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Belt charging system for the 35 MV Vivitron accelerator
414
Nuclear Instruments and Methods in Physics Research A268 (1988) 414-418 North-Holland, Amsterdam
BELT CHARGING SYSTEM FOR THE 35 MV VIVITRON ACCELERATOR Jean-Marie HELLEBOID Centre de Recherches Nucléaires, BP 20, 67037 Strasbourg-Cedex, France
A Van de Graaff belt charging system has been chosen for the Vivitron. Although classical in its principle and conservative in its design, it includes different new features that will be discussed in detail. The main electrical and mechanical characteristics are also reviewed together with the status of the project .
1. Introduction The Vivitron [1], now being built at the CRN, is a 35 MV tandem electrostatic accelerator for which a charging system is being studied [2,3] . The conceptual design has led us to choose a Van de Graaff type belt system that includes different new features (some of them fundamental), which make of it an original and specific system well suited to the Vivitron. The main electrical and mechanical characteristics have been calculated so as to achieve the desired performance, taking into account various constraints . A test bench is being built on which mechanical and electrical behaviour are to be demonstrated . Detailed realization studies should begin soon.
2. Requisite characteristics and related constraints The charging system of the Vivitron should deliver a current of 500 g.A for terminal voltages up to 35 MV with the usual (or better) stability of large Van de Graaff tandems ; this value is obtained by extrapolation from a current of 320 gA for the MP tandem at 18 MV with a corona current of 40 ILA. It should fit in a transverse electrically and mechanically limited space located between the accelerating tube, the column electrodes and the longitudinal insulating plates (fig. 1). Intermediate fixtures and supports, if needed must also be placed into this transversally confined space in the dead sections (which are only 220 mm long) i.e. at most every 2.82 m. The distance along the machine axis between the terminal and ground is - 25 m on either side of the terminal . Moreover, the system, being inside the column, should include the least number of conducting pieces 0168-9002/88/$03 .50 C Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Fig. 1. Column cross-section and belt location in two successive dead sections: A (left) and B (right). possible in the active (E longitudinal) sections so as to respect general Vivitron philosophy . 3. Choice of a belt charging system Although novel systems can be imagined, it has been judged more reasonable to confine ourselves to those at present used in large tandems . Both a chain and a belt system have been considered and the latter has been retained for numerous reasons of which the main ones are as follows : - extensive experience with a successful belt system developed on the Strasbourg MP tandem ;
J.-M.
T R A N S V E R S E
415
Helleboid / Belt charging system for the 35 MV Vivuron
U N I F 0 R M I T Y
Fig. 2. Measured transverse uniformity of the charge density a deposited at atmospheric air onto a belt by two identical arrangements of different ionisers : either usual stainless steel screens or brushes of 8 pm carbon fibers . modest intensity requirement (I=500 ttA) for the Vivitron compared with 700 uA (2 x 350) that can be obtained from the up and down charge system on the MP ; planned incorporation in the design of different new features issuing from original ideas [2,3] that we think will improve on present performances, particularly in terms of field tolerance, parasitic charge level, and related reliability; recent availability of a reasonably priced, probably satisfactory, new charging belt material from a Swedish firm [4,5].
