High-Magnetic-Field Research and Facilities: [Final Report] (1979)

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5 Magnet Design and Materials INTRODUCTION The goals identified in Chapter 3 on Scientific Opportunities to ensure the continued progress of research using high magnetic fields were 1. Continuous operation up to approximately 75 T; 2. Quasi-static pulse operation up to approximately 100-200 T, with fields roughly constant for 10-100 msec; 3. Pulse operation up to 1000 T (nondestructive) for times in the micro- second range; and 4. Very short (destructive) pulse operation in the nanosecond range up to 10,000 T (100 megagauss). Our task in magnet design and materials was to determine the resources necessary to attain these far-reaching goals, as well as to consider ways that currently available resources might be used in the near term to extend capa- bilities to intermediate or more modest goals. The Panel examined three categories of field generation: (a) continuous high fields with super- conducting and resistive systems, (b) quasi-static pulse systems of inter- mediate-field range, and (c) very-short-duration ultrahigh field systems. We then offered several recommendations for a national program to advance toward new field levels and toward maximum utilization of those fields. For much of this century scientists and engineers have worked to extend magnetic-field levels beyond those readily available at any given time. Con- tinuous-field generation, for example, progressed from the large iron magnets of the 1920's to 10 T in the 1930's, to 20 T in the early 1960's, and to 30 T in the late 1970's. Each new major increase required significant time and effort to develop technologies, test concepts, and marshal the major new commitment of resources that was needed. This is a time of new scientific opportunity for high-field research. The availability of hybrid-magnet steady fields approaching 30 T and the relatively widespread use of superconducting 55

56 HIGH-MAGNETIC-FIELD RESEARCH AND FACILITIES magnets to 15T, some with extraordinary resolution and stability, have generated a broad scientific base for high-field research. An important development that is occurring, and which must be taken into account, is the growing sophistication and scale of large power supplies and large superconducting magnets, both of which have received substantial support from fusion research activities. Thus a scale of resources previously unavailable to the high-field research community is developing. To capitalize on the scientific and technological opportunities, we offer the following con- clusions. CONCLUSIONS After examining the field-magnitude goals set forth in Chapter 3 on Scientific Opportunities, we have assessed the resources required to extend the current state of the art toward these goals. We find that it is technically feasible to reach a continuous field of 75 T and quasi-static fields in the 100-T range. We find that pulse operation in the microsecond domain is feasible to 1000T (and beyond), but that the limit of nondestructive techniques is approxi- mately 200 T. We find also that fields appreciably beyond 1000 T will be very difficult to achieve. Nonrepeatable, high nonuniform field environments to 5000 T are not impossible to imagine, however; nor is it impossible that nonrepeatable local fields in intense plasma discharges might reach 10,000 T. On the basis of the preceding considerations, we conclude that a vigorous program should begin that will ultimately lead to 75-T continuous fields. As a major first step, an in-depth feasibility and cost analysis should be undertaken to determine the status of appropriate methods for producing steady-state, highly homogeneous fields of 75 T. This effort should include a study of high-field superconducting magnets, resistive magnets, and hybrid systems, which combine superconducting outer coils and water-cooled inserts, and which represent the approach most likely to result in the highest steady-state fields, and as such should receive priority attention. Intermediate goals be- tween the available 30 T and the desired 75 T should be pursued; goals of 45 T and 60 T would divide the interval in thirds. We further conclude that an important goal is the extension of the upper- field limit for superconducting materials and coils, which will ultimately con- tribute to the production of 75 T through the hybrid approach; in the shorter term, such coils will contribute to special applications such as high-resolution nuclear magnetic resonance (NMR), for which water-cooled or hybrid mag- nets are less well suited. Given the current status of technology (for example, high-field large- volume superconductors and existing power supplies) a 75-T project would

