thus reducing the electric stress [5] . In the ANU design, the combined bracket, resistor mount, shield mount, serves this purpose at one end, and a titanium electrode performs the function at the other end (see fig. 1) . The ANU design employs a pair of 40 kV, 110 mm long, Welwyn resistors for each column gap with each resistor mounted in its own spark protection assembly.
Thus there are no hitermediate junctions between individual resistors as in the Daresbury design where five 14 kV resistors are connected in line. Such intermediate junctions do not have their field environments controlled by electrodes. Five resistor strings also put more mechanical load on the brass cap to resistive element
junction . The tolerance on the custom-made Welwyn resistors is also specified to ensure that the cantilever mount is
strong and will keep the resistor near to the shield tube axis . This allows the centering of the free-end titanium electrode . A crucial feature of the design is the ability to securely tighten the resistor into its mount and to the titanium electrode, without exerting torque on the joint between the resistor ceramic and the brass end cap. The titanium electrode serves to decouple rf energy around the resistor element and to act as a spark gap to limit the voltage across the resistor. Titanium is chosen
Fig . 2. The resistor assemblies mounted in the 14UD, showing a pair of resistors connected in series by a wire loop, spanning each gap and the large distance achievable between assemblies for adjacent gaps .
D. C.
Weisser / Resistor assemblies for NEC accelerators
to minimise the cantilevered weight and surface spark damage. An important feature of the ANU system is the close mounting of the pair of resistors spanning a gap as seen in fig . 2, which is similar to that employed in the Vivirad installation [6] . During a spark to ground in an accelerator, the resistors can be subjected to at least three Pinds of transient electric overvoltage. Firstly the electromagnetic pulse of the spark can radiatively couple to the resistor in two ways. The transient magnetic field will induce a current proportional to the area enclosed by the resistors and their gap. This area is much smaller for a close mounted pair than for conventional resistor string geometry. Secondly, the transient electric field will induce a current proportional to the length of the resistor configuration. This length is also much shorter for the close mounted pair geometry. Thirdly, a string of resistors must be attached to parts of the accelerator of the order of 300-500 mm apart, for example column posts. During a spark, one post might well break down before the one at the other end of the resistor string. This could put a megavolt transient across the resistors . Close mounting avoids this too . The advantage of the ANU system over the Vivirad configuration is that the pairs of resistor assemblies in adjacent gaps may be mounted far away from one another . Thus there is no problem with high electric fields between components grading adjacent gaps .
Unit 14 of the Canberra 14U D accelerator in February 1988. The rest of the machine, including the accelerator tube in Unit 14, was graded by corona point assemblies. The accelerator was used to test the new compressed geometry tube [7] and achieved 16.7 MV. The machine was then in normal service for experiments, operating at up to 15.5 MV, for the remainder of the year. It suffered many full column sparks. The resistors were removed in March 1989 and their resistances measured both at 5 kV and at 3 V. The results of the 3 V-tests are shown in fig. 3. The resistances of all 34 resistors were within the tolerance of ~- 2%, originally specified. During the test year, new resistor assemblies were designed and manufactured for installation in the entire machine. All 3044 resistors were installed by March 1989 . The full complement of resistors are now operating up to 30 kV on each column resistor and 15 kV on each tube resistor. 5. Summary A resistor protection system that exploits many of the design features of existing devices has been developed and tested. It adds to them essential improventents, to produce assemblies which have demonstrated excellent resistance stability. The concepts have been successfully applied to the entire machine both for the 25 mm pitch column and the 12.7 mm pitch tube.
4. Test results A set of 34, 800 ± 2% MSZ Welwyn resistors were mounted in ANU protection assemblies and installed in Specifications 2%
---t
15
0
N 10 m
0
âm
.0
i 0
~»»viiii~i nww
790
800
Acknowledgements The upgrade program has been made possible by the sacrifice and encouragement of the staff of the Nuclear Physics Department. The leadership and advice of T.R. Ophel was instrumental in th s program. Crucial design contributions have been made by A . Cooper, A. Muirhead, D. Stewart and H. Wallace. The project depended upon the enthusiastic participation of the R.S. Phys. S. workshop and especially P. Akeri, the Manager, and R. Cruickshank, head of numerically controlled machining. The leadership and care of R. Turkentine and the hard work of R. Ball, J. Bockwinkel, A . Harding, A. Hays, J. Heighway, A. Rawlinson and H. Tran were essential ,, the success of the enterprise. It gives the pleasure to acknowledL the cooperation of M. Mitchell and M. Hollis. The consistent interest and support of Professor John Carver is gratefully acknowledged.
