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Initial operating experience with beams at the Daresbury tandem
Nuclear Instruments and Methods in Physics Research 220 (1984) 149-153 North-Holland, Amsterdam
149
INITIAL OPERATING EXPERIENCE WITH BEAMS AT THE DARESBURY TANDEM T.W. A I T K E N , T. J O Y and H . G . P R I C E
SERC, Daresbury Laboratory, Daresbury, Warrington WA4 4AD, UK
The Daresbury tandem has operated for nuclear physics experiments for 5 months accelerating ions up to mass 58 and operating with beam at terminal voltages up to 20.1 MV. The performance of the machine with beam is described. Details of the accelerator tube conditioning are presented with a description of many of the components involved in beam production.
1. Introduction
3. Deconditioning effects and damage to the tube
The NSF tandem at the Daresbury Laboratory [1] began operation for experiments late in 1982. In these proceedings Tait [2] described the general progress made in commissioning the accelerator. This paper gives details of the performance of the machine with beam, covering conditioning, deconditioning and damage to the tube, the types of beam accelerated, transmission efficiency, the performance of the charge state separator, the beam energy stabiliser, stripper foils and general experience with the computer control system.
The tube has been opened up to dry nitrogen on several occasions. Modules which have been previously conditioned, recondition to 1.4 MV in under half an hour. Those not exposed require only slight reconditioning. Four modules have had to be replaced as a result of damage caused during sparking at levels around 18-19.5 MV. Following damage these modules sparked at levels around 1.0 to 1.2 MV when run individually without any sign of reconditioning. Examination showed that the damage was internal; this is described by Eastham [5]. The damage was located at modules near the intershield and the end of the column where it is possible that transient voltages are at a maximum. In view of this the gradient of the four relevant modules has been reduced and the spark gap protection afforded by the column locally increased over that already provided for this version of the tube. Since then no further tube damage has been experienced at voltages up to 20.1 MV. Regular reconditioning of the modules is carried out mainly as a check on performance and this typically taken 8-12 h. The most significant deconditioning effect has been found to be on the input module at the entrance to the low energy tube when running beam and usually this requires to be reconditioned each week. The effect was worst with an injected beam of oxygen and the most probable cause is likely to be scattering of beam from the aperture near the tube entrance on to the tube aperture surface. The effect of an SF6 leak is shown in fig. 1. Without beam no serious problem seemed to exist but with a 48Call beam injected scattering produced the highly distorted gradient shown. Damage to the module above the intershield was caused by allowing the module to approach its conditioned level. This demonstrates how important it is to operate well below this level. It is possible to monitor the column gradient using the colunto currents and alarm limits have now been set to warn of such a situation arising again.
2. Conditioning of the accelerator tube Voltage was first applied to the tube [3] on 18th April 1982. Using the shorting mechanisms each of the 36 modules was individually conditioned. Microdischarge activity [4] characterised by rapid bursts of X-rays which gradually decrease in overall intensity at a fixed applied voltage, set in at levels of 700-800 kV and the modules were conditioned at a rate of about 100 k V / h initially to 1.1 MV. In only three cases did modules show an uncharacteristic behaviour and this was attributed to particle contamination or field emission sites in these modules. Conditioning was only possible in these cases by allowing sparking to occur. Because of the low energy involved these discharges were always gentle enough to ensure that no tube damage was sustained. The modules were all then conditioned gradually to successively higher levels reaching in most cases 1.4 MV without any problems before the tube was run as a whole with all sections live. It is now routine to condition all modules up to at least 1.4 MV before running at high voltage. This gives a safety factor of 25% between the conditioned level and the operating level of 19-20 MV. Conditioning modules up to 1.5 MV has recently been carried out enabling operation at over 20 MV to be achieved. 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
IV. NEW ACCELERATORS
150
T.W. Aitken et al. / Initial experience at the Daresbury tandem
l.3
.Y.. ~60-TI _~ ]
"
~f
f
12 i
g i "oO"m ~- I -
"
~
W
.
t1,0 ~
LWL
S
L
Beam direction
0 50
W
o.8 ,3 /
Fig. 1. The effect of an SF6 leak.
