*Present address: Institute for Solid State Physics, Universityof Tokyo, 5-1-5 Kashiwanoha,Kashiwa, Chiba 277-8581, Japan.Fax: #81-471-36-33-35.E-mail address: fritz.herlach@fys.kuleuven.ac.be (F. Herlach).

Physica B 294}295 (2001) 500}504

Innovations and trends in magnet laboratories and techniques

Fritz Herlach*

K.U.Leuven } Natuurkunde, Lab. voor Vaste-Stowysika en Magnetisme, Celestijnenlaan 200 D, B-3001 Leuven, Belgium

Abstract

This is a survey of the contributions from 23 magnet laboratories presenting their facilities in a special poster session.The presentations are focused on innovation and future development of magnet laboratory facilities, magnet technologyand experimental techniques. Highlights are the 45T hybrid magnet, quiet water-cooled magnets with "elds above 30T,a large variety of pulsed magnets with exceptional performance, and megagauss "elds from explosive-driven #uxcompression, electromagnetic #ux compression and the single-turn coil. � 2001 Elsevier Science B.V. All rightsreserved.

Keywords: Magnet laboratories; Experimental techniques; Research magnets

1. Introduction

At the HFM-88 conference [1], a special sessionwas dedicated to the presentation of magnet laborat-ories and magnet technology. At subsequent con-ferences, this topic was present to di!erent degrees.For the RHMF-2000 conference, all major magnetlaboratories were invited to present innovations oftheir facilities. The response was large, 23 laborator-ies submitted abstracts for this special session. Withso many contributions, this could only be realised asa poster session. This was enhanced by an introduc-tory lecture which is the basis of this paper.

2. Steady-state magnets

Francis Bitter has been the foremost pioneer ofwater-cooled high-"eld magnets. It is remarkable

that his design with a stack of `Bitter platesa is stillused today. `Polyhelixa magnets were introducedas an e$cient alternative, but the robust and simpleBitter design seems to win out in the end. ModernBitter magnets may consist of several sections(`Poly-Bittera); in recent years remarkable progresswas made by reshaping the cooling holes forimproved strength and by dimensioning the water-cooling system for low-noise operation. This result-ed in a 33T water-cooled magnet and a 45T hybridmagnet, both at the NHMFL [2].Steady-"eld magnets are the most valuable

research tools for convenient experimentation. Thestandard bore used to be 50mm, but for the higher"elds the bore had to be reduced to about 30mm.The required high power of order 20MW can onlybe provided at large national or international facili-ties. The major facilities worldwide are present atthis conference: NHMFL Tallahassee (40MW) [2],Grenoble (25MW) [3], Nijmegen (20MW) [4],Tsukuba (15MW) [5], Sendai (8MW) [6], Wroc-law (5MW) [7] and Hefei (3MW) [8]. Mostof these laboratories provide extensive and

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well-equipped user facilities, and there are engineer-ing groups for advanced magnet design and mater-ials development.Operation of these facilities is expensive, among

others because of the electricity bills. Therefore, thedevelopment of high-"eld superconducting mag-nets is pursued at several laboratories. The nexttargets are the 25 T magnet [5,9] and the high-performance magnet for 1GHz nuclear magneticresonance [2,5]. One relative inconvenience ofsuperconducting magnets is the consumption oflarge amounts of liquid helium. This is eliminatedin the so-called cryogen-free magnets that incor-porate a closed-circuit refrigerator [5,6]. In thelong run, it can be expected that facilities for highDC "elds will be dominated by superconductingmagnets, with a few hybrid magnets worldwide forthe highest steady "elds up to 50 or even 60T.

3. Long-pulse magnets

The pulse duration of a long-pulse magnet is ofthe order 1 s. Sometimes these are called quasi-sta-tionary, but this is an illusion because from an op-erational point of view these have all the sameadvantages and disadvantages that are associatedwith pulsed operation. By contrast to compact mag-nets with shorter pulse duration that are energisedby capacitor banks, the long-pulse magnets have theuseful feature of a programmable pulse shape. A bet-ter name would therefore be `controlled waveformamagnets. A disadvantage is the residual ripple fromthe controlled recti"cation of alternating current(from the mains or from #ywheel generators).The pioneering prototype for all controlled

waveform magnets is the 40T magnet that wasbuilt at the University of Amsterdam in 1969 [10];this is still in operation. Among recent projects tobuild 60T long-pulse magnets, only the LosAlamos magnet has been realised [11]. This mag-net is a valuable research tool for experiments thatcannot be accomplished in the 10ms range of com-pact magnets, for example the measurement of spe-ci"c heat. Other applications are research onmetallic samples and at extremely low temperature.For the larger laboratories that cannot a!ord the