150
4. Characteristics and limitations of present belt systems In all the present systems, the maximum current is always such smaller than the theoretical capacity calculated from fundamental electrostatic laws for an ideal system [2]. The limiting factors are: - superimposed longitudinal acceleration field; - nonuniformity in the field due to the discrete geometry of nearby conductors ; - nonuniformity of the charge deposit giving high local values [6] (fig . 2) ; - variable, uncontrolled, charge collection efficiency [6] (fig . 3) ; - possible high level of triboelectric parasitic charge generation [7]; - other well known perturbations (dust, protrusions, humidity, vibrations . . . ). 5. Conceptual design of the Vivitron charging system main new features 5.1 . Tandem belt arrangement It has been decided to run a 520 mm wide belt through the full length of the machine, so extending to the charging system the symmetry of the accelerating tube and column tandem arrangement . In this way the -- 100 m full belt length can be considered as twice the normal up- and down-charged belt in a tandem, and so electrically divided into four oppositely charged portions, each running from ground to the terminal and then back to ground (fig. 4). For a given total current, the charge density on the belt is then only half of the value for a conventional system, as is also the net transverse electrical limit from the charge deposit. In addition, the belt is tensioned from the ends of the pressure vessel, avoiding longitudinal stresses that the column could not supported. 5.2. Decoupled belt structure
H U L 4 `"
When two portions of a belt facing each other without internal gradient rods are equally and oppositely 50
Ground
Terminal Electrode
++++++++++++++
Ground
---------------------
0 COLLECTED CURRENT
(VA)
Fig. 3. Measured efficiency of charge collection for four arrangements of two different ionisers : either a usual stainless steel screen or a brush of 8 pm carbon fibers .
Fig. 4. Symmetrical belt arrangement with four oppositely charged portions, each running from ground to the terminal and then back to ground . III. CHARGING/INSULATING/RELATED TECHNOLOGY
416
J.-M.
Helleboid / Belt charging system for the 35 MV Vioitron d
E E
1 2
. E = E
E3 - with
n
vB
d
T D
e-
_d T D ET
ET _-
-200
E3 _D-d D ë o D
Fig. 5. Principle of the decoupled structure: two facing belt portions equally but oppositely charged induce quasi-uniform fields . The farer the nearby conductors (D) compared with belt portions distance (d), the higher the internal field E3 (maximum value = a/co) the lower the external field E l = Ez . charged, the transverse field is quasi-uniform if the charge density is uniform. Moreover, for a given charge level a and a fixed nearby conductor geometry (D = const) (fig. 5), the closer the two runs of the belt the higher the internal quasi-uniform electric field. Con-
-100
Fig. 7. Measured breakdown voltage for different geometries in SF6 under 56 psi gauge pressure.
versely the distorted external electric field, strongly influenced by nearby conductors, is then lower. This constitutes the decoupling effect . For d << D (fig. 6), the charging system and the rest of the accelerator run quasi-independently, which permits one to adapt external conductors almost without regard to the presence of a charging system, while taking advantage of the better voltage holding for a quasi-uniform field (fig . 7). External gradient rods can thus be eliminated, which enhances the decoupling effect by increasing the distance D and reduces the number of conducting pieces inside the column. 5.3 . Charge commutations and equilibrium
Fig. 6. Potential and field lines for a typical decoupled structure arrangement in an accelerating column. Nearby conduc tors (not shown) are located horizontally in the upper and lower parts of the figure .
As usual, the outside face of the belt is charged. In a decoupled structure, the field limitation is directly related to charge density levels as well as to their symmetry. Because of that, a rather simple but well controlled commutation system has been designed (fig . 8), limiting the number of ionisers, especially of "collection ionisers" whose efficiency is not well defined [6]. So, three out of four commutations (da = f 2a) are each effected by only one current regulated ioniser playing both a collecting ( + a --> 0) and a charging (0 - f a) role, somewhat like the system in the Felici machines . The fourth commutation is carried out, as in present accelerators, by a collecting commutation unit (-a - ±8a) followed by a current regulated charging ioniser (da = a) . As in the Strasbourg MP tandem, the collecting unit consists of a so-called "French connection" array of a first, polarised ioniser followed by a second, grounded
J.-M.
Helleboid / Belt charging system for the 35 MV Vivitron
417
metric two ply polyester with nitril rubber covering and can be spliced easily . The lifetime may be shorter than that of HVEC belts but still could be quite adequate. Long term experience is lacking. 6. Main characteristics Fig. 8. Charge commutation system for the Vivitron for a total current I.