4. Types of beam accelerated
S- Source L - Lenses W- Waist
L
Table 1 lists the ion beams accelerated for use in experiments with details of the energies a n d intensities used. The figures in brackets indicate the m a x i m u m currents so far analysed. The He ions were produced by the lithium charge exchange source while the remainder were produced by a Middleton sputter source modified to use the reflex geometry in order to be able to use small diameter pills. In m a n y of the cases enriched pill material or bleeder gas was used.
if)
W
5. Beam transmission efficiency Beams up to an intensity of 2/~A have been injected into the t a n d e m without causing appreciable loading a n d these have been transmitted with good efficiency. Fig. 2 shows a schema of the b e a m envelope through the machine. Ions are kept within the 25 m m diameter tube aperture by means of sets of magnetic quadrupole triplets a n d the b e a m is waisted by these at the i n p u t to the tube, the 1st stripper, the charge state separator slits, the 2nd stripper, towards the end of the high energy tube and the object point of the analysing magnet as shown.
Fig. 2. Schema of the beam envelope through the machine.
Table 1 Summary of beams used up to March 1983 Beam
Term (MV)
Charge
Output (MeV)
MeV/nucleon
Analysed current (nA)
4He 6Li 7Li 12C 160 170 t80 29Si 32S 34S 36S 48Ca 48Ti 58Ni
10-17 15-15.3 15-17 17.6 10.7-16 11.1 11-18.7 12 15-16.5 15-16.5 15-16.5 16.7-18.3 17.5-18.3 15.3
2+ 3 3÷ 5+ 6 +/7 + 6+ 6+/7 + 8+ 9+ 9+ 9+ 11 + 11 ÷ 11 +
30-51 60~-61 60-68 106 75-150 78 77-150 108 150-165 150-165 150-165 200-220 210-220 184
7.5-13.0 10-10.2 8.6-9.7 8.8 4.7-9.4 4.6 4.3-8.3 3.7 4.7-5.2 4.4-4.9 4.2-4.6 4.2-4.6 4.4-4.6 3.2
120-450 120-190 190-(500) 60 60-(3000) 120-330 12-400 40-80 80 10-30 10-30 36 37 10-(110)
T. IV. Aitken et al. / Initial experience at the Daresbury tandem
In practice, obtaining waists at the correct positions is carried out by using apertures or slits at the waist positions and maximising the intensity transmitted through these in sequence through the machine. In general standard settings can be predicted which virtually guarantee beam through the machine for any ion, molecule, energy and charge state and only require slight optimisation to maximise beam transmission. Problems with the beam scanners in our application have been overcome after extensive modifications and seven of the thirteen scanners are now working with good reliability. Lack of the availability of scanners held up initial beam studies. These have now shown that the beam is being transmitted in the correct manner. The output intensity is limited as expected by the acceptance of the machine. Calculation shows this to be 4 ~r mm mrad (MeV) 1/2 for 160 ions injected at 0.32 MeV with an 18 MV terminal voltage. When the input beam is limited to this the transmission efficiency is virtually 100%. Since about 30% of the output of the Middleton source can be outside this emittance it is tempting to try to increase the output beam by trying to inject the larger emittance especially for the more difficult ion species but this is of course unfruitful. Brighter sources would be valuable but by using enriched materials all the experimental requirements for output intensities have so far been met.
480a at 17.12 MeV
--
Theory ,, This experiment
"6
0
.
8
9
.
.
10
.
11 12 Charge state
~
_36
[
r
13
14
Locus of best _ focus for e states
<
E 32-
"5 28 o 24 30
40
50
Inner current (A)
Fig. 3. A calibration run identifying the charge states from 48CaH stripped at 17.9 MeV by a 5/~g/cm2 carbon foil.
