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Acceptance tests of the Legnaro XTU tandem
30
Nuclear Instruments and Methods in Physics Research 220 (1984) 30-36 North-Holland, Amsterdam
ACCEPTANCE
TESTS OF THE LEGNARO
XTU TANDEM
C. S I G N O R I N I
*, G. B E Z Z O N , F. C E R V E L L E R A ,
P. S P O L A O R E
and R.A. RICCI *
Laboratori Nazionali di Legnaro, INFN, Legnaro, Padova, ltaly
In this paper we report on the acceptance tests of the XTU tandem Van de Graaff accelerator, installed at the "Laboratori Nazionali di Legnaro", and on the first year of operation. The accelerator has been shown to work at up to 15.6 MV, with a sulphur beam. The present routine operation is around 14 MV.
I. Introduction The X T U tandem accelerator was accepted by I N F N from H V E C in November 1981; the accelerator has operated with a heavy ion beam (32S) at up to 15.6 MV with a beam current of 330 pnA. In its first year of operation the accelerator has mainly operated at 14 MV with heavy ion beams (A > 32). The most important dates for the installation were the following: middle of 1975 purchase order for the 16 MV tandem accelerator placed by I N F N ; * Also: Istituto di Fisica dell'UniversitY, Padova, Italy.
June 1977 building started; February 1979 accelerator installation started; May 1980 column successfully tested up to 20.3 MV; early 1981 first beam through the accelerator; July 1981 acceptance test with protons up to 15.0 MV; October 1981 acceptance test with sulphur at 15.6 MV and with iodine at 15.0 MV.
2. Acceptance tests The basic generator configuration during the tests and also during present operation is essentially un-
Fig. 1. Details of the column resistor installation in section number 4. The protecting shields can be observed. 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
C. Signorini et al. / Acceptance tests of the Legnaro XTU tandem
31
Fig. 2. Details of the shorting string passing through one dead section.
changed from that reported previously [1-5]. The most important features are the following: (a) The accelerating tube consists of 8 standard 72" long, 14" diameter, stainless steel tubes, (b) The high voltage terminal is charged by a single laddertron running at 12 m s -1 in the high energy "column above the accelerating tubes (maximum current capacity 400/xA [3-5] using up and down charge). (c) The voltage divider consists of: 600 MI2 H V E C blue resistors (constructed from 20 single Welwyn elements in series) in sections 1, 2, 3, 6, 7 and 8, together with separate voltage dividers for sections 4 and 5 consisting of: 1200 M~2 blue H V E C resistors on the column, 1200 M~2 Caddock resistors on tube number 4, and 1200 Mg/ RPC resistors on tube number 5. In addition, aluminum shields are installed between some of the resistors to protect them from surges. All the blue resistors have shields every two elements in each half section closest to the high voltage terminal and every three elements in the remaining part of the sections. Fig.
1 shows as an example the arrangement in column section number 4. Resistors on tubes 4 and 5 have shields between all elements. (d) The terminal is equipped with 2 ion getter pumps, each of 250 1 s - 1 and one titanium pump. There is also provision to admit a very small controlled gas flow into the tubes from the terminal (so-called quenching gas). (e) A shorting system was installed for separate conditioning of each accelerating tube. The system consists of rods inserted radially from above at each dead section plus a string made half of metal (¼" diameter) and half of nonconducting multi-strand material (composite glass-teflon fibers by Dodge E-761-314, Oak Materials Group - Fluor Glass Division, N Y 12090) running axially. A photograph of the connection of the string at one dead section is shown in fig. 2. For all the tests negative ion beams of 140 keV (120 kV preacceleration plus 20 kV extraction) were delivered by a G I C model 834 sputter source. An off-axis duoplasmatron source was not used, as originally
Table 1 Summary of beam acceptance test data for the XTU tandem. (All data were obtained with the sputter source mod. 834 from General Ionex. The data in parentheses refer to the contract specifications. It was agreed to run the proton beam at reduced voltage and current mainly for radiation safety reasons. Each run lasted 2 to 3 h) Ion type
Terminal voltage (MV)
Analyzed current (pnA)
Stripper
H 32S
15.0 (16.0) 15.6 (16.0) 15.0 (16.0) 15.5
700 (5000) 330 (250) 120 (100) 50
C-foil 5/.~g c m gas gas C-foil 5/~g c m
127I
160
Terminal pumps status
-2
-2
Transmission
Ti ball
Ion pumps
off on on off
off on on off
50% 37% 50% 53%
I. STATUS .REPORTS
32
C. Signorini et al. / Acceptance tests of the Legnaro XTU tandem f
T PROTON I(AM lUG AT~ID.O MY
t
,
,
HIBH ENEROY VACUUM
LOW ENERGY VACUUM/
t~,
.L
0
L
0.00
1.01
1.50 ELAPSED TIME |HOURS)
Fig. 3. Vacuum reading during the protons run acceptance test.