810 M a
Fig. 3. ANU resistor test. Measured resistance values after one year of operation in Unit 14. Welwyn special product 'F'-series, 40 kV and T48TU, 50 kV. Terminal voltages up to 16.7 MV . Original specification ± 2% .
References
[l] T.R. Ophel, D.C. Weisser, A. Cooper, L .K. Fifield and G .D. Putt, Nucl . Instr . and Meth. 217 (1983) 383. Ill . NEW DEVELOPMENTS
D.C Weisser / Resistor assemblies for NEC accelerators [2] D.C . Weisser, Nucl . Instr . and Meth . A268 (1988) 419 . [3] T .W . Aitken and R. Thorn, Rev. Phys . Appl . 12 (1977) 1517 . [4] T. Joy, private communication . [5] J .W . Noé, Symp. of North Eastern Accelerator Personnel, 1986, eds . E.D. Berners, U . Garg and C .P . Browne (World Scientific Publ . Co .) p . 168 .
[6] J .C. Oberlin, G . Heng and M . Letournel, Nucl . Instr . and
Meth. A244 (1986) 35 . [7] D.C . Weisser and M.D . Malev, these Proceedings (5th Int . Conf. on Electrostatic Accelerators and Associated Boosters, Strasbourg- Heidelberg, 1989) Nucl . Instr. and Meth. A287 (1990) 64.
RESISTOR ASSEMBLIES * FOR NEC ACCELERATORS D.C. WEISSER
Nuclear Physics Department, Australian National University, Canberra, Australia
Resistor assemblies have been under test in the 14UD Pelletron at the Australian National University in Canberra, Australia since February 1988, with no failures. A full set of resistors was installed in March 1989 and their performances has been satisfactory. The design incorporates coaxial shielding and individual mounting resulting in excellent flexibility in installation and maintenance . A pair of resistors is used to span each insulated gap . The design can be easily adapted to other types of accelerators .
1 . Introduction A new resistor protection assembly, which incorporates the features of several existing designs is described . It is adapted to both the 25 mm pitch of the column and to the 12 .7 mm pitch of the NEC accelerator tube. The individual mounting of resistors for each gap avoids high electric fields between units . Installation, inspection and replacement are also convenient . The resistors are mounted as adjacent pairs which minimises their effective aerial size . Each resistor experiences a cylindrical electric field and is protected from overvoltage by an electrode which combines the function of a decoupling capacitor and a spark gap . Thirty-four resistors were tested for one year in one un:L of the 14UD Pelletron at Canberra and performed excellently . The entire machine was subsequently equipped with production versions of the assemblies .
2. The case for resistors The 14UD Pelletron at the Australian National University ANU, Canberra has operated well over its voltage specification and for more than 5000 h a year, for many years, using a corona point system for voltage distribution . The replacement of such a successful system has only been consid ~-d because of several intrin-
sic problems with corona in SF. : 1) Corona systems, operating in SF6 create ,gaseous n corrosive breakdown products which attack nylon
pellets in Pelletron chains 2) Solids are formed from the corrosion of metals by the breakdown products on corona point tips and on the facing planes . These solids are liberated as free particulates which can detonate sparks . An Australian Provisional Patent with world wide coverage has been applied for . 0168-9002/90/$03 .50 0 Elsevier Science Publishers B .V . (North-Holland)
3) Corona point systems exhibit large variations in voltages across nominally identical gaps due to the sensitivity of the corona current to the sub-millimeter geometry of the point to plane region and to the buildup of solid breakdown products on the corona point tips [2]. 4) Corona point assemblies have a two to three year lifetime in the AN U Pelletron and, although relatively inexpensive to purchase, require substantial
shutdown time to replace . 5) Corona systems in fixed SF, pressure, operate satisfactorily only over ù limited range of gradients . In principle, resistors have none of these disadvantages . However, tLey should be adopted only if they can be protected from spark damage, so that their operational life is of the order of ten years .
