VLHC BASED ON COOLED IRON INTERMEDIATE FIELD
SUPERCONDUCTING MAGNETS
A.D.Kovalenko, JINR, Dubna, Russia

Abstract
A new approach to the VLHC cryomagnetic systemdesign is considered. The magnets are based on asuperconducting one-layer winding at T=4.5K and an ironyoke/shield at T=(50÷80)K. The experimental results
obtained in this direction at the Laboratory of HighEnergies, JINR are presented. Essential features of theredesigned Nuclotron-type and cosine θ -type magnets areanalysed. Other key problems of the VLHC construction
are also discussed.

1 INTRODUCTION
Works on a search for and an analysis of the most costeffective cryomagnetic systems of proton and ion highenergy synchrotrons/colliders over an energy range ofmuch larger than the LHC started at the Laboratory ofHigh Energies (LHE), JINR in 1995. Our first result,presented at the Mini-Symposium “New Low-Cost
Approaches to High Energy Hadron Colliders atFermilab” in Indianapolis, was based on the extrapolationof the operating parameters of the Nuclotron
cryomagnetic system, as well as on the results of R&D
works at miniature iron-dominated 2T field SC-magnets
obtained at the LHE since 1975, for a 100 TeV
synchrotron/collider [1]. A more detailed design of thisconcept was reported at EPAC’95 in Barcelona [2] and at
HEACC’98 [3]. The option of a window-frame
2
20x30 mm (v × h) twin aperture (side-by-side) magnet
with a cold iron yoke and a one-turn NbTi
superconducting bus drived by I=34 kA per each aperture
was considered and discussed. By the time ofHEACC’98, the steering committee for a future verylarge hadron collider was appointed to coordinate efforts
in the United States to achieve a superconducting ppcollider with approximately 100 TeV cm and an34-2-1
approximately 10 cm sec luminosity [4]. Three
working groups, namely: magnet technology, accelerator
technology and accelerator physics, were established. The
progress achieved at our Laboratory and some new ideas
were presented at both the Workshops 1 and 2 on theVLHC Magnet Technology in Port Jefferson (November16-18, 1998) and in Batavia (May 24-26, 2000). The
option presented in Port Jefferson[5] and called “8”magnet was different from the previous one in its yoke
configuration and cooling system. A window-frame twin
aperture “bottom-on-top” yoke suitable for combinedfunction magnet operation was proposed. The possibility
to operate without a beam pipe and a high vacuum

pumping system was also discussed. Nevertheless, all of
the above - mentioned “Nuclotron-type” options of theVLHC were based on a cold iron/cold bore/LTS-coilapproach. The next step towards a more effective
cryomagnetic system can be made by applying a newconcept- an intermediate temperature iron SC-magnet [6].

2 IRON YOKE AT INTERMEDIATETEMPERATURE
The iron yoke is an important element of the magnet. It
plays two major roles in superconducting accelerator
magnets: a) it enhances the value and improves thequality of a magnetic field in the magnet bore, and b) itworks as a magnetic screen confining a return magnetic
flux and protecting the filed in the bore against thedistortion caused by variations of the external magnetic
environment. The greater part of SC-magnets designed
for high energy proton/ion synchrotrons is based on acold yoke (T=4.2÷4.5K). These are the magnets
developed for HERA, Nuclotron, UNK, SSC, RHIC,
LHC and for the most of high field VLHC options. There
are also three cases: a Tevatron, a Double-“C”
Transmission line magnet [7] and a double aperture 11T
Nb3Sn dipole magnet at Fermilab [8]. using an iron yoke
at T=300K. The main disadvantage of the magnet with aniron yoke at T ≅ 4.5 K is its huge cold mass. In the case of“warm iron” options, the cold mass has a minimum value.
Nevertheless, the field enhancement due to magnetic flux
concentration by the yoke is much smaller (for cosine
theta magnets). Another problem is the complexity of acryostat with a superconducting winding at T ≅ 4.2 K
inside.
The advantages of the above cases can be realized if
the temperature of the iron yoke (or its main part) has anintermediate level, say T=50÷80 K. Such an approach iseasily applied to different types of yokes: window-frame,cosine theta for single and double apertures, as well.
The proposed layout of the cosine θ magnet is shownin Fig.1. The superconducting winding is pressed between
two tubes a beam pipe and a cold mass shell. A coolant
passes through the space between the tubes to provide anoperating temperature. The beam screen is installed inside
the beam pipe to absorb the synchrotron radiation power.
Its temperature is much higher than that of the beam pipe.
(20 ÷ 80 K). The cold mass shell outer diameter made2-3 mm smaller than the internal diameter of the ironyoke. So, there is a gap of 1-1.5 mm between them after
assembling. The needed fixation of the cold mass and thealignment of the SC-winding, inside the yoke can be3 EXPERIMENTAL TEST OF THE 80 K
OPTION

Fig.1.
Intermediate
temperature
superconducting dipole magnet.