As the Strasbourg MP is capable of running with a charge density of 3 nC/cmz, the Vivitron belt speed has been fixed at 10 m/s (24.5 m/s for the MP) for which a = 2.6 nC/cmZ for I = 500 ILA. Commutation will be ensured in a first stage by conventional but thinner screen assemblies (Id = 4 mil, 120 mesh). The nominal power to be transmitted is - 25 kW, including the electrical work Pe = 17 .5 kW at 35 MV and 500 luA, the gas friction Pt - 3 kW for 8 atm (abs) and 10 m/s, and provision for an extra P = 4.5 kW if needed . These values can be compared with P. = 6 kW, Pt = 14 kW and P = 2 kW for the MP at 18 MV and 24.5 m/s. The 520 mm wide, - 100 m full length belt is driven by both end pulleys, each powered by a 37 kW motor, sharing the load so as to avoid extreme values of mechanical tension. Its path is forced by 10 pairs of guides (fö = 112 mm rollers or gas blowers) in every second dead section and at least one pair in the terminal (fig . 9) . Their arrangement has been chosen, taking into account the characteristics of the Swedish belt, so as to ensure a decoupling factor - 0.25 while keeping a correct running whatever the load, with a minimum number of guides. The minimum average belt tension is 520 daN at which more than 40 kW could be transmitted.
one, ensuring a reasonably low residual charge So on the belt. Taking into account a maximum error of ± da = ± di/lo for each controlled ioniser, charge levels will not differ by more than 3da+2Sa. 5.4. Reduced belt speed As a consequence of these features, the belt speed can be significantly lowered from the usual value while keeping a reasonable field constraint for a given current . Because the lost power from gas friction is a v3, this choice will greatly reduce parasitic triboelectric charges. 5.5. Belt support All along the column, in the dead sections and in the terminal, the belt could be supported by rollers as in the MP. Nevertheless gas support with "SF, cushions" is also being considered . This would suppress solid friction and related parasitic charge and avoid bearing problems . 5.6. Belt material
7. Present status
Use of a hundred meter long HVEC belt would be unreasonable. Luckily a new material has recently been used as charging belt [4] and appears to be quite satisfactory [5]. Reasonably priced, this material is an asym-
The conceptual design has been completed, -fundamental choices decides, and the main electrical and mechanical characteristics determined.
column
T = 520 daN (Max - 690) min (615 to 765)I 435 < T < 605
T_
P+Pu 4v
_ P _ Pu T
4v
0
- 0 .25
c' .
315 mm
- 37
d/-6 (decoupling)
electrodes
kW
d < 120 mm
P u
D
5,64 m accelerating tube
P-Pu
T+
= 450 mm
¢ 315 mm 37 kW
4v
- P + Pu
T-
4v
-.. 50 m
Fig. 9. Belt arrangement for the Vivitron . Mechanical belt tension is indicated at different locations as a function of consumed power. P is the electrical and mechanical friction power, P. is the possible set up power in the terminal electrode where the tension has its mean value T. III . CHARGING/INSULATING/RELATED TECHNOLOGY
418
J.-M. Helleboid / Belt charging system for the 35 MV Vtuitron
Fig. 10. Prototype of a gas blower support for the belt .
Fig. 11 . 30 m, one-to-one, charging system test bench . A prototype gas blower support has been developed in collaboration with industry and is ready for tests (fig. 10). A one-to-one scale test bench for a 54 m full length belt is being built (fig . 11) for studying the two motor drive, dynamic running, gas support, power generation, and the electrical behaviour of the system, and particularly of the belt, but in air without voltage. Detailed studies should begin soon.
References [1] M. Letournel and the Vivitron Group, these Proceedings (7th Tandem Conference) Nucl . Instr. and Meth. A268 (1988) 295. [2] J.M. Helleboid and H. Bertein, Revue Générale de l'Electricité 9 (1985) 650. [3] J.M. Helleboid, Internal Report CRN-VIV-28 (1986). [4] B. Hemryd, Uppsala University annual report (1984) . [5] P. Arndt, Rev. Sci. Instr. 57 (5) (1986) 735. [6] Ch . Frieh and J.M . Helleboid, Internal Report CRN-VIV-22 (1985). [7] J. Bach and J.M . Helleboid, Internal Report CRN-VIV-15 (1984) .