planned, because of major damage sustained by the source at the beginning of the test runs. No serious limitation resulted from the use of the sputter source, despite its known larger emittance. In fact we greatly appreciated [6] the flexibility of this source for changing quickly between different beams for the various cheeks through the accelerator. Acceptance tests were run for proton, sulphur and iodine beams; a summary of the data is given in table 1. Tests of the proton specifications were run first in July
1981. In this first run some limitations from sparking were found beyond 15 MV and we decided to limit this test to this voltage. In addition, the beam current was limited to 700 nA mainly for radiation safety. This run lasted for about 1.5 h with three sparks. During the operation a chart recorder continuously monitored the low energy and the high energy vacuum. Fig. 3 shows the vacuum record of the proton run. Base vacuum was around 7 × 10 -7 Torr on the low energy side and 3 × 10 -7 Torr on the high energy side.
SULPHUR SEA. HUH AT -15.5 MY
HIGH EHEHHYYACHU.
\
/
! ¢
LGW.,,.,AC"U.
O,S
Fig. 4. Vacuum reading during the sulphur run acceptance test.
,!,
,Is
ELAPSED TIME IHDUHG)
2.0
33
C. Signorini et al. / Acceptance tests of the Legnaro XTU tandem i
Ionln[ n t A I
nnH AT ~ I | . n
iV
HI6# [n[R6Y VACUUM
LOW [ l [ l n T
VACnnM
I 1.0
0.5
1.5
2.0
(LAPSED TIMEIHOURS)
Fig. 5. Vacuum reading during the iodine run acceptance test.
After the proton run, the column was thoroughly checked and cleaned with particular care at all the spring contacts between the column and tubes. The voltage holding capability of each single tube was checked using the short circuit system. The data are shown in table 2 and are compared with data obtained later during routine operation. The sulphur beam specifications were run twice. There was no problem in taking the tandem up to - 15 MV. In order to go beyond 15 MV it was necessary to raise the voltage slowly with beam through the accelerator; in most cases a small quantity of quenching gas was very useful in reducing tube activity and in achieving a more stable voltage. The first sulphur test lasted - 2 h around 15.5 MV with 4 sparks; a 5 /~g cm -2 carbon stripper was used together with some quenching gas. N o measurable effect was observed in the vacuum which remained at 4 × 10 -7 Torr on the low energy side and 2 × 10 -7 Torr on the high energy side. Transmission of the beam through the accelerator was 42%. The second run was made using the gas stripper. The voltage was raised from 15.1 MV up to 15.7 MV for a 4 h period, with some sparks. The base low energy and high energy vacuum was 5 x 10 -7 Torr and 3 x 10 -7 Torr at the beginning (opening of stripper gas) and dropped down to 4 × 10 - 7 Torr and 2 × 1 0 - ' Torr
Table 2 Single tube conditioning data (voltages in MV) Date
1
2
3
4
5
6
7
8
October 1981 October 1982
3.8 3.5
4.3 4.2
4.3 4.2
3.9 4.2
4.3 4.0
4.2 4.2
4.2
4.3 4.2
respectively, after - 2 h operation. All the terminal pumps were on. In this run the transmission was 37%. A portion (2 h) of the vacuum readings recorded during this last run is shown in fig. 4. The iodine beam run was made from 14.8 MV up to 15.1 MV over a 6 h period with some sparks. It was decided to stop the tests at this voltage to avoid too much sparking and possible damage to the generator components. With stripper gas the vacuum was always 4 x 10 -7 T o r t and 3 x 10 -7 Torr in the low energy and high energy sides, respectively. The vacuum reading data over a 2 h period are shown in fig. 5. Transmission through the accelerator was 50%. Some other runs were made at 12 MV with 32S beam and the gas stripper. Transmissions of up to 78% were recorded.