151
6. The charge state selector This device [6] consists of an offset magnetic quadrupole triplet in the centre terminal following the first stripper. The dipole field components bend the required charge state in a dog-leg returning it on to the central axis while the quadrupole fields focus this charge state through defining slits on the axis after the separator. Fig. 3 shows a calibration run in which the charge states from 48Call stripped at 17.9 MeV by a 5 # g / c m 2 carbon foil are identified. The plot gives the currents required to focus each charge state at the image slits and also the fraction in each state. In the case shown the charge state distribution agrees with that predicted by the formula of Sayer [7]. The operation of the separator in practice is entirely predictable. Current settings can be generated for any ion and charge state required and normally the settings only require to be altered very slightly to adjust the output beam direction.
7. Beam energy stabilisation The output beam energy is kept stable by a feedback loop operating between the analysing slits and the laddertron downcharge as shown in fig. 4. Currents intercepted by the slits are fed to the control computer via logarithmic preamplifiers at a sample rate of 10 samples per second together with a beam on/off logic signal. Processing is then carried out by a minicomputer program based on that developed for the Pilot Machine [8] extended to use the slit signals and to switch automatically from slit to GVM stabihsation when the beam is switched on and off. The frequency of 1 Hz at which the loop gain falls to zero is determined by the rapid increase in phase delay caused by the 0.15 s time delay between change in downcharge and its effect on the terminal voltage. Despite this limited response the performance of the loop has been most impressive. Once the comer frequencies and gain had been set for optimum response the system has worked without the necessity for any further changes for a period of four months with beam currents in the range 1 to 3000 nA. The automatic switching works without any noticeable transients and copes with any type of beam interruption from short ion source trips to the insertion of low energy Faraday cups for long periods. The beam intensity through the stabilising slits is normally constant to within a few per cent and estimates of the energy stability from the slit output signals indicate an energy stability of better than +1 part in 104. Experimental measurements using apparatus with a resolution of + 1 in 6000 confirm this showing no measurable component due to beam energy fluctuations. The stability of the beam is the result of three main factors, the use of an electrostatically suppressed tube giving an extremely IV. NEW ACCELERATORS
152
T. IV. Aitken et al. / Initial experience at the Daresbury tandem
II
GVM . . . . 1 (Terminal to intershield) GVM 2 (Intershield to vessel) ----7
i
I
Mini computer
ge
_
inductor
I
._~ Down charge
inductor kV
Error _ . . Other charging system voltage Infra- red link
i
Laddertron
Keyboard
lysing net
InlT\eractive d sp ay
Top slit u
n Bottom slit
Fig. 4. Schematic diagram of s~t stabi~ser.
stable output beam position, the use of a laddertron to eliminate belt ripple and the presence of the intershield acting as a large smoothing capacitance. The machine within the intershield is solely responsible for short term energy variations and hence the feedback loop has parameters independent of the characteristics of the machine outside the intershield. 8. Foils The foils used have all been 5 # g / c m 2 slackened glow discharge carbon foils of the " D a r e s b u r y " type [O]. These have shown excellent performance giving a useful lifetime of 10 h for 48Ca at 18 MeV with an intensity of 100 nA. The lifetime was determined by the gradual spread in energy loss in the foil due to thickening or distortion rather than by sudden foil rupture. Studies are being carried out using the G V M to measure the energy loss and rotating wire scanners before and after the analysing magnet to measure the spread in energy loss in the foil during its useful lifetime. First results using a S beam are not well understood and more work is required. These studies will be extended to other types of ions and foils.
source, injection system and low energy beam line, the second controls the machine and high energy beam line beam handling components and the third controls the machine charging system including the energy stabiliser. Each mini supports an interactive colour TV display, two control knobs and can present information on oscilloscopes or analogue meters [10]. Experience with the system has shown that the operators find it easy to use and that the sequential allocation of the two control knobs to the beam handling devices from a master mimic diagram gives sufficient " f e e l " to make beam optimisation easy. Software routines have been developed to gang supplies together so that the two knobs provide for more complex control functions than simply that of one parameter. A fourth minicomputer provides alarms and a limited data logging facility. At present beam handling parameters have to be set up and recorded manually but it is planned to connect the minis to a midicomputer so that this can be done automatically. The midi will relieve the loading on the minis which are at present working at their limits and also provide a much more detailed safety surveillance than at present.