Nuclear Instruments and Methods in Physics Research A268 (1988) 414-418 North-Holland, Amsterdam
BELT CHARGING SYSTEM FOR THE 35 MV VIVITRON ACCELERATOR Jean-Marie HELLEBOID Centre de Recherches Nucléaires, BP 20, 67037 Strasbourg-Cedex, France
A Van de Graaff belt charging system has been chosen for the Vivitron. Although classical in its principle and conservative in its design, it includes different new features that will be discussed in detail. The main electrical and mechanical characteristics are also reviewed together with the status of the project .
1. Introduction The Vivitron [1], now being built at the CRN, is a 35 MV tandem electrostatic accelerator for which a charging system is being studied [2,3] . The conceptual design has led us to choose a Van de Graaff type belt system that includes different new features (some of them fundamental), which make of it an original and specific system well suited to the Vivitron. The main electrical and mechanical characteristics have been calculated so as to achieve the desired performance, taking into account various constraints . A test bench is being built on which mechanical and electrical behaviour are to be demonstrated . Detailed realization studies should begin soon.
2. Requisite characteristics and related constraints The charging system of the Vivitron should deliver a current of 500 g.A for terminal voltages up to 35 MV with the usual (or better) stability of large Van de Graaff tandems ; this value is obtained by extrapolation from a current of 320 gA for the MP tandem at 18 MV with a corona current of 40 ILA. It should fit in a transverse electrically and mechanically limited space located between the accelerating tube, the column electrodes and the longitudinal insulating plates (fig. 1). Intermediate fixtures and supports, if needed must also be placed into this transversally confined space in the dead sections (which are only 220 mm long) i.e. at most every 2.82 m. The distance along the machine axis between the terminal and ground is - 25 m on either side of the terminal . Moreover, the system, being inside the column, should include the least number of conducting pieces 0168-9002/88/$03 .50 C Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Fig. 1. Column cross-section and belt location in two successive dead sections: A (left) and B (right). possible in the active (E longitudinal) sections so as to respect general Vivitron philosophy . 3. Choice of a belt charging system Although novel systems can be imagined, it has been judged more reasonable to confine ourselves to those at present used in large tandems . Both a chain and a belt system have been considered and the latter has been retained for numerous reasons of which the main ones are as follows : - extensive experience with a successful belt system developed on the Strasbourg MP tandem ;
J.-M.
T R A N S V E R S E
415
Helleboid / Belt charging system for the 35 MV Vivuron
U N I F 0 R M I T Y
Fig. 2. Measured transverse uniformity of the charge density a deposited at atmospheric air onto a belt by two identical arrangements of different ionisers : either usual stainless steel screens or brushes of 8 pm carbon fibers . modest intensity requirement (I=500 ttA) for the Vivitron compared with 700 uA (2 x 350) that can be obtained from the up and down charge system on the MP ; planned incorporation in the design of different new features issuing from original ideas [2,3] that we think will improve on present performances, particularly in terms of field tolerance, parasitic charge level, and related reliability; recent availability of a reasonably priced, probably satisfactory, new charging belt material from a Swedish firm [4,5].
150
4. Characteristics and limitations of present belt systems In all the present systems, the maximum current is always such smaller than the theoretical capacity calculated from fundamental electrostatic laws for an ideal system [2]. The limiting factors are: - superimposed longitudinal acceleration field; - nonuniformity in the field due to the discrete geometry of nearby conductors ; - nonuniformity of the charge deposit giving high local values [6] (fig . 2) ; - variable, uncontrolled, charge collection efficiency [6] (fig . 3) ; - possible high level of triboelectric parasitic charge generation [7]; - other well known perturbations (dust, protrusions, humidity, vibrations . . . ). 5. Conceptual design of the Vivitron charging system main new features 5.1 . Tandem belt arrangement It has been decided to run a 520 mm wide belt through the full length of the machine, so extending to the charging system the symmetry of the accelerating tube and column tandem arrangement . In this way the -- 100 m full belt length can be considered as twice the normal up- and down-charged belt in a tandem, and so electrically divided into four oppositely charged portions, each running from ground to the terminal and then back to ground (fig. 4). For a given total current, the charge density on the belt is then only half of the value for a conventional system, as is also the net transverse electrical limit from the charge deposit. In addition, the belt is tensioned from the ends of the pressure vessel, avoiding longitudinal stresses that the column could not supported. 5.2. Decoupled belt structure
H U L 4 `"
When two portions of a belt facing each other without internal gradient rods are equally and oppositely 50
Ground
Terminal Electrode
++++++++++++++
Ground
---------------------
0 COLLECTED CURRENT
(VA)
Fig. 3. Measured efficiency of charge collection for four arrangements of two different ionisers : either a usual stainless steel screen or a brush of 8 pm carbon fibers .