3. Review of the major troubles during the installation
It seems worthwhile here to summarize the major troubles we had to face during the installation and which ended up in delays to the installation itself. (1) During the column high voltage tests, above - 18 MV, several equipotential (half) rings fell down, forcing a tank opening. The method of attaching these rings to the column had to be improved. The vibrations (rather high), induced by the laddertron into all the column components and the electrostatic forces, were triggering this effect. (2) The laddertron charging system had two major failures (broken insulator links) already described in previous articles [4,5]. (3) A pressure leak appeared [5] in the bellows connecting the high voltage terminal lens (an electroI. STATUS REPORTS
34
C. S i g n o r i n i
et al. / A c c e p t a n c e
tests of the Legnaro
XTU
tandem
), o
+ :-...,
2 A
,<
---7, v i
~-
~a
× ¢-q
e~
~. . . . . ~ Bs m
l
o
× rq fi
e~
22
o. u
i
e. o
It l
e~ o
m
..... 7, .o
z
r~
-7 ,,5
.~
35
C. Signorini et al. / Acceptance tests of the Legnaro XTU tandem
static quadrupole triplet) to the high energy side; the entire lens had to be replaced since in our design the bellows is welded to the lens housing to conserve space. (4) Tube number one was severely limited [5] in voltage holding capability (~< 2.4. MV). Any attempt to raise its voltage by increasing the charging current only caused an increase in radiation. This tube had to be replaced. (5) We often had resistor problems. During the tube conditioning the blue resistors were in many cases badly arced inside the epoxy between single elements and consequently reduced in value. This trouble could be eliminated by saw cuts about 2 cm deep in the resistor block on the side opposite to the spark gaps. The addition of aluminum shields, mentioned previously,
has partly reduced the problem. Resistor failure still occurs during routine operation as described below.
4. Routine operation During 1982 the accelerator operated for about 40% of the time; the remaining time was utilized for the installation of the radiation monitoring system, safety interlocks and experimental equipment. The operational voltage has been generally around 14 MV. Operation above 14 MV was avoided in order to prevent excessive damage from surges to the various components inside the generator. The sparking rate is typically several sparks per day (3 to 9).