10. Conclusion 9. Control system The beam handling and charging systems are controlled by three minicomputers. One controls the ion
The N S F tandem at Daresbury is now operating with beam at voltages up to 20 MV with excellent stability. The limits of the machine have not yet been
T. W. Aitken et al. / l n i t i a l experience at the Daresbury tandem
fully explored and it is expected that with gradual development and a reasonable level of associated R and D even better performance will be achieved.
References [1] R.G.P. Voss, in: Proc. III Int. Conf. on Elect. Tech., Oak Ridge (IEEE Press, New York, 1981). [2] N.R.S. Tait, these proceedings (6th Tandem Conf.), p. 54. [3] T. Joy, in: Proc. III Int. Conf. on Electr. Tech., Oak Ridge (IEEE Press, New York, 1981).
153
[4] D.A. Eastham and R. Thorn, J. Phys. D: 11 (1978) 1149. [5] D.A. Eastham, these proceedings (6th Tandem Conf.), p. 101. [6] D.A. Eastham, T. Joy and N.R.S. Tait, in: Daresbury Laboratory Preprint D L / N S F / P 5 (1973). [7] R.O. Sayer, Rev. de Phys. Appl. 12 (1977) 1543. [8] T.W. Aitken, I. Goodall and K. Spurling, Nucl. Instr. and Meth. 153 (1978) 333. [9] D.A. Whitmell, B.H. Armitage, D.W.L. Tolfree and N.R.S. Tait, A new Daresbury Lab. Preprint DL/NSF/P86 (Accelerator) (1978). [10] J.C. Beech et al., these proceedings (6th Tandem Conf.), p. 170.
IV. NEW ACCELERATORS
149
INITIAL OPERATING EXPERIENCE WITH BEAMS AT THE DARESBURY TANDEM T.W. A I T K E N , T. J O Y and H . G . P R I C E
SERC, Daresbury Laboratory, Daresbury, Warrington WA4 4AD, UK
The Daresbury tandem has operated for nuclear physics experiments for 5 months accelerating ions up to mass 58 and operating with beam at terminal voltages up to 20.1 MV. The performance of the machine with beam is described. Details of the accelerator tube conditioning are presented with a description of many of the components involved in beam production.
1. Introduction
3. Deconditioning effects and damage to the tube
The NSF tandem at the Daresbury Laboratory [1] began operation for experiments late in 1982. In these proceedings Tait [2] described the general progress made in commissioning the accelerator. This paper gives details of the performance of the machine with beam, covering conditioning, deconditioning and damage to the tube, the types of beam accelerated, transmission efficiency, the performance of the charge state separator, the beam energy stabiliser, stripper foils and general experience with the computer control system.
The tube has been opened up to dry nitrogen on several occasions. Modules which have been previously conditioned, recondition to 1.4 MV in under half an hour. Those not exposed require only slight reconditioning. Four modules have had to be replaced as a result of damage caused during sparking at levels around 18-19.5 MV. Following damage these modules sparked at levels around 1.0 to 1.2 MV when run individually without any sign of reconditioning. Examination showed that the damage was internal; this is described by Eastham [5]. The damage was located at modules near the intershield and the end of the column where it is possible that transient voltages are at a maximum. In view of this the gradient of the four relevant modules has been reduced and the spark gap protection afforded by the column locally increased over that already provided for this version of the tube. Since then no further tube damage has been experienced at voltages up to 20.1 MV. Regular reconditioning of the modules is carried out mainly as a check on performance and this typically taken 8-12 h. The most significant deconditioning effect has been found to be on the input module at the entrance to the low energy tube when running beam and usually this requires to be reconditioned each week. The effect was worst with an injected beam of oxygen and the most probable cause is likely to be scattering of beam from the aperture near the tube entrance on to the tube aperture surface. The effect of an SF6 leak is shown in fig. 1. Without beam no serious problem seemed to exist but with a 48Call beam injected scattering produced the highly distorted gradient shown. Damage to the module above the intershield was caused by allowing the module to approach its conditioned level. This demonstrates how important it is to operate well below this level. It is possible to monitor the column gradient using the colunto currents and alarm limits have now been set to warn of such a situation arising again.