Fig. 4. Symmetrical belt arrangement with four oppositely charged portions, each running from ground to the terminal and then back to ground . III. CHARGING/INSULATING/RELATED TECHNOLOGY
416
J.-M.
Helleboid / Belt charging system for the 35 MV Vioitron d
E E
1 2
. E = E
E3 - with
n
vB
d
T D
e-
_d T D ET
ET _-
-200
E3 _D-d D ë o D
Fig. 5. Principle of the decoupled structure: two facing belt portions equally but oppositely charged induce quasi-uniform fields . The farer the nearby conductors (D) compared with belt portions distance (d), the higher the internal field E3 (maximum value = a/co) the lower the external field E l = Ez . charged, the transverse field is quasi-uniform if the charge density is uniform. Moreover, for a given charge level a and a fixed nearby conductor geometry (D = const) (fig. 5), the closer the two runs of the belt the higher the internal quasi-uniform electric field. Con-
-100
Fig. 7. Measured breakdown voltage for different geometries in SF6 under 56 psi gauge pressure.
versely the distorted external electric field, strongly influenced by nearby conductors, is then lower. This constitutes the decoupling effect . For d << D (fig. 6), the charging system and the rest of the accelerator run quasi-independently, which permits one to adapt external conductors almost without regard to the presence of a charging system, while taking advantage of the better voltage holding for a quasi-uniform field (fig . 7). External gradient rods can thus be eliminated, which enhances the decoupling effect by increasing the distance D and reduces the number of conducting pieces inside the column. 5.3 . Charge commutations and equilibrium
Fig. 6. Potential and field lines for a typical decoupled structure arrangement in an accelerating column. Nearby conduc tors (not shown) are located horizontally in the upper and lower parts of the figure .
As usual, the outside face of the belt is charged. In a decoupled structure, the field limitation is directly related to charge density levels as well as to their symmetry. Because of that, a rather simple but well controlled commutation system has been designed (fig . 8), limiting the number of ionisers, especially of "collection ionisers" whose efficiency is not well defined [6]. So, three out of four commutations (da = f 2a) are each effected by only one current regulated ioniser playing both a collecting ( + a --> 0) and a charging (0 - f a) role, somewhat like the system in the Felici machines . The fourth commutation is carried out, as in present accelerators, by a collecting commutation unit (-a - ±8a) followed by a current regulated charging ioniser (da = a) . As in the Strasbourg MP tandem, the collecting unit consists of a so-called "French connection" array of a first, polarised ioniser followed by a second, grounded
J.-M.
Helleboid / Belt charging system for the 35 MV Vivitron
417
metric two ply polyester with nitril rubber covering and can be spliced easily . The lifetime may be shorter than that of HVEC belts but still could be quite adequate. Long term experience is lacking. 6. Main characteristics Fig. 8. Charge commutation system for the Vivitron for a total current I.