Fig. 7. Photograph of the laddertron installation at the tank high energy base (fig. 7a) and at the terminal (fig. 7b). I. STATUS REPORTS
36
C. Signorini et al. / Acceptance tests of the Legnaro XTU tandem
One measurement lasting several hours (part of an excitation function) has taken place with 32S at 14.5 MV. The stability of the generator is very satisfactory, typically + 1.5 kV; this allows very weak beams out of the injector (e.g. 3 nA of 46Ti) to be accelerated. Fig. 6 shows an example of the behaviour versus time of various accelerator parameters (low and high energy vacuum, low and high energy column current and terminal voltage) during routine operation at around 14 MV with a sulphur beam. The beam used most often has been sulphur. Heavier beams, such as Ti, Ni, Cu, and Br have also been accelerated. The stripper was usually carbon (5 ~g cm-2 foils), used in order to achieve high charge states and consequently higher energy. Transmission was typically around 25% with no particular effort made to achieve higher transmission efficiency. Recently, with a 5aNi beam at 14 MV a transmission of 55% was measured. Some relevant data on specific tandem components collected from the experience of one year of operation are described in the following: (a) Laddertron. We have accumulated 7600 h of operation with no basic troubles other than the broken links previously mentioned. In the Summer of 1982, during a scheduled maintainance, two nylon links were found during inspection to have arced on the surface on the side next to the bushing hole and were replaced. Fig. 7 shows some details of the laddertron installation. (b) Accelerating tubes. The tubes have around 4000 h of operation. Tubes number 4 and 5 (next to the terminal) each have one glass insulator arced across by sparks and, subsequently, shorted out for safety reasons. Some recent data (October 1982) on single tube voltage holding capabilities are shown in table 2. The situation, especially for tube 1, is now worse than it was prior to the final acceptance test. (c) Gas recovery system. The major components are two 125 HP, 3 stage, piston compressors with teflon seals and a roughing system consisting of two Roots blowers (first 2600 m3/h, second 2600 m3/h, by Edwards S.p.A., Milano), followed by a two stage compressor (first 1000 m3/h, second 600 m3/h, by Northey, Parkstone, Poole, Dorset, England). The turn around time for the SF6 insulating gas is about 24 h (760 m3 tank volume with an initial gas pressure of around + 7.0 bar, a final SF6 gas pressure around 1.0 mbar and a final air pressure around 0.5 mbar). In the last 24 tank openings (from June 1981 until March 1983) we had an average gas loss of - 1 5 0 kg per cycle. No particular care is
taken to obtain a low moisture content in the gas system dew point being typically around - 6 0 ° F , since the results of the column and tube tests were rather insensitive to this parameter. The gas is recirculated about two days per week, mainly to collect the decomposition products in the dryers. (d) Resistors. The situation is acceptable, but not particularly good. Including operation in 1983 (40% of the time, mainly at 14 MV) the results are as follows: - 600 MI2 resistors: 15% have been found to have changed by more than 20% of the original value, usually lowered because of arcing between the single 30 MI2 Welwyn elements. This failure is cured with a saw cut as previously discussed. - 1200 M~2 tube resistors: about 20% were found to have changed, mainly to a value about 20% lower than the original value. 1200 M$2 column resistors: about 15% had changed, mainly to 2 value 20% higher than the original value. (e) Spring contacts between column and tubes equipotential planes. Serious problems occurr at the contacts between the springs and the aluminum equipotential rods. At this contact point, as already reported from several other installations, a strong arcing often occurs in SF6. The problem, which is rather serious, especially in the first accelerating section, is presently being cured by covering the aluminum rod at the contact portion with a thin sheet of phosphor bronze, 2 cm wide. Summarizing, we believe that the present voltage limitation around 14 MV originates from several persistent small sources, such as loose contacts, bad resistors, and activities inside the tubes, having a tendency to appear together and, consequently, trigger sparks.
References
[1] R.A. Ricci and C. Signorini, Nucl. Instr. and Meth. 146 (1977) 93. [2] C. Signorini, Rev. Phys. Appl. 12 (1977) 1361. [3] R.A. Ricci and C. Signorini, Nucl. Instr. and Meth. 184 (1981) 35. [4] F. Cervellera and C. Signorini, Nucl. Instr. and Meth. 184 (1981) 49. [5] C. Signorini and R.A. Ricci, IEEE Trans. Nucl. Sci. CH 1639-4/81, p. 12; Proc. 3rd Int. Conf. on Electrostatic Accelerator Technology,Oak Ridge, USA, ed., J.A. Martin (1981). [6] P. Spolaore and C. Signorini, IEEE Trans. Nucl. Sci. Ch 1639-4/81, p. 65; Proc. 3rd Int. Conf on Electrostatic Accelerator Technology, Oak Ridge, USA ed., J.A. Martin (1981).