Magnet Design and Materials 57 be beyond the resources available to the magnet user community. However, developments in superconducting materials, technology, and the size of power supplies are all being pushed by other research areas such as fusion; therefore, a combination of technologies that would lead to an economical solution may be available in the not unreasonably distant future. The tech- nical and economic considerations of generating 75-T steady-state fields should be continuously re-examined to guide research and re-evaluate oppor- tunities for substantial progress. We further conclude that quasi-static fields up to 75-1 00T are feasible and can be economically attained using existing technology and large pulse supplies similar to those in the fusion program. These fields, of approximately nominal 1-sec duration, can allow preliminary experiments in preparation for the steady field, which would become available in the longer-range future. The coil technology has much in common with the necessary steady-field technology; thus it would stimulate progress toward that goal. Additionally we conclude that increased support for one or more centers, where there is experience with ultrahigh fields (1000 T and above) is required to stimulate experimental work and the development of facilities in this currently underused area. The technology is largely in place, and a program should therefore be initiated as soon as possible. Experimental techniques should be perfected and key experiments undertaken to demonstrate the applicability of the short-duration fields to the very significant experiments that have been proposed. Existing major investments in high-power pulse equipment should be utilized to the maximum extent. However, in choosing a facility to support, one must take into account the quality and the breadth of the scientific support group necessary to do the work. Hence existing facili- ties need not necessarily dictate the natural location of such expanded support. CONTINUOUS-FIELD GENERATION Superconducting Magnets-Present Status These magnets have come of age since the early 1960's when the first small- volume, nominal 10-T magnet appeared. The magnets listed by the Subpanel on High Magnetic Field Facilities and Users represent the current state of the art. The advent of these reasonable-volume fields above 14 T has made possi- ble the extension of high-field research to a number of centers. Prior to these developments, continuous fields of this level were available only in those central facilities that had large power supplies.

58 HIGH-MAGNETIC-FIELD RESEARCH AND FACILITIES 400 - IOT I2.5T I5T FIELD (TESLA) I7.5T FIGURE 1 Approximate cost of 5-cm bore superconducting coils in the field range from 10 to 17.5 T. Although fields of 10 T are easily attained with superconducting magnets, the cost of producing fields substantially above 10 T rises rapidly. Figure 1 gives an approximate cost curve for 5-cm bore coils. There is a factor of 5 increase between 10 T and 15T, and an additional factor of 3 rise when one goes to 17.5 T, the highest field generated to date by a superconducting magnet. As the capital investment in a superconducting magnet increases, so does the investment in support equipment and in the necessary operating cost to fully utilize it, as described in Chapter 6. Failure to provide such operating expenses in the past has limited the full utilization of many superconducting installations. Superconducting magnets have advantages beyond their independence from large central power supplies. In principle, they can be placed in a per- sistent mode, thus being free from time variations. This plus the ability to carefully control the winding process can lead to extremely high-homogeneity fields for NMR. Many NMR grade systems in the 6-8 T range are in use, but work is just beginning in the higher field ranges. An NMR system recently installed at Carnegie-Mellon University produces 14 T in a 9.5-cm bore with an uncompensated homogeneity of 2 x 1CT5 G over the central centimeter. This has been compensated to allow resolution of

Magnet Design and Materials 59 0.4 Hz at 600 MHz. The magnet system, exclusive of spectrometer, costs approximately $250,000. Further improvements of NMR will undoubtedly result when multifilamentary Nb3Sn is available and persistent current joints can be developed. Superconducting Magnets-Future Possibilities In the near term, multifilamentary Nb3Sn materials will be sufficiently well developed to provide an alternative for the Nb3Sn or V3Ga tapes now used in coils. This advance should lead to faster sweep times (now limited by helium loss rates to many minutes for a full up-down sweep), potentially higher homogeneity, and persistent mode coils. Also in the near term, one can expect some extension of the maximum field generated by superconductors. If the best V3Ga tapes are used, a field of 20 T can be generated in a 3.2-cm bore for a magnet cost of approximately a factor of 2.5 times higher than at 17.5 T. The cost of such a magnet would be about $700,000. 20 30 FIELD (TESLA) FIGURE 2 Limiting winding current density assumed for advanced super- conducting coils. The limiting useful field for various developed and specula- tive materials are indicated.