huge budgets required for state-of-the-art DC mag-

nets, the next best thing is a long-pulse magnet(apart from superconducting magnets, of course).This approach is pursued at Vienna [12], Wroclaw[7] and Hefei [8]. Most pulsed magnets are cooledwith liquid nitrogen. At Hefei, the "rst large water-cooled pulsed magnet is in the planning. Recently,a new type of long-pulse magnet has been success-fully tested at Grenoble [3]. This magnet is ener-gised from inductive storage capable of storing72MJ; the "eld coil is located inside the storage coilin order to take additional bene"t from the "eld ofthis coil. In a successful test, 5.9MJ were transfer-red from 15.1MJ accumulated in the storage coil;the peak "eld was 57.2T (including 3.6T from thestorage coil) in a 24mm bore at an energy of 1.8MJand with a pulse duration of 77ms (at B'40T). AtDresden-Rossendorf [14], an ambitious project isunder consideration to build a very large capacitorbank of order 50MJ that could become the Euro-pean counterpart of the Los Alamos installation.

4. Compact non-destructive pulsed magnets

These magnets are called non-destructive be-cause they are operated close to the limit where themagnetic forces will destroy the coil. The largelong-pulse magnets are farther away from destruc-tion because these are more expensive and there-fore operated at more conservative stress levels.A typical compact magnet is energised by a capaci-tor bank with energy of order 100 kJ}2MJ; it hasa mass of a few kilograms and therefore can behandled with relative ease. A skilled person canmanufacture such a magnet in less than a week;therefore a failed magnet can be easily replaced. Ofcourse, it is good practice to monitor the magnetscarefully and to never use more energy than reallyneeded for any experiment, in order to keep thee!ect of the occasional explosion within limits (thisis one more reason for the conservative design oflong-pulse magnets where huge energies are in-volved).

4.1. User magnets

Announcements of record "elds have to be takenwith a grain of salt; pulsed-"eld designers are

F. Herlach / Physica B 294}295 (2001) 500}504 501

notorious optimists! The upper limit at the presentstate of the art of user magnets is 60T for up to25ms in a bore of order 20mm. If pulsed up to thislimit, these magnets may last for not more than 100pulses. For 55T magnets, a service life of up to 1000pulses has been reported. 70T user magnets witha 10mm bore and 10ms pulse duration shouldbecome available soon. These need a capacitorbank of at least 500 kJ at 10 kV. Further progresscan be expected, but this will require much e!ortand patience.Magnets of this type are now in operation at

a large number of laboratories that is steadilyincreasing [9}23]. One of the "rst was Foner'slaboratory at MIT that is still in operation [24],and the largest so far is the Toulouse laboratorythat has a long-standing tradition [22].Most pulsed "eld laboratories used to develop

and build their own coils. More recently, pulsedmagnets have become commercially available, to-gether with complete facilities [25,26]. Some of thenew laboratories prefer indeed to buy their coils[12,14,15]. The capacitor bank at Frankfurt wasbuilt by Oxford Instruments, and the bank to beinstalled at Nijmegen will be built by Metis.

4.2. The drive towards 100T

Since several years, projects have been pursuedfor developing a non-destructive 100T magnet.This is a valid challenge because the limit of inevi-table destruction is in the vicinity of 100T("1MG). However, most magnet designers stillmay underestimate the di$culties that have to beovercome. Experienced designers share the generalfeeling that with present coil construction tech-niques a brick wall is hit around 80T. New mater-ials and coil construction techniques will be needed.In Europe, a group that calls itself EUROMAG-

TECH was founded at the time of the HMF-88conference. This received continuous support fromthe European Commission for developing high-"eld magnets and the needed high-tech materials.A feasibility study sponsored by the EC and by theEUPRO (European Union of Physics ResearchOrganisations) came to the conclusion that a 100Tmagnet could be built in principle but this shouldbe done in stages. The latest project in this series

(ARMS"Advanced Research Magnet Systems) istherefore aimed at setting up an 80T user magnetat Toulouse where a large capacitor bank is avail-able [27].In the US, the approach is more direct. The

availability of the large #ywheel generator at LosAlamos [11] has inspired the necessary optimismto design a very ambitious 100T magnet. It isindeed taken for granted that any 100T user mag-net will consist of a large outer coil that providesa background "eld with long-pulse duration, andan inner short-pulse coil that is made of advancedmaterials and energised by a capacitor bank. TheARMS magnet is based on the same principle.At Osaka, on the other hand, an e!ort will be

made to design a more compact 100T magnet,energised by the available capacitor banks [19].

5. Destructive magnets for megagauss 5elds

Fields in excess of 100T cannot be generatedwithout destruction of the magnet. The emphasis ishere on experimental techniques that work in theshort time interval (of order microsecond or less)before the magnet is destroyed by the combinede!ect of magnetic forces and Joule heating. Withincreasing "eld, the extent of destruction rangesfrom the destruction of a single-turn coil that leavesthe sample intact, to an explosion that leaves a bigcrater in the ground.