Nuclear Instruments and Methods in Physics Research 220 (1984) 30-36 North-Holland, Amsterdam
ACCEPTANCE
TESTS OF THE LEGNARO
XTU TANDEM
C. S I G N O R I N I
*, G. B E Z Z O N , F. C E R V E L L E R A ,
P. S P O L A O R E
and R.A. RICCI *
Laboratori Nazionali di Legnaro, INFN, Legnaro, Padova, ltaly
In this paper we report on the acceptance tests of the XTU tandem Van de Graaff accelerator, installed at the "Laboratori Nazionali di Legnaro", and on the first year of operation. The accelerator has been shown to work at up to 15.6 MV, with a sulphur beam. The present routine operation is around 14 MV.
I. Introduction The X T U tandem accelerator was accepted by I N F N from H V E C in November 1981; the accelerator has operated with a heavy ion beam (32S) at up to 15.6 MV with a beam current of 330 pnA. In its first year of operation the accelerator has mainly operated at 14 MV with heavy ion beams (A > 32). The most important dates for the installation were the following: middle of 1975 purchase order for the 16 MV tandem accelerator placed by I N F N ; * Also: Istituto di Fisica dell'UniversitY, Padova, Italy.
June 1977 building started; February 1979 accelerator installation started; May 1980 column successfully tested up to 20.3 MV; early 1981 first beam through the accelerator; July 1981 acceptance test with protons up to 15.0 MV; October 1981 acceptance test with sulphur at 15.6 MV and with iodine at 15.0 MV.
2. Acceptance tests The basic generator configuration during the tests and also during present operation is essentially un-
Fig. 1. Details of the column resistor installation in section number 4. The protecting shields can be observed. 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
C. Signorini et al. / Acceptance tests of the Legnaro XTU tandem
31
Fig. 2. Details of the shorting string passing through one dead section.
changed from that reported previously [1-5]. The most important features are the following: (a) The accelerating tube consists of 8 standard 72" long, 14" diameter, stainless steel tubes, (b) The high voltage terminal is charged by a single laddertron running at 12 m s -1 in the high energy "column above the accelerating tubes (maximum current capacity 400/xA [3-5] using up and down charge). (c) The voltage divider consists of: 600 MI2 H V E C blue resistors (constructed from 20 single Welwyn elements in series) in sections 1, 2, 3, 6, 7 and 8, together with separate voltage dividers for sections 4 and 5 consisting of: 1200 M~2 blue H V E C resistors on the column, 1200 M~2 Caddock resistors on tube number 4, and 1200 Mg/ RPC resistors on tube number 5. In addition, aluminum shields are installed between some of the resistors to protect them from surges. All the blue resistors have shields every two elements in each half section closest to the high voltage terminal and every three elements in the remaining part of the sections. Fig.
1 shows as an example the arrangement in column section number 4. Resistors on tubes 4 and 5 have shields between all elements. (d) The terminal is equipped with 2 ion getter pumps, each of 250 1 s - 1 and one titanium pump. There is also provision to admit a very small controlled gas flow into the tubes from the terminal (so-called quenching gas). (e) A shorting system was installed for separate conditioning of each accelerating tube. The system consists of rods inserted radially from above at each dead section plus a string made half of metal (¼" diameter) and half of nonconducting multi-strand material (composite glass-teflon fibers by Dodge E-761-314, Oak Materials Group - Fluor Glass Division, N Y 12090) running axially. A photograph of the connection of the string at one dead section is shown in fig. 2. For all the tests negative ion beams of 140 keV (120 kV preacceleration plus 20 kV extraction) were delivered by a G I C model 834 sputter source. An off-axis duoplasmatron source was not used, as originally
Table 1 Summary of beam acceptance test data for the XTU tandem. (All data were obtained with the sputter source mod. 834 from General Ionex. The data in parentheses refer to the contract specifications. It was agreed to run the proton beam at reduced voltage and current mainly for radiation safety reasons. Each run lasted 2 to 3 h) Ion type
Terminal voltage (MV)
Analyzed current (pnA)
Stripper
H 32S
15.0 (16.0) 15.6 (16.0) 15.0 (16.0) 15.5
700 (5000) 330 (250) 120 (100) 50
C-foil 5/.~g c m gas gas C-foil 5/~g c m
127I
160
Terminal pumps status
-2
-2
Transmission
Ti ball
Ion pumps
off on on off
off on on off
50% 37% 50% 53%
I. STATUS .REPORTS
32
C. Signorini et al. / Acceptance tests of the Legnaro XTU tandem f
T PROTON I(AM lUG AT~ID.O MY
t
,
,
HIBH ENEROY VACUUM
LOW ENERGY VACUUM/
t~,
.L
0
L
0.00
1.01
1.50 ELAPSED TIME |HOURS)
Fig. 3. Vacuum reading during the protons run acceptance test.