60 HIGH-MAGNETIC-FIELD RESEARCH AND FACILITIES 8M 7M 6M 5M 4M 3M 2M I M 20 25 FIELD (TESLA) 30 FIGURE 3 Estimated cost of 3-cm bore superconducting coils for very high fields using the current density limitations of Figure 2. Material costs are based on current NB3Sn conductors scaled by the field to which they are exposed. Looking farther ahead, one can speculate on the impact of applying higher-critical-field materials. There are, of course, materials with critical fields in the 50-T range. Experience with all materials developed to date suggests, however, that although the higher-critical-field materials obviously extend the maximum possible fields, they tend to have the same constant product of field times critical current as the lower-field materials. Thus any given wire will carry half the current at 20 T as it did at 10 T, independent of what superconductor is used in the wire. If we make some reasonable assumptions about the cost and properties of future materials, we can project the cost of magnets above 20 T. We assume that (a) consideration of coil protection limits overall coil current density to 10 x 103 A/cm2; (b) this limiting current density can be used up to 20 T only and must be degraded at a constant B x / above that point as increasing fractions of the conductor must be occupied by the superconductor; (c) any portion of the winding at diameters greater than 40 cm must drop the limiting current density to 7.5 X 103 A/cm2, again for considerations of protection; and (d) any new materials will have the same cost per ampere- meter as Nb3Sn, and like Nb3Sn will require greater thickness of material, thus greater cost, as the field goes up and the current-carrying capacity drops. These limiting assumptions are illustrated in Figure 2, together with the ap- propriate field ranges for the developed and anticipated materials.

Magnet Design and Materials 61 The cost curve up to 30 T resulting from these assumptions appears in Figure 3. The cost of a 30-T magnet is clearly very high; in fact, it compares unfavorably with the cost of alternative generation techniques such as hybrid systems (discussed in the section that follows). Figure 3, for fields above 20 T, further assumes the commercial development of materials that exist now only as small samples. Unfortunately, there is reason to expect that this commercialization will not happen, because there is no market of sufficient scale to warrant the expensive development. Niobium-titanium and nio- bium-tin have been developed in response to markets in the fields of high energy, fusion, and rotating machines. Research magnets, no matter how ambitious, would not compare with the scale of these applications. If we accept the basic assumptions that went into Figure 2, it appears that continuous-field generation beyond 20 T would turn to other than super- conducting systems, for example, resistive or resistive-superconducting hy- brid systems. If fields must have special characteristics, such as high reso- lution or extraordinary spatial homogeneity, the superconducting approach might be carried further in spite of the unfavorable cost comparison. It is vital to consider also what might happen if the assumption underlying Figure 2 does not hold. The most far-reaching effect would be a future violation of the constant B x / principle. This constancy is based on defect- type flux-pinning, hence more on experimental than fundamental arguments. Considerable effort has gone into maximizing current-carrying capacity in NbTi, Nb3Sn, and V3Ga. The prospect that fundamentally different pinning mechanisms will be discovered that will apply to the higher-field materials cannot be ruled out, but the probability does not seem high. We also note that the allowable overall current density in the magnets would not be altered by such a new mechanism. Overall current density is determined more by stability and protection considerations; however, the amount of super- conductor required per unit coil cross section would diminish, and that would be expected to have an impact on the wire cost. Resistive and Hybrid Magnets-Present Status Until the economic arguments in the previous section change, continuous fields much above 20 T will continue to be generated by resistive water- cooled magnets or resistive inserts boosted by external superconducting coils. These boosted, or hybrid, systems are a good combination, because they place the resistive elements on the inside where the field is high and the power requirements lowest, and the superconductor on the outside where the field is low enough but the power requirements highest. The current state of the art for resistive and hybrid systems is given in Table 3.