2. Conditioning of the accelerator tube Voltage was first applied to the tube [3] on 18th April 1982. Using the shorting mechanisms each of the 36 modules was individually conditioned. Microdischarge activity [4] characterised by rapid bursts of X-rays which gradually decrease in overall intensity at a fixed applied voltage, set in at levels of 700-800 kV and the modules were conditioned at a rate of about 100 k V / h initially to 1.1 MV. In only three cases did modules show an uncharacteristic behaviour and this was attributed to particle contamination or field emission sites in these modules. Conditioning was only possible in these cases by allowing sparking to occur. Because of the low energy involved these discharges were always gentle enough to ensure that no tube damage was sustained. The modules were all then conditioned gradually to successively higher levels reaching in most cases 1.4 MV without any problems before the tube was run as a whole with all sections live. It is now routine to condition all modules up to at least 1.4 MV before running at high voltage. This gives a safety factor of 25% between the conditioned level and the operating level of 19-20 MV. Conditioning modules up to 1.5 MV has recently been carried out enabling operation at over 20 MV to be achieved. 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
IV. NEW ACCELERATORS
150
T.W. Aitken et al. / Initial experience at the Daresbury tandem
l.3
.Y.. ~60-TI _~ ]
"
~f
f
12 i
g i "oO"m ~- I -
"
~
W
.
t1,0 ~
LWL
S
L
Beam direction
0 50
W
o.8 ,3 /
Fig. 1. The effect of an SF6 leak.
4. Types of beam accelerated
S- Source L - Lenses W- Waist
L
Table 1 lists the ion beams accelerated for use in experiments with details of the energies a n d intensities used. The figures in brackets indicate the m a x i m u m currents so far analysed. The He ions were produced by the lithium charge exchange source while the remainder were produced by a Middleton sputter source modified to use the reflex geometry in order to be able to use small diameter pills. In m a n y of the cases enriched pill material or bleeder gas was used.
if)
W
5. Beam transmission efficiency Beams up to an intensity of 2/~A have been injected into the t a n d e m without causing appreciable loading a n d these have been transmitted with good efficiency. Fig. 2 shows a schema of the b e a m envelope through the machine. Ions are kept within the 25 m m diameter tube aperture by means of sets of magnetic quadrupole triplets a n d the b e a m is waisted by these at the i n p u t to the tube, the 1st stripper, the charge state separator slits, the 2nd stripper, towards the end of the high energy tube and the object point of the analysing magnet as shown.
Fig. 2. Schema of the beam envelope through the machine.