3. Spark protection assembly design Resistor protection schemes have now developed to the extent that long lifetimes have been demonstrated in several different installations. The goal of the present effort is to adopt the best features of the successful devices and combine them into assemblies tailored to the geometry of the 14U D Pelletron accelerator . The devices must be cheap tc manufacture, mechanically and electrically reliable and easy to install and inspect . Two field configurations were considered, the planar field, as exemplified in the Daresbury [3] and Hahnn W fS _ 1 Meitner Institute [4] designs, and the coaxial fac ad used at Rochester [5] and Brookhaven [6] . Both field-configurations have demonstrated success in protecting -esistors for long periods . The Daresbury [3,4] installation has been effective for more than four years in a machine with 20 MV sparks and large stored energy . The cylindrical protection devices at Brookhaven have a comparable record . Coaxial protection at Brookhaven replaced a planar system, which was deficient in that it Ill . NEW DEVELOPMENTS
D. C. Weisser / Resistor assemblies for NEC accelerators COLUMN RESISTOR ASSEMBLY WELWYN RESISTOR, 40KV, 982:2%Mi2 RESISTIVE ELEMENT
TITANIUM CAPACITIVE ELECTRODE SPARK GAP
MOUNTING CLAMP
TUBE RESISTOR ASGEMBLY
WELWYN RESISTOR, 20KV, 300'_2%Mt2
Fig. l . Resistor assemblies for both 25 and 12 .7 mm pitches.
allowed resistance values to change. This might be taken to imply that the coaxial geometry afforded better protection. It should be pointed out, however, that the planar version at Brookhaven did not employ fully enclosing spark gaps as does the Daresbury design . The present decision to opt for coaxial geometry was based in pay t on the Brookhaven experience and, in at least equal measure, on the other advantages of the coaxial option, that is, ease of manufacture, installation and inspection . Fig . 1 shows ANU resistor assemblies for the 25 rnrn pitch of the column and the 12 .7 mm pitch of the tube .
The electric field at the ends of the resistors have been regions of high stress leading to failure [4]. Such failures have been eliminated by enclosing the last couple of centimeters of the resistor in a metal electrode. The electrode reduces the electric field at the junction between the resistive element and the brass cap. As well, during transient overvoltage situations, the electrode can couple current into the first few convolutions or turns off the resistive element, thus reducing the electric stress [5] . In the ANU design, the combined bracket, resistor mount, shield mount, serves this purpose at one end, and a titanium electrode performs the function at the other end (see fig. 1) . The ANU design employs a pair of 40 kV, 110 mm long, Welwyn resistors for each column gap with each resistor mounted in its own spark protection assembly.
Thus there are no hitermediate junctions between individual resistors as in the Daresbury design where five 14 kV resistors are connected in line. Such intermediate junctions do not have their field environments controlled by electrodes. Five resistor strings also put more mechanical load on the brass cap to resistive element
junction . The tolerance on the custom-made Welwyn resistors is also specified to ensure that the cantilever mount is
strong and will keep the resistor near to the shield tube axis . This allows the centering of the free-end titanium electrode . A crucial feature of the design is the ability to securely tighten the resistor into its mount and to the titanium electrode, without exerting torque on the joint between the resistor ceramic and the brass end cap. The titanium electrode serves to decouple rf energy around the resistor element and to act as a spark gap to limit the voltage across the resistor. Titanium is chosen
Fig . 2. The resistor assemblies mounted in the 14UD, showing a pair of resistors connected in series by a wire loop, spanning each gap and the large distance achievable between assemblies for adjacent gaps .