62 HIGH-MAGNETIC-FIELD RESEARCH AND FACILITIES TABLE 3 State of the Art of Resistive and Hybrid Magnets Field (T) Bore (cm) Power (MW) Type 30 3.2 10 Hybrid 25 3.2 5 Hybrid 20 3.2 2.5 Hybrid 23.5 3.2 10 Resistive 19.5 5.4 10 Resistive 18.5 3.2 5 Resistive 16 5.4 5 Resistive Table 3 indicates that resistive magnets with power supplies of at least 5.0-MW capacity and hybrid magnets of at least 2.5-MW capacity can produce field magnitudes higher than the 20-T level that is available relatively eco- nomically from superconducting magnets. However, there are certain advan- tages to resistive magnets, even below 20 T, largely as a result of sweep speed. In central facilities where there are power supplies, the hybrid can be readily applied to boost any field level limited by the available power. It can also be used to allow multiple experiments by reducing the power neces- sary for a given field, thus releasing power for parallel operations. A hybrid magnet to generate 30 T with 10-MW power requires an investment of ap- proximately $300,000 for a 7.5-T boost superconducting coil and another $200,000 for installation and suitable closed-cycle refrigeration system. If 30 T were desired and no facilities existed, one would have to examine the total capital cost, which would depend on the field generated by the superconductor and the amount generated by the power supply. If 8 T were chosen for the superconductor, representing a modest technological under- taking, 10 MW of power would be required, which would call for an invest- ment of approximately $5 million for power supplies and $0.5 million for the superconducting system. If one chose 12 T for the superconductor, a reason- able step forward, the superconducting system cost would increase to ap- proximately $1.7 million, but the power requirement would drop to $3.3 million. The total cost for the latter systems would thus be about 10 percent lower. A central power supply can be time-shared with a number of magnets in a cost-effective way. Water-cooled magnets are inexpensive; a 5-MW, 18.5-T repeat magnet costs about $20,000 to construct. A larger central facility such as the National Magnet Laboratory has 24 high-field magnets time-sharing a central 10-MW power supply that can be subdivided into four 2.5 units. Two hybrid systems will extend the maximum field to 30 T and allow two 25-T magnets to run simultaneously.

Magnet Design and Materials 63 Resistive and Hybrid Magnets-Future Possibilities An ambitious near-term goal for hybrid systems might be to extend an NMR environment beyond what can be reached with superconductors. For ex- ample, a field of 25 T in a 10-cm-bore hybrid magnet could be generated with 4.0 MW of power and a 15-T superconducting boost coil. The field could be stabilized by series regulation and field feedback stabilizer coils, with a final stage of passive stabilization by means of a thin superconducting shield made from a high-critical-field material. A longer-term quite ambitious goal would be a 75-T hybrid magnet. The equivalent magnetic pressure at 75 T is 22,000 atm. We must remember, however, that magnetic stress is not a fundamental limit for solenoids. If a magnet is divided into independent nested solenoidal elements, the stress can be limited to an arbitrary level by controlling the field generated by each subelement. The inner elements are more efficient, of course, and the less field they can generate, the more must be generated in elements further out. Nevertheless, fields of arbitrary level could be generated. The limit then is not fundamental but practical. Does the magnet grow too power-consuming, or does it become impossibly large and expensive? The present highest continuous field, of 30 T, could be extended to much higher fields if the necessary, very large power supplies were available. Using heat-transfer rates and copper alloys reinforced with interleafed steel sheets, both of which are used in the 30-T hybrid magnet, one can extrapolate power requirements to a field level of 75 T. Figure 4 shows such an extrapolation, with the amount of reinforcing progressively increased as the field increases. Were such a coil, having an outer diameter of 1 m, operated without an external superconducting booster section, it would require 165 MW continu- ously to generate 75 T. Even with an ambitious 20-T superconducting booster coil, 89 MW would still be required. Higher-field outer superconducting sections will further reduce the power supply required but will also be very expensive. As with the pure super- conducting coils discussed in the previous section, a fundamental change in the empirical rule that B x / = constant must occur to allow substantial cost reduction. Although such a 75-T system is technically feasible, its cost clearly would be high. A 100-MW supply of adequate stability would cost about $50 mil- lion. A 20-T booster coil of 1-m inner diameter is estimated at an additional $25 million at a minimum. Power costs for a 100-MW supply would be $5000/h at $0.05/kWh. The largest continuous dc supply suitable for research magnets now in existence is 50 MW (Princeton Plasma Physics Laboratory) and is used for nuclear fusion machines. The National Magnet Laboratory (NML) and Gre-