planned, because of major damage sustained by the source at the beginning of the test runs. No serious limitation resulted from the use of the sputter source, despite its known larger emittance. In fact we greatly appreciated [6] the flexibility of this source for changing quickly between different beams for the various cheeks through the accelerator. Acceptance tests were run for proton, sulphur and iodine beams; a summary of the data is given in table 1. Tests of the proton specifications were run first in July
1981. In this first run some limitations from sparking were found beyond 15 MV and we decided to limit this test to this voltage. In addition, the beam current was limited to 700 nA mainly for radiation safety. This run lasted for about 1.5 h with three sparks. During the operation a chart recorder continuously monitored the low energy and the high energy vacuum. Fig. 3 shows the vacuum record of the proton run. Base vacuum was around 7 × 10 -7 Torr on the low energy side and 3 × 10 -7 Torr on the high energy side.
SULPHUR SEA. HUH AT -15.5 MY
HIGH EHEHHYYACHU.
\
/
! ¢
LGW.,,.,AC"U.
O,S
Fig. 4. Vacuum reading during the sulphur run acceptance test.
,!,
,Is
ELAPSED TIME IHDUHG)
2.0
33
C. Signorini et al. / Acceptance tests of the Legnaro XTU tandem i
Ionln[ n t A I
nnH AT ~ I | . n
iV
HI6# [n[R6Y VACUUM
LOW [ l [ l n T
VACnnM
I 1.0
0.5
1.5
2.0
(LAPSED TIMEIHOURS)
Fig. 5. Vacuum reading during the iodine run acceptance test.
After the proton run, the column was thoroughly checked and cleaned with particular care at all the spring contacts between the column and tubes. The voltage holding capability of each single tube was checked using the short circuit system. The data are shown in table 2 and are compared with data obtained later during routine operation. The sulphur beam specifications were run twice. There was no problem in taking the tandem up to - 15 MV. In order to go beyond 15 MV it was necessary to raise the voltage slowly with beam through the accelerator; in most cases a small quantity of quenching gas was very useful in reducing tube activity and in achieving a more stable voltage. The first sulphur test lasted - 2 h around 15.5 MV with 4 sparks; a 5 /~g cm -2 carbon stripper was used together with some quenching gas. N o measurable effect was observed in the vacuum which remained at 4 × 10 -7 Torr on the low energy side and 2 × 10 -7 Torr on the high energy side. Transmission of the beam through the accelerator was 42%. The second run was made using the gas stripper. The voltage was raised from 15.1 MV up to 15.7 MV for a 4 h period, with some sparks. The base low energy and high energy vacuum was 5 x 10 -7 Torr and 3 x 10 -7 Torr at the beginning (opening of stripper gas) and dropped down to 4 × 10 - 7 Torr and 2 × 1 0 - ' Torr
Table 2 Single tube conditioning data (voltages in MV) Date
1
2
3
4
5
6
7
8
October 1981 October 1982
3.8 3.5
4.3 4.2
4.3 4.2
3.9 4.2
4.3 4.0
4.2 4.2
4.2
4.3 4.2
respectively, after - 2 h operation. All the terminal pumps were on. In this run the transmission was 37%. A portion (2 h) of the vacuum readings recorded during this last run is shown in fig. 4. The iodine beam run was made from 14.8 MV up to 15.1 MV over a 6 h period with some sparks. It was decided to stop the tests at this voltage to avoid too much sparking and possible damage to the generator components. With stripper gas the vacuum was always 4 x 10 -7 T o r t and 3 x 10 -7 Torr in the low energy and high energy sides, respectively. The vacuum reading data over a 2 h period are shown in fig. 5. Transmission through the accelerator was 50%. Some other runs were made at 12 MV with 32S beam and the gas stripper. Transmissions of up to 78% were recorded.
