The CEPC-SPPC Project, a Trip to China

CEPC-SPPC: Circular Electron Positron Collider – Super Proton-Proton Collider

Ernie Malamud

In the fall of last year I was “called out of retirement” to assist in the development of a pre-CDR (Conceptual Design Report) for the ambitious Chinese CEPC-SPPC proposal. [1] My role was mainly to smooth out the English, but I was also able to draw on many years of accelerator experience and the preparation of similar large reports [2, 3] to be able to make numerous suggestions on content details. It gave me great satisfaction to be able to do so.

I was invited by Weiren Chou, my Fermilab colleague of many years to join this effort. I spent two weeks at the Institute for High Energy Physics (IHEP) of the Chinese Academy of Sciences. IHEP is a comfortable place to work. It is located in an animated quarter of Beijing; the Guest House is comfortable; the onsite restaurant is excellent and I had a well-equipped office in the IHEP main building.

The pre-CDR has been written by a large group, mostly young hard-working Chinese physicists and engineers. The design study author list currently has 258 names from 44 institutions. Most of the institutions are Chinese with IHEP the lead institution. It is impressive how rapidly the CEPC-SPPC project has evolved. Also it impressed me that these many different Chinese physicists and engineers were able to express themselves in a language that is certainly not native to them and quite different from Chinese. As I worked my way through the text it was apparent that each chapter and in many cases each section of each chapter was written by a different person or group with their own stylistic quirks.

The discovery of the Higgs and its relatively low mass (126 GeV) revived interest in large-circumference circular colliders. (See also the article on the FCC Study [4] in this newsletter.) And once having a large circumference tunnel, it also revives our dream of a super high-energy hadron collider, a dream that died with the demise of the SSC.

The CEPC-SPPC pre-CDR concentrates on the accelerator physics (chapter 4) and the technical systems (chapter 5) required to achieve the performance goals of the e+e- Higgs factory. The pp machine is not yet developed in detail. An important feature of CEPC-SPPC is that the tunnel is large enough so that the pp ring can be put in place without disturbing the lepton collider; both physics programs could be run simultaneously, and there is the exciting option of ep and eA collisions.

The CEPC-SPPC facility will be 50 to 100 km in circumference. Most of the detailed work in the report assumes C=54.7 km. The e+e- working parameters are cm energy 240 GeV and integrated luminosity per IP per year of 250 fb^-1. Higgs bosons are produced mainly through the e+e- → ZH reaction. At CEPC the Higgs can be detected through the recoil mass method by reconstructing only the Z without including the recoiling H in the event reconstruction. Therefore, Higgs production can be disentangled from its decay in a model independent way. Moreover, the environment at a lepton collider allows clean exclusive measurement of Higgs decay channels. The CEPC will have an impressive reach in probing Higgs properties. With an integrated luminosity of 5 ab-1, over one million Higgs will be produced.

Important decisions for the CEPC were to have a one-ring collider so both electrons and positrons travel in the same beam pipe, and to have a full-energy Booster. There are both advantages and disadvantages to these choices. The one-ring collider choice is similar to BEPC-I, LEP and the CESR. An alternative design, which is preferred for beam physics considerations and machine operation, but which costs more, is to use two beam pipes as in BEPC-II, PEP-II, KEKB and DAFNE. Two-beam pipes could give higher luminosity because a larger number of bunches are allowed.

The CEPC will have 8 arcs and 8 straight sections. Four straight sections, about 1 km each, are for the interaction regions and RF; another four, about 850 m each, are for RF, injection and beam dump. The lengths of these straight sections are determined by taking into account the future needs of the huge detectors and complex collimation systems of the SPPC. The total length of the straight sections is about 14% of the ring circumference, similar to the LHC. Among the four IPs, IP1 and IP3 will be used for e+e- collisions, whereas IP2 and IP4 are reserved for pp collisions. The tunnel will be underground, 50 - 100 m deep, to accommodate these three ring accelerators: the CEPC collider, the SPPC collider, and a full energy booster for the CEPC. Therefore, the tunnel must be big, about 6 m in width because it is planned to keep the CEPC ring in the tunnel when the SPPC is built and operates. While the two colliders will be mounted on the floor, the booster will hang from the ceiling, similar to the Recycler in the Main Injector tunnel at Fermilab.