64 HIGH-MAGNETIC-FIELD RESEARCH AND FACILITIES ZOO I50 I00 o Q. 50 RESISTIVE /HYBRID / IOT HYBRID 20 T 25T 50T CENTRAL FIELD 75T FIGURE 4 Power-supply requirements for 3-cm bore resistive and hy- brid magnets. The two hybrid curves are for 10-T and 20-T booster fields, respectively. noble each have 10-MW supplies, the largest devoted to high-field research applications. We can expect progress in the intermediate range between the 30 T now possible and 75 T, which is feasible but certainly a long-range goal. Moving from 30 to 35 T is feasible by simply increasing the present 30-T hybrid booster field from 7.5 to 12.5 T. An investment of approximately $1.5 mil- lion would be required. If one were able to use the Princeton 50-MW supply for a research magnet, it is possible that a field of 50 T could be achieved for an investment of approximately $5 million in a 10-T booster superconducting coil to surround a 40-T, 50-MW water-cooled insert. QUASI-STATIC PULSE SYSTEMS Present Status Although continuous fields are necessary for many experiments and always desirable, certain experiments can be conducted in shorter times. Pulse times

Magnet Design and Materials 65 on the order of 1 sec could represent an approach for experiments that bene- fit from continuous fields. These long pulses can be powered by various techniques. In the University of Amsterdam facility, a 6-MW controlled recti- fier supply switches directly onto the mains. A field of 40 T can be produced for 0.1 sec in a 2-cm bore. Similar parameters have been obtained at the NML by silicon-controlled-rectifier switching of the fully excited rotating dc generator output. At Toulouse, a third approach uses a 1.25-MJ capacitor bank and a crowbar circuit to produce a 0.1-sec rise and a 1-sec fall to a peak of40Tina2.5-cmbore. These coils operate adiabatically and generally use precooling with liquid nitrogen or, in Amsterdam, with liquid neon, to limit the temperature rise. Recool times of present devices are on the order of 1 h, but increased atten- tion to obtaining high repetition rates could reduce this to the 5-min range. Because the pulse rate is limited, particular attention must be given to multi- ple-channel data collection and sophisticated diagnostics, which are an im- portant part of any limited-time-scale facility and can dominate the cost of experimental equipment. To achieve repeatable pulses will be vitally im- portant if such pulse fields are to be used for NMR and other high-resolution experiments. Future Possibilities Quasi-static pulse facilities could represent the best chance to extend present experiments well above 30 T during the next decade. This possibility is greatly enhanced because of the increasing use of large pulse supplies for fusion experiments. The NML, for example, is installing a 200-MW, 200-MJ pulse supply for the Alcator fusion experiment. It is of interest to examine what such a supply can offer for quasi-static pulse experiments in small volumes. Precooled magnets that depend on thermal inertia are limited by the prod- uct of magnet current density squared times the pulse time (/2r). The larger the scale of the magnet for a given field, the lower the resultant current density; hence, the longer the pulse can be without overheating. The larger the scale, the larger the energy source must be, but the longer the pulse can be held. This relationship is illustrated in Figure 5, which shows two scales, magnet weights of 500 and 4000 kg, and gives the field achievable at various pulse times, subject to a given temperature limit. The scale curves also repre- sent constant-energy requirements. We note that 50 T can be generated for 1 sec if a coil has a mass of 4000 kg and if a 200-MJ energy source is avail- able. A peak power of 200 MW would be required. The curve also indicates that 75 T could be held for 0.5 sec with the same temperature rise, but a 400-MW peak power supply would be required.

66 HIGH-MAGNETIC-FIELD RESEARCH AND FACILITIES I00 T - 75 T 50 T 25T W = 4000 kg 200 MJ 0.5 I.0 SECONDS I.5 FIGURE 5 Field versus pulse time for 5-cm bore LN2-cooled long-pulse coils. The two curves are for constant magnet weights and stored energies. The peak resistive powers required can be estimated by dividing the stored energy by the pulse time. We note also that the resistive continuous-field coils in Figure 3 could be driven on a pulse basis. The magnets are more complex than the LN2 pre- cooled coils and require large coolant pumps, but fields can be held for times determined by the energy storage of the supply rather than by the heat capacity of the coils. The cost of the long-pulse magnets is relatively low. A 50-T, 1-sec magnet would cost about $100,000. A large pulse supply involving a rotating alter- nator capable of delivering 200 MJ at a 200-MW peak power level would cost approximately $10 million. The long-pulse magnets, particularly using present fusion program power supplies, represent the least expensive entry into the field range between 30 and 75 T. Realization of these ambitious pulse coils will represent a major magnet design challenge for the next few years. This program could speed the attainment of continuous fields in this range by stimulating technological exploration and providing significant new opportunities for physics research.