Table 1 Summary of beams used up to March 1983 Beam
Term (MV)
Charge
Output (MeV)
MeV/nucleon
Analysed current (nA)
4He 6Li 7Li 12C 160 170 t80 29Si 32S 34S 36S 48Ca 48Ti 58Ni
10-17 15-15.3 15-17 17.6 10.7-16 11.1 11-18.7 12 15-16.5 15-16.5 15-16.5 16.7-18.3 17.5-18.3 15.3
2+ 3 3÷ 5+ 6 +/7 + 6+ 6+/7 + 8+ 9+ 9+ 9+ 11 + 11 ÷ 11 +
30-51 60~-61 60-68 106 75-150 78 77-150 108 150-165 150-165 150-165 200-220 210-220 184
7.5-13.0 10-10.2 8.6-9.7 8.8 4.7-9.4 4.6 4.3-8.3 3.7 4.7-5.2 4.4-4.9 4.2-4.6 4.2-4.6 4.4-4.6 3.2
120-450 120-190 190-(500) 60 60-(3000) 120-330 12-400 40-80 80 10-30 10-30 36 37 10-(110)
T. IV. Aitken et al. / Initial experience at the Daresbury tandem
In practice, obtaining waists at the correct positions is carried out by using apertures or slits at the waist positions and maximising the intensity transmitted through these in sequence through the machine. In general standard settings can be predicted which virtually guarantee beam through the machine for any ion, molecule, energy and charge state and only require slight optimisation to maximise beam transmission. Problems with the beam scanners in our application have been overcome after extensive modifications and seven of the thirteen scanners are now working with good reliability. Lack of the availability of scanners held up initial beam studies. These have now shown that the beam is being transmitted in the correct manner. The output intensity is limited as expected by the acceptance of the machine. Calculation shows this to be 4 ~r mm mrad (MeV) 1/2 for 160 ions injected at 0.32 MeV with an 18 MV terminal voltage. When the input beam is limited to this the transmission efficiency is virtually 100%. Since about 30% of the output of the Middleton source can be outside this emittance it is tempting to try to increase the output beam by trying to inject the larger emittance especially for the more difficult ion species but this is of course unfruitful. Brighter sources would be valuable but by using enriched materials all the experimental requirements for output intensities have so far been met.
480a at 17.12 MeV
--
Theory ,, This experiment
"6
0
.
8
9
.
.
10
.
11 12 Charge state
~
_36
[
r
13
14
Locus of best _ focus for e states
<
E 32-
"5 28 o 24 30
40
50
Inner current (A)
Fig. 3. A calibration run identifying the charge states from 48CaH stripped at 17.9 MeV by a 5/~g/cm2 carbon foil.
151
6. The charge state selector This device [6] consists of an offset magnetic quadrupole triplet in the centre terminal following the first stripper. The dipole field components bend the required charge state in a dog-leg returning it on to the central axis while the quadrupole fields focus this charge state through defining slits on the axis after the separator. Fig. 3 shows a calibration run in which the charge states from 48Call stripped at 17.9 MeV by a 5 # g / c m 2 carbon foil are identified. The plot gives the currents required to focus each charge state at the image slits and also the fraction in each state. In the case shown the charge state distribution agrees with that predicted by the formula of Sayer [7]. The operation of the separator in practice is entirely predictable. Current settings can be generated for any ion and charge state required and normally the settings only require to be altered very slightly to adjust the output beam direction.
7. Beam energy stabilisation The output beam energy is kept stable by a feedback loop operating between the analysing slits and the laddertron downcharge as shown in fig. 4. Currents intercepted by the slits are fed to the control computer via logarithmic preamplifiers at a sample rate of 10 samples per second together with a beam on/off logic signal. Processing is then carried out by a minicomputer program based on that developed for the Pilot Machine [8] extended to use the slit signals and to switch automatically from slit to GVM stabihsation when the beam is switched on and off. The frequency of 1 Hz at which the loop gain falls to zero is determined by the rapid increase in phase delay caused by the 0.15 s time delay between change in downcharge and its effect on the terminal voltage. Despite this limited response the performance of the loop has been most impressive. Once the comer frequencies and gain had been set for optimum response the system has worked without the necessity for any further changes for a period of four months with beam currents in the range 1 to 3000 nA. The automatic switching works without any noticeable transients and copes with any type of beam interruption from short ion source trips to the insertion of low energy Faraday cups for long periods. The beam intensity through the stabilising slits is normally constant to within a few per cent and estimates of the energy stability from the slit output signals indicate an energy stability of better than +1 part in 104. Experimental measurements using apparatus with a resolution of + 1 in 6000 confirm this showing no measurable component due to beam energy fluctuations. The stability of the beam is the result of three main factors, the use of an electrostatically suppressed tube giving an extremely IV. NEW ACCELERATORS
152
T. IV. Aitken et al. / Initial experience at the Daresbury tandem
II
GVM . . . . 1 (Terminal to intershield) GVM 2 (Intershield to vessel) ----7
i
I
Mini computer
ge
_
inductor
I
._~ Down charge
inductor kV
Error _ . . Other charging system voltage Infra- red link
i
Laddertron
Keyboard
lysing net
InlT\eractive d sp ay
Top slit u
n Bottom slit
Fig. 4. Schematic diagram of s~t stabi~ser.