D. C.
Weisser / Resistor assemblies for NEC accelerators
to minimise the cantilevered weight and surface spark damage. An important feature of the ANU system is the close mounting of the pair of resistors spanning a gap as seen in fig . 2, which is similar to that employed in the Vivirad installation [6] . During a spark to ground in an accelerator, the resistors can be subjected to at least three Pinds of transient electric overvoltage. Firstly the electromagnetic pulse of the spark can radiatively couple to the resistor in two ways. The transient magnetic field will induce a current proportional to the area enclosed by the resistors and their gap. This area is much smaller for a close mounted pair than for conventional resistor string geometry. Secondly, the transient electric field will induce a current proportional to the length of the resistor configuration. This length is also much shorter for the close mounted pair geometry. Thirdly, a string of resistors must be attached to parts of the accelerator of the order of 300-500 mm apart, for example column posts. During a spark, one post might well break down before the one at the other end of the resistor string. This could put a megavolt transient across the resistors . Close mounting avoids this too . The advantage of the ANU system over the Vivirad configuration is that the pairs of resistor assemblies in adjacent gaps may be mounted far away from one another . Thus there is no problem with high electric fields between components grading adjacent gaps .
Unit 14 of the Canberra 14U D accelerator in February 1988. The rest of the machine, including the accelerator tube in Unit 14, was graded by corona point assemblies. The accelerator was used to test the new compressed geometry tube [7] and achieved 16.7 MV. The machine was then in normal service for experiments, operating at up to 15.5 MV, for the remainder of the year. It suffered many full column sparks. The resistors were removed in March 1989 and their resistances measured both at 5 kV and at 3 V. The results of the 3 V-tests are shown in fig. 3. The resistances of all 34 resistors were within the tolerance of ~- 2%, originally specified. During the test year, new resistor assemblies were designed and manufactured for installation in the entire machine. All 3044 resistors were installed by March 1989 . The full complement of resistors are now operating up to 30 kV on each column resistor and 15 kV on each tube resistor. 5. Summary A resistor protection system that exploits many of the design features of existing devices has been developed and tested. It adds to them essential improventents, to produce assemblies which have demonstrated excellent resistance stability. The concepts have been successfully applied to the entire machine both for the 25 mm pitch column and the 12.7 mm pitch tube.
4. Test results A set of 34, 800 ± 2% MSZ Welwyn resistors were mounted in ANU protection assemblies and installed in Specifications 2%
---t
15
0
N 10 m
0
âm
.0
i 0
~»»viiii~i nww
790
800
Acknowledgements The upgrade program has been made possible by the sacrifice and encouragement of the staff of the Nuclear Physics Department. The leadership and advice of T.R. Ophel was instrumental in th s program. Crucial design contributions have been made by A . Cooper, A. Muirhead, D. Stewart and H. Wallace. The project depended upon the enthusiastic participation of the R.S. Phys. S. workshop and especially P. Akeri, the Manager, and R. Cruickshank, head of numerically controlled machining. The leadership and care of R. Turkentine and the hard work of R. Ball, J. Bockwinkel, A . Harding, A. Hays, J. Heighway, A. Rawlinson and H. Tran were essential ,, the success of the enterprise. It gives the pleasure to acknowledL the cooperation of M. Mitchell and M. Hollis. The consistent interest and support of Professor John Carver is gratefully acknowledged.
810 M a
Fig. 3. ANU resistor test. Measured resistance values after one year of operation in Unit 14. Welwyn special product 'F'-series, 40 kV and T48TU, 50 kV. Terminal voltages up to 16.7 MV . Original specification ± 2% .
References
[l] T.R. Ophel, D.C. Weisser, A. Cooper, L .K. Fifield and G .D. Putt, Nucl . Instr . and Meth. 217 (1983) 383. Ill . NEW DEVELOPMENTS
D.C Weisser / Resistor assemblies for NEC accelerators [2] D.C . Weisser, Nucl . Instr . and Meth . A268 (1988) 419 . [3] T .W . Aitken and R. Thorn, Rev. Phys . Appl . 12 (1977) 1517 . [4] T. Joy, private communication . [5] J .W . Noé, Symp. of North Eastern Accelerator Personnel, 1986, eds . E.D. Berners, U . Garg and C .P . Browne (World Scientific Publ . Co .) p . 168 .
[6] J .C. Oberlin, G . Heng and M . Letournel, Nucl . Instr . and
Meth. A244 (1986) 35 . [7] D.C . Weisser and M.D . Malev, these Proceedings (5th Int . Conf. on Electrostatic Accelerators and Associated Boosters, Strasbourg- Heidelberg, 1989) Nucl . Instr. and Meth. A287 (1990) 64.