3. Review of the major troubles during the installation
It seems worthwhile here to summarize the major troubles we had to face during the installation and which ended up in delays to the installation itself. (1) During the column high voltage tests, above - 18 MV, several equipotential (half) rings fell down, forcing a tank opening. The method of attaching these rings to the column had to be improved. The vibrations (rather high), induced by the laddertron into all the column components and the electrostatic forces, were triggering this effect. (2) The laddertron charging system had two major failures (broken insulator links) already described in previous articles [4,5]. (3) A pressure leak appeared [5] in the bellows connecting the high voltage terminal lens (an electroI. STATUS REPORTS
34
C. S i g n o r i n i
et al. / A c c e p t a n c e
tests of the Legnaro
XTU
tandem
), o
+ :-...,
2 A
,<
---7, v i
~-
~a
× ¢-q
e~
~. . . . . ~ Bs m
l
o
× rq fi
e~
22
o. u
i
e. o
It l
e~ o
m
..... 7, .o
z
r~
-7 ,,5
.~
35
C. Signorini et al. / Acceptance tests of the Legnaro XTU tandem
static quadrupole triplet) to the high energy side; the entire lens had to be replaced since in our design the bellows is welded to the lens housing to conserve space. (4) Tube number one was severely limited [5] in voltage holding capability (~< 2.4. MV). Any attempt to raise its voltage by increasing the charging current only caused an increase in radiation. This tube had to be replaced. (5) We often had resistor problems. During the tube conditioning the blue resistors were in many cases badly arced inside the epoxy between single elements and consequently reduced in value. This trouble could be eliminated by saw cuts about 2 cm deep in the resistor block on the side opposite to the spark gaps. The addition of aluminum shields, mentioned previously,
has partly reduced the problem. Resistor failure still occurs during routine operation as described below.
4. Routine operation During 1982 the accelerator operated for about 40% of the time; the remaining time was utilized for the installation of the radiation monitoring system, safety interlocks and experimental equipment. The operational voltage has been generally around 14 MV. Operation above 14 MV was avoided in order to prevent excessive damage from surges to the various components inside the generator. The sparking rate is typically several sparks per day (3 to 9).