stable output beam position, the use of a laddertron to eliminate belt ripple and the presence of the intershield acting as a large smoothing capacitance. The machine within the intershield is solely responsible for short term energy variations and hence the feedback loop has parameters independent of the characteristics of the machine outside the intershield. 8. Foils The foils used have all been 5 # g / c m 2 slackened glow discharge carbon foils of the " D a r e s b u r y " type [O]. These have shown excellent performance giving a useful lifetime of 10 h for 48Ca at 18 MeV with an intensity of 100 nA. The lifetime was determined by the gradual spread in energy loss in the foil due to thickening or distortion rather than by sudden foil rupture. Studies are being carried out using the G V M to measure the energy loss and rotating wire scanners before and after the analysing magnet to measure the spread in energy loss in the foil during its useful lifetime. First results using a S beam are not well understood and more work is required. These studies will be extended to other types of ions and foils.
source, injection system and low energy beam line, the second controls the machine and high energy beam line beam handling components and the third controls the machine charging system including the energy stabiliser. Each mini supports an interactive colour TV display, two control knobs and can present information on oscilloscopes or analogue meters [10]. Experience with the system has shown that the operators find it easy to use and that the sequential allocation of the two control knobs to the beam handling devices from a master mimic diagram gives sufficient " f e e l " to make beam optimisation easy. Software routines have been developed to gang supplies together so that the two knobs provide for more complex control functions than simply that of one parameter. A fourth minicomputer provides alarms and a limited data logging facility. At present beam handling parameters have to be set up and recorded manually but it is planned to connect the minis to a midicomputer so that this can be done automatically. The midi will relieve the loading on the minis which are at present working at their limits and also provide a much more detailed safety surveillance than at present.
10. Conclusion 9. Control system The beam handling and charging systems are controlled by three minicomputers. One controls the ion
The N S F tandem at Daresbury is now operating with beam at voltages up to 20 MV with excellent stability. The limits of the machine have not yet been
T. W. Aitken et al. / l n i t i a l experience at the Daresbury tandem
fully explored and it is expected that with gradual development and a reasonable level of associated R and D even better performance will be achieved.
References [1] R.G.P. Voss, in: Proc. III Int. Conf. on Elect. Tech., Oak Ridge (IEEE Press, New York, 1981). [2] N.R.S. Tait, these proceedings (6th Tandem Conf.), p. 54. [3] T. Joy, in: Proc. III Int. Conf. on Electr. Tech., Oak Ridge (IEEE Press, New York, 1981).
153
[4] D.A. Eastham and R. Thorn, J. Phys. D: 11 (1978) 1149. [5] D.A. Eastham, these proceedings (6th Tandem Conf.), p. 101. [6] D.A. Eastham, T. Joy and N.R.S. Tait, in: Daresbury Laboratory Preprint D L / N S F / P 5 (1973). [7] R.O. Sayer, Rev. de Phys. Appl. 12 (1977) 1543. [8] T.W. Aitken, I. Goodall and K. Spurling, Nucl. Instr. and Meth. 153 (1978) 333. [9] D.A. Whitmell, B.H. Armitage, D.W.L. Tolfree and N.R.S. Tait, A new Daresbury Lab. Preprint DL/NSF/P86 (Accelerator) (1978). [10] J.C. Beech et al., these proceedings (6th Tandem Conf.), p. 170.
IV. NEW ACCELERATORS