iron2-2.5T

made in a different manner. Such a kind of the problem
has been solved under the Tevatron design. Our case ismuch simpler because the support structure is completely
inside the vacuum vessel. Due to a small weight andsurface of the cold mass and not so high electromagnetic
displacing forces, the heat flow through the support
structure does not exceed 0.1 W/m. There is no separate
liquid nitrogen or gaseous helium screen because the yokeplays this role. The support of the yoke is also muchsimpler.
In accordance with our basic vision, a magnetic field of
2-2.5 T is the most reasonable value for a very large
hadron collider. The main limiting factor is synchrotron
radiation from the beam. The radiated power is expressed
as:
-21 2
2
P = 4.7 ⋅ 10 γ B N0 (W),2where γ = E/M0c – Lorentz-factor, M0 – proton rest
mass, c- speed of light, B – bending magnetic field in [T],N0 – the total number of accelerated protons.
The calculations made in [9] for both low (B=2.1 T)
and high (B=12.5 T) field options of 2x50 TeV vlhc, givethe value of losses 0.12 W/m and 2.2 W/m, respectively
34-2 -1
at a luminosity of 10 cm s . For much higher energy,
say 2 x 1000 TeV, the radiated power from the beamincreases up to about 50 W/m for the low field case and toabout 1000 W/m for the high field one. To compensate aheat flow of 50 W/m effectivily the radiation screen at
T=20-50 K can be used. It was made at a temperature
difference of 1.9 K/4.7 K at the LHC. Assuming a beam
radiation screen aperture of ∅ 35 mm, we can determine
the internal diameter of the SC-winding as 50 mm.
Notice that the separation of the SC-coil from the mainpart of the yoke allows an easy replacement of a lowtemperature superconductor by a high temperature one
without the reconstruction of magnet “hardware”.

First tests of the modified Nuclotron dipole were
performed at the LHE in December 2000. The standard
elements of the Nuclotron cryomagnetic unit are used,
namely: a window-frame iron yoke 1400 mm in length,
the sizes of the window are 146 × 56,2 mm; a sadle-shapesuperconducting two-layer winding made of a hollowtube NbTi superconducting cable. A more detailed
description of the Nuclotron magnets was publishedearlier [10]. A general view of the modified magnet inside
the cryostat is shown in Fig.2. The SC-coil is reinforced
by a fiberglass epoxy bandage 0.5 mm in thickness. The
sizes of the window increased up to 59 mm – vertical and
151 mm – horizontal. The adjustable support/alignment
shtifts passing through the holes in the yoke were used tofix the coil. As a result, the direct thermal contact
between the SC-coil cooled by two-phase He at T=4.5 K
and the yoke cooled by liquid N2 at T=80 K wasdecreased. The magnet was ramped by current pulses up
to I ≅ 5.9 kA, with a repetition rate up to f ≅ 1 Hz. The
maximum magnetic field in the aperture was B ≅ 1.87 T
and dB/dt ≅ 4 T/s. The measured static heat flow(~1.8 W/m) was close to the calculated one.

Fig.2. View of the modified Nuclotron dipole.
It should be noticed that the dynamic heat releases inthe cold mass were at the level of 25-30 W or about twiceas less in compare to the original Nuclotron dipole.
The investigation is continuing in cooperation with GSI(Darmstadt).

4 SUPERCOLLIDERS OR COSMICRAYS?
The only tools for the physics quest into an energy
range of tens and hundreds TeV’s are hadronsupercolliders. There is no even idea how to construct an+ + e e - or µ µ -collider of comparable energies. On theother hand, it is interesting to imagine a reasonable ornatural limit for a possible future hadron collider.
Cosmic rays are the “Alma mater” of elementary
particle physics. Many discoveries of new elementary
particles were made at the accelerator given to us by theNature – cosmic rays. The highest energy of a proton,
20whenever observed in cosmic rays, is E = (3,0 +−00,,3654 ) 10
eV [11]. Is it possible to attain higher energies at a “handmade” hadron collider? If we assume the collider of22x1000 TeV, the equivalent energy: E= 2E cm /M0c2 will
be about 2 ⋅1021 eV! The natural limit for such collider isthe length of the accelerator/collider ring. Let’s take themagnetic field value of about 2T. In this case, thecircumference of the accelerator ring should be about
104 km. Is there a place on the Earth where such a ringcan be built? The answer is shown in Fig.3. The imagine
ring surrounding the Sakhara desert has just the samelength.

insufficient at present. The author’s understanding of thisproblems is very clear. Nevertheless, I would say, there iseven a wonderfull coincidence of such different
circumstances as:- the needed dimensions of the accelerator ring andthe envelope of the area;- excellent stable geology, the absence of industrial
ground motion- a long underground tunnel passing around the area
can be designed not only for the accelerator but also
for interstate connections,- much Sun energy, much gaseous helium, muchocean water not far from the main part of the ring.
So, it looks very attractive. The efforts of the worldcommunity in designing such a project can lead to generalprogress of at the continent as well as to the progress of ahigh energy accelerators for a long future.

6 ACKNOWLEDGEMENT
The author is grateful to Drs. H.Khodzhibagiyan,N.Agapov, P.Isaev for stimulating discussions as well as
to Mrs. N.Korotkova, L.Sveshnikova and L.Barabash forthier help in preparation of the manuscript.

REFERENCES

Fig.3. Main parameter of a 2 x 1000 TeV hadron collider.

5 CONCLUSION
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