Fig. 7. Photograph of the laddertron installation at the tank high energy base (fig. 7a) and at the terminal (fig. 7b). I. STATUS REPORTS
36
C. Signorini et al. / Acceptance tests of the Legnaro XTU tandem
One measurement lasting several hours (part of an excitation function) has taken place with 32S at 14.5 MV. The stability of the generator is very satisfactory, typically + 1.5 kV; this allows very weak beams out of the injector (e.g. 3 nA of 46Ti) to be accelerated. Fig. 6 shows an example of the behaviour versus time of various accelerator parameters (low and high energy vacuum, low and high energy column current and terminal voltage) during routine operation at around 14 MV with a sulphur beam. The beam used most often has been sulphur. Heavier beams, such as Ti, Ni, Cu, and Br have also been accelerated. The stripper was usually carbon (5 ~g cm-2 foils), used in order to achieve high charge states and consequently higher energy. Transmission was typically around 25% with no particular effort made to achieve higher transmission efficiency. Recently, with a 5aNi beam at 14 MV a transmission of 55% was measured. Some relevant data on specific tandem components collected from the experience of one year of operation are described in the following: (a) Laddertron. We have accumulated 7600 h of operation with no basic troubles other than the broken links previously mentioned. In the Summer of 1982, during a scheduled maintainance, two nylon links were found during inspection to have arced on the surface on the side next to the bushing hole and were replaced. Fig. 7 shows some details of the laddertron installation. (b) Accelerating tubes. The tubes have around 4000 h of operation. Tubes number 4 and 5 (next to the terminal) each have one glass insulator arced across by sparks and, subsequently, shorted out for safety reasons. Some recent data (October 1982) on single tube voltage holding capabilities are shown in table 2. The situation, especially for tube 1, is now worse than it was prior to the final acceptance test. (c) Gas recovery system. The major components are two 125 HP, 3 stage, piston compressors with teflon seals and a roughing system consisting of two Roots blowers (first 2600 m3/h, second 2600 m3/h, by Edwards S.p.A., Milano), followed by a two stage compressor (first 1000 m3/h, second 600 m3/h, by Northey, Parkstone, Poole, Dorset, England). The turn around time for the SF6 insulating gas is about 24 h (760 m3 tank volume with an initial gas pressure of around + 7.0 bar, a final SF6 gas pressure around 1.0 mbar and a final air pressure around 0.5 mbar). In the last 24 tank openings (from June 1981 until March 1983) we had an average gas loss of - 1 5 0 kg per cycle. No particular care is
taken to obtain a low moisture content in the gas system dew point being typically around - 6 0 ° F , since the results of the column and tube tests were rather insensitive to this parameter. The gas is recirculated about two days per week, mainly to collect the decomposition products in the dryers. (d) Resistors. The situation is acceptable, but not particularly good. Including operation in 1983 (40% of the time, mainly at 14 MV) the results are as follows: - 600 MI2 resistors: 15% have been found to have changed by more than 20% of the original value, usually lowered because of arcing between the single 30 MI2 Welwyn elements. This failure is cured with a saw cut as previously discussed. - 1200 M~2 tube resistors: about 20% were found to have changed, mainly to a value about 20% lower than the original value. 1200 M$2 column resistors: about 15% had changed, mainly to 2 value 20% higher than the original value. (e) Spring contacts between column and tubes equipotential planes. Serious problems occurr at the contacts between the springs and the aluminum equipotential rods. At this contact point, as already reported from several other installations, a strong arcing often occurs in SF6. The problem, which is rather serious, especially in the first accelerating section, is presently being cured by covering the aluminum rod at the contact portion with a thin sheet of phosphor bronze, 2 cm wide. Summarizing, we believe that the present voltage limitation around 14 MV originates from several persistent small sources, such as loose contacts, bad resistors, and activities inside the tubes, having a tendency to appear together and, consequently, trigger sparks.
References
[1] R.A. Ricci and C. Signorini, Nucl. Instr. and Meth. 146 (1977) 93. [2] C. Signorini, Rev. Phys. Appl. 12 (1977) 1361. [3] R.A. Ricci and C. Signorini, Nucl. Instr. and Meth. 184 (1981) 35. [4] F. Cervellera and C. Signorini, Nucl. Instr. and Meth. 184 (1981) 49. [5] C. Signorini and R.A. Ricci, IEEE Trans. Nucl. Sci. CH 1639-4/81, p. 12; Proc. 3rd Int. Conf. on Electrostatic Accelerator Technology,Oak Ridge, USA, ed., J.A. Martin (1981). [6] P. Spolaore and C. Signorini, IEEE Trans. Nucl. Sci. Ch 1639-4/81, p. 65; Proc. 3rd Int. Conf on Electrostatic Accelerator Technology, Oak Ridge, USA ed., J.A. Martin (1981).