The 400 GeV Proton Synchrotron (SPS)

Excerpt from the CERN Annual Report 1976
by John Adams

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The 400 GeV Super Proton Synchrotron (SPS) is the latest and largest of the CERN accelerators. Its early history is so recent that there is no need to dwell on it in this review. A first design was put forward to Council in 1964 and, in a considerably modified form, the project was finally approved in February 1971. During the last five years, the design and construction of this giant machine has progressed very well, and in the spring of this year the commissioning stage was reached.

The first stage of the commissioning, which started on 5 April 1976, was to extract a proton beam from the PS machine at 10 GeV energy, using the continuous extraction scheme, and transport it down the injection transfer line to the injection straight-section of the SPS machine, a distance of about 800 m. All the beam elements of this transfer system were set up to their calculated values and the beam arrived at a temporary beam dump placed near the SPS within a few millimetres of the required position. Once the temporary dump was removed, the beam continued through the injection system of the SPS and on to the orbit of the machine.

The second stage of commissioning was to allow the beam to go once round the SPS. This stage began and finished on 3 May, about a month after commissioning started. The beam went all round the 7 km circumference of the machine and arrived on a first-turn beam stopper only a few millimetres off target. Using the closed-orbit correcting dipoles, which are installed all round the machine, some minor adjustments were made to the transverse position of the beam at a few places around the circumference of the machine, and then the first-turn beam stopper was removed to allow the beam to circulate freely round and round the machine. Much to the delight of the accelerator builders, the beam circulated with very little loss of intensity for over 10 000 turns after which it was dumped on to an internal beam dump.

These tests confirmed that there were no obstructions in the vacuum system of the SPS and that the 1000 or more elements of the magnet system had been made within the prescribed tolerances and were correctly aligned in the machine tunnel. In many ways this was the most important test of all, since any errors in these respects would have taken months to rectify.

The third stage of commissioning began on 6 May and concerned the acceleration of the circulating beam. During three runs up to 10 May, the trapping of the circulating beam in the RF buckets of the SPS was achieved, and the phase and radial loops of the RF servomechanisms were closed and operated. The protons were accelerated only a few GeV during these tests; just sufficient to check the trapping efficiency, which turned out to be better than 60% . During these test it became apparent that more work had to be done on understanding and controlling the movement of the working point of the machine in the Q plane before the acceleration tests could sensibly continue.

Consequently, the runs on 12 and 14 May concentrated on this problem with very encouraging results. The measurements showed that the SPS is a very clean machine in the sense that higher-order resonances, due to unintentional sextupole and octupole fields around the 7 km circumference of the machine are only weakly present, and that the magnet fields are nearly pure dipole and quadrupole, as one would hope. During these measurements the chromaticity of the machine at injection was measured and then adjusted, using the sextupole correcting magnets installed for this purpose all around the machine. The octupole correcting magnets which are also incorporated in the SPS were used to suppress a resistive wall instability which became noticeable at moderate intensities.

The acceleration tests were continued again on 25 and 26 May, and this time, after checking the beam trapping and control system, the beam was accelerated through to transition energy, first up to 50 GeV and then to 80 GeV. The reason acceleration was stopped at 80 GeV and the beam dumped, was that only two of the twelve magnet power supplies were ready at that time.

Tests were resumed on 3 and 4 June, at which time six power supplies were available and operating under control from the main control room. After removing a transient shift in Q early on during the acceleration cycle, it was found possible to accelerate a proton beam with an intensity of 2.2 x 10E12 p/p up to 200 GeV without any measurable loss. The beam intensity injected into the SPS was 3 x 10E12 p/p so the trapping efficiency was over 70 %. Thus in a matter of two months from the beginning of commissioning, the SPS had reached 200 GeV energy.

Four more power supplies were available for the runs on 10 and 11 June, but it was found that at a magnet cycle corresponding to an energy of about 240 GeV, instabilities set in in the power supply system and all attention was focused on this problem to clear the way to higher energies.

A circulating proton beam of 400 GeV energy was first achieved in the SPS on 17 June 1976, which happened to be the date of the Council session.

Having reached the design energy of 400 GeV, the next step was to extract the circulating proton beam from the machine. The simplest extraction method is to extract all the beam in one revolution, which takes 23 microseconds, and this was successfully achieved for the first time on 9 July. During the rest of July and August the circulating beam intensity was increased from 10E12 protons per pulse to 5 x 10E12.

In September the slow extraction system was brought into operation and all the circulating beam extracted from the machine over a period of 700 ms.

On 3 November, a circulating beam intensity of 10E13 protons per pulse at 400 GeV energy was achieved for the first time. Early in December 4 x 1OE12 protons per pulse were extracted from the SPS at 400 GeV energy.

In parallel with the commissioning of the SPS machine, the primary and secondary beams in the West Experimental Area were being installed and tested, and the experiments themselves brought into operation.

The extracted proton beam from the SPS machine was first brought to the primary proton targets, Tl, T3 and T5, which feed the secondary beams in the West Hall, on 22 October. On 3 November, the secondary beams Sl and H1 were powered for the first time and transported particles to four experiments in the West Hall. A few days later experiment WA4 was supplied with an electron beam which was then converted into photons. By the end of November the experiment WA10 was fed with a hadronic secondary beam and the secondary beam H1B was used to feed experiment WA3. Also an attenuated proton beam was brought to the hyperon beam Yl and operated together with experiment WA2.

Finally, in the middle of December, the narrow-band neutrino beam was tested successfully and experiment WA1 started to collect data on neutrino events.

The situation at the end of the year was that all the secondary beams in the West Experimental Area were commissioned, with the exception of the wide-band neutrino beam, which is scheduled for April 1977, and the experiments have all received test beams and checked out their apparatus.

Next year, on 7 January 1977, the SPS will start scheduled running, and the experimental research programme will start in earnest.

Now that the SPS accelerator is working, it is interesting to look back at the document CERN/958/Rev. which was used by the Council to launch the 300 GeV Programme in February 1971. In that document certain conditions were specified for the 300 GeV Programme which can be used as a check list to see how well the instructions of the Council have been followed.

In paragraph III.1 of the document, the aim of the Programme was stated to be a proton synchrotron which would give an incident proton energy of at least 300 GeV and an internal beam intensity of at least 10E12 protons per second. Both these conditions have been met. The incident proton energy is 400 GeV and the circulating beam intensity is 1.2 x 10E12 protons per second.

In paragraph III.4 of the document, it is required that research should start at 300 GeV or at an intermediate energy in the existing West Hall during the sixth year of the Programme. The sixth year ends on 19 February 1977. As reported above, the experimental programme will start in the West Area on 7 January 1977 both at 200 GeV and at 400 GeV energy. So this requirement has also been met.

Lastly, the document gives the estimated cost of the 300 GeV Programme as 1150 million Swiss francs at 1970 costs, and states that this is the financial ceiling for the Programme. It is now estimated that the Programme can be completed for less than 1150 million Swiss francs at 1970 costs, so this requirement will also be satisfied.

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