5.1. The single-turn coil

Since the single-turn coil was introduced as a re-search instrument [28], much progress has beenmade, in particular with experimental techniques.Recently, two new facilities with single-turn coilshave been built: one at the Humboldt University atBerlin [13] and the other at the new Kashiwacampus of the University of Tokyo [16]. The Hum-boldt facility features the highest voltage used so far(60 kV) and infrared diagnostics with the unprece-dented resolution of 0.5% of the transmitted radi-ation. The Kashiwa facility builds on the 20 yearsof experience gained at the Roppongi campuswhere the ISSP megagauss Laboratory wasformerly located. There are two single-turn coil

502 F. Herlach / Physica B 294}295 (2001) 500}504

installations, one with the coil horizontal and theother vertical, supported by the most modern diag-nostic instrumentation. In both facilities, record"elds are above 300T and user "elds up to 250T, atliquid helium temperature.

5.2. Electromagnetic yux compression

The principal installation at the Kashiwa facilityis a large 5#1.5MJ capacitor bank for electro-magnetic implosion [16]. This also builds on theexperience gained at Roppongi where a 600T re-cord "eld was achieved. The new capacitor bankhas a smaller internal inductance and is aimed atgenerating "elds in excess of 750T. The supportinginstrumentation is outstanding, and cryogenic tem-peratures are available even in the implosion sys-tem.

5.3. Explosive-driven yux compression

Sarov is at present the only place in the worldwhere explosive-driven #ux compression deviceswith high performance are developed [29]. Thedevelopment at Sarov is based on the creative workof A.D. Sacharov and A.I. Pavlovskii and theircollaborators. The cylindrical #ux compression de-vices rely on so-called cascades for stabilising theimplosion. This enabled the generation of up to10MG in a `standarda device that is now widelyused for experiments in solid-state physics. Thedevices are used in the international `Kapitzaaprogramme at Sarov and also in the `DIRACaseries of experiments at Los Alamos. The devicedevelopment has culminated in a generator thathas achieved 28MG. At present, work is under wayto consolidate this success and to make the highest"elds available for experiments.

6. Conclusion and outlook

Magnet laboratories worldwide are in a healthystate of development. The NHMFL [2,11] is nowin full operation and has set new milestones inmagnet development: the 33T water-cooledmagnet, the 45 T hybrid and the 60T long-pulsemagnet. At Nijmegen, a completely new magnet

laboratory will be built. Together with the largelaboratories at Grenoble, Tsukuba and Sendai, thiswill certainly meet the demand for DC "elds in theforeseeable future. For some laboratories, opera-tion at high levels of continuous power would bea heavy burden on the budget; these are going intwo alternative directions, long-pulse magnets [7,8]and superconductingmagnets [5,6,9]. The conveni-ence of operating superconducting magnets isgreatly enhanced by the development of the `cryo-gen-freea magnets that are most practical self-con-tained research instruments [5,6]. In the long run,it can be expected that superconducting magnetsfor higher "elds will become available; even anothermajor breakthrough in superconductor technologyis possible if not likely.Pulsed "eld laboratories are steadily increasing

in number and size; most of the recently establishedlaboratories have a capacitor bank of the order ofmegajoule and several measuring stations that canbe used in parallel. Great improvements in instru-mentation technology have facilitated experi-mentation with pulsed "elds. Another trend is theminiaturisation of experiments down to the micro-and nano-metre scale. This has direct bene"ts be-cause decreasing the bore of the magnet is the mosteconomical and practical way to obtain higher"elds. Another factor that enables higher "elds isthe reduction of pulse duration. Fields above 100Tpersist only for a few microseconds or even frac-tions of a microsecond. The development of fastdigital recording instruments and optical tech-niques has made it possible to do precise experi-ments at high sensitivity even in the microsecondrange.Two important parameters in solid-state

research are temperature and pressure. Much pro-gress in these disciplines has been made recently.Liquid �He is now commonly used, and the "rstdilution refrigerators have been installed in pulsedmagnets [11,18,19,22]. One of the three laborator-ies that make up the NHMFL is dedicated toresearch at extremely low temperatures [30].High-pressure devices have been constructed that"t into the small bore of pulsed magnets and thatare made frommaterials with su$ciently high resis-tivity to let the pulsed "eld penetrate. One exampleis described in detail at this conference [17].

F. Herlach / Physica B 294}295 (2001) 500}504 503

Finally, miniaturisation of the magnets themsel-ves may o!er many advantages. The ultimateminiaturisation is achieved by fabricating coils withlithographic techniques, possibly together with thesamples. First steps in this direction have beentaken [31], but here is a large and interesting terri-tory to be explored.

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