The top level parameters for the pp collider are less certain. The energy, of course, depends on the final ring circumference and achievable magnetic fields in production magnets. With the 54.7 km circumference, 70 TeV cm can be realized if the magnets, probably constructed from a combination of Nb3Sn and Hi-Temperature SC coils can operate at 20 T. A luminosity goal of 10³⁵ is stated in Chapter 7 (Upgrade to the SPPC) but there is considerable debate in the HEP community of what it needs to be for the physics one is aiming at but also the ability of future detectors to handle these high luminosities. Center of mass  energies will depend on the final choice of circumference and what can be achieved in the magnet R&D program. These energies are a factor of 5 to 7 jump from LHC, which itself is a factor of 7 jump from the Tevatron. So this is a logical future step as humanity pushes forward on the energy frontier.

More than 65% of the Booster and Main Ring circumference will consist of dipole magnets. Therefore, the magnet cost becomes an important issue, especially the dipole magnets. Since the field of the dipole magnets is very low, as in LEP's dipole magnets, steel-concrete cores will be used to make the yokes. Advantages for steel-concrete cores are cost reduction since concrete substitutes for 75% of the steel and also by increasing the magnetic induction in the iron, the magnets are less sensitive to variations in iron quality and in particular to the coercive force.

There are numerous possibilities for the site. As was learned from the Fermilab and the SSC site selection processes, choosing a site is complex and many factors are involved. In Chapter 9 of the report a candidate site near Qinghuangdao, a city of 2 million, about 300 km east of Beijing, has been chosen. This site has excellent geology and other advantages. Chapter 9 is interesting reading because the site selection criteria, geology, water, site access, possible construction methods are outlined in detail, and then applied to this “candidate” site.

After considering the details of the technical systems as well as the civil construction it is possible to make a tentative time line for the CEPC for a 5-year R&D phase followed by a 7 year construction period. The most expensive technical systems of the CEPC are: (1) the superconducting RF (SRF) system; (2) the RF power source; and (3) the cryogenic system and a large part of the R&D budget will focus on these systems. The CEPC SRF system will be one of the largest and most powerful SRF accelerator installations in the world. To succeed with designing, fabricating, commissioning and installation of such a system, a significant investment in R&D, infrastructure and personnel development is necessary. The total RF stations provide 12 GeV of RF voltage. The collider will use 384 650-MHz five-cell cavities in 96 cryomodules for the collider and 256 nine-cell 1.3-GHz cavities in 32 cryomodules for the Booster. All the cavities will be cooled in a liquid-helium bath at 2 K. Thus the cryogenic system is a major component of CEPC. The majority of the R&D budget (58%) will be invested in the “big three” systems – SRF, RF power source and cryogenics.

In addition to the capital construction cost, the operations cost is another major issue. It is mainly determined by the power consumption to operate the CEPC. When the Tevatron was running, the average total power usage at Fermilab was 58 MW. When the LHC was running, CERN used 183 MW (average over 2012). The consensus for operating a future circular Higgs factory is that the power should not exceed 300 MW, in which 100 MW is for the synchrotron radiation. In other words, wall plug efficiency of 1/3.

This report only describes a few of the highlights of my trip to China and a few of the major parameters of the CEPC-SPPC facility. The full report [1] will be published soon so the interested reader can learn more details. This is work in progress. Parameters may change but the basic concept is sound. I look forward to another two weeks in Beijing in March as we work to finalize the report after a February review.

References:

[1]. “CEPC-SPPC, Preliminary Conceptual Design Report. To be released as IHEP-AC-2015-001
[2] National Accelerator Laboratory, original design report, 1967 (co-author of two of the chapters)
[3] Design Study for a Staged Very Large Hadron Collider, Fermilab TM-2149, June 4, 2001. (an editor of the report as well as participating in the VLHC studies over several years).
[4] Michael Benedikt and Frank Zimmermann, “Future Circular Collider (FCC) Study,” in this newsletter issue.

Ernie Malamud (马欧尼), spent three decades at Fermilab participating in high energy physics experiments and accelerator design and construction. He is a Fermilab Scientist Emeritus, is on the adjunct faculty at the University of Nevada, and a member of the CEPC-SPPC Design Study Group.

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