As the Strasbourg MP is capable of running with a charge density of 3 nC/cmz, the Vivitron belt speed has been fixed at 10 m/s (24.5 m/s for the MP) for which a = 2.6 nC/cmZ for I = 500 ILA. Commutation will be ensured in a first stage by conventional but thinner screen assemblies (Id = 4 mil, 120 mesh). The nominal power to be transmitted is - 25 kW, including the electrical work Pe = 17 .5 kW at 35 MV and 500 luA, the gas friction Pt - 3 kW for 8 atm (abs) and 10 m/s, and provision for an extra P = 4.5 kW if needed . These values can be compared with P. = 6 kW, Pt = 14 kW and P = 2 kW for the MP at 18 MV and 24.5 m/s. The 520 mm wide, - 100 m full length belt is driven by both end pulleys, each powered by a 37 kW motor, sharing the load so as to avoid extreme values of mechanical tension. Its path is forced by 10 pairs of guides (fö = 112 mm rollers or gas blowers) in every second dead section and at least one pair in the terminal (fig . 9) . Their arrangement has been chosen, taking into account the characteristics of the Swedish belt, so as to ensure a decoupling factor - 0.25 while keeping a correct running whatever the load, with a minimum number of guides. The minimum average belt tension is 520 daN at which more than 40 kW could be transmitted.
one, ensuring a reasonably low residual charge So on the belt. Taking into account a maximum error of ± da = ± di/lo for each controlled ioniser, charge levels will not differ by more than 3da+2Sa. 5.4. Reduced belt speed As a consequence of these features, the belt speed can be significantly lowered from the usual value while keeping a reasonable field constraint for a given current . Because the lost power from gas friction is a v3, this choice will greatly reduce parasitic triboelectric charges. 5.5. Belt support All along the column, in the dead sections and in the terminal, the belt could be supported by rollers as in the MP. Nevertheless gas support with "SF, cushions" is also being considered . This would suppress solid friction and related parasitic charge and avoid bearing problems . 5.6. Belt material
7. Present status
Use of a hundred meter long HVEC belt would be unreasonable. Luckily a new material has recently been used as charging belt [4] and appears to be quite satisfactory [5]. Reasonably priced, this material is an asym-
The conceptual design has been completed, -fundamental choices decides, and the main electrical and mechanical characteristics determined.
column
T = 520 daN (Max - 690) min (615 to 765)I 435 < T < 605
T_
P+Pu 4v
_ P _ Pu T
4v
0
- 0 .25
c' .
315 mm
- 37
d/-6 (decoupling)
electrodes
kW
d < 120 mm
P u
D
5,64 m accelerating tube
P-Pu
T+
= 450 mm
¢ 315 mm 37 kW
4v
- P + Pu
T-
4v
-.. 50 m
Fig. 9. Belt arrangement for the Vivitron . Mechanical belt tension is indicated at different locations as a function of consumed power. P is the electrical and mechanical friction power, P. is the possible set up power in the terminal electrode where the tension has its mean value T. III . CHARGING/INSULATING/RELATED TECHNOLOGY
418
J.-M. Helleboid / Belt charging system for the 35 MV Vtuitron
Fig. 10. Prototype of a gas blower support for the belt .
Fig. 11 . 30 m, one-to-one, charging system test bench . A prototype gas blower support has been developed in collaboration with industry and is ready for tests (fig. 10). A one-to-one scale test bench for a 54 m full length belt is being built (fig . 11) for studying the two motor drive, dynamic running, gas support, power generation, and the electrical behaviour of the system, and particularly of the belt, but in air without voltage. Detailed studies should begin soon.
References [1] M. Letournel and the Vivitron Group, these Proceedings (7th Tandem Conference) Nucl . Instr. and Meth. A268 (1988) 295. [2] J.M. Helleboid and H. Bertein, Revue Générale de l'Electricité 9 (1985) 650. [3] J.M. Helleboid, Internal Report CRN-VIV-28 (1986). [4] B. Hemryd, Uppsala University annual report (1984) . [5] P. Arndt, Rev. Sci. Instr. 57 (5) (1986) 735. [6] Ch . Frieh and J.M . Helleboid, Internal Report CRN-VIV-22 (1985). [7] J. Bach and J.M . Helleboid, Internal Report CRN-VIV-15 (1984) .