Magnet Design and Materials 67 SHORT-PULSE SYSTEMS: FIELD GREATER THAN 100 T Nondestructive Coil Approaches-State of the Art For the foreseeable future, fields greater than 100 T must be produced on a pulsed basis. Thus, in addition to the usual specifications such as volume and field homogeneity, other factors must be considered, for example, the time variation of the field, the number of times the magnet can be used before destruction, and the survival of test samples and diagnostic equipment. All of these factors usually worsen as the field magnitudes increase. Pulse fields obtained from capacitor discharges have been the principal method of generating fields beyond those available from continuous fields. A small bank of 10 kJ can generate fields of the order of 75 T for a 50-Msec half period in a 5-mm bore and have been used since the early 1950's. A more ambitious bank of 100 kJ can typically be used to generate 65 T in a 2-cm bore for a 650-Msec half period. These systems are seldom operated as faculties but are usually considered part of a particular experiment. This need not be the case if personnel and dedicated instrumentation are made available to operate as a facility. Nondestructive Coil Approaches-Future Possibilities The future goal of nondestructive short-pulse coils appears to be 100-200 T. M. Date, of Osaka University, obtained several shots with peak fields greater than 100T (pulse width 175 //sec) before destroying a specially de- signed, force-reduced coil at 107 T. Based on these encouraging results, he has received funds from the Japanese Ministry of Education to build a 1.5-MJ, 30-kV capacitor bank that should be capable of generating magnetic fields of 150-T peak (1-msec pulse width) over a 6-mm id x 10-mm long volume. If one assumes a sinusoidal waveform, the magnetic fields should exceed 90 percent of the peak for nearly 300 Msec. Experimental samples and diagnostic equipment normally would not be destroyed in such an arrangement. Although it is not likely that coils of this type could have an indefinite lifetime, they probably can be designed ultimately to withstand many shots, particularly at the somewhat lower field of 120 T. We note that Date's pro- posed capacitor bank is roughly equivalent to half the Los Alamos Scyllac bank fired at half voltage. It is possible that with continued coil development, use of the full Scyllac bank would generate long pulsed fields appreciably above 150 T over larger volumes. In principle, one can extend the nondestructive concept to arbitrarily high fields by following the multiple-section-coil approach. The principle is simply to generate only as much field with a given section as the strength of that section can support, and to generate the balance of the field with successive

68 HIGH-MAGNETIC-FIELD RESEARCH AND FACILITIES IOGJ IOOOMJ IOOMJ O cr UJ IOMJ IMJ 2 CM BORE NON-DESTRUCTIVE PULSE COILS OSAKA MEGAGAUSS FIGURE 6 Energy requirements for multishell nondestructive pulse coils. The Osaka magnet project is indicated. shells. The higher the field, the less can be generated by the inner elements where the field is high, and the more must be generated in shells farther out, but at the cost of rapidly increased energy demands. Figure 6 extends the

Magnet Design and Materials 69 three-shell approach used in Osaka to more shells. Using the strongest ductile material available, we note that more than 100MJ are required to exceed 200 T. Fast capacitor banks cost some $500,000 to $1 million per MJ. The largest high-speed capacitor bank ever built is the 10-MJ 60-kV bank at Los Alamos, for the Scyllac fusion experiment. The banks for the large laser experiments are comparable. Destructive Cofl Approaches-State of the Art Currently there are several ways to produce 120-300 T fields over short times, all of which result in coil destruction. The simplest systems involve capacitive discharge either directly into a single turn coil, an imploding wire or shell, or a transformer coupled to a liner that implodes, compressing flux into a smaller volume. Calculations suggest that single-turn coils should be capable of fields approaching 800 T, but to date fields have been limited to 300 T. Typical pulse times near the peak field are about 1 ^sec. The im- ploding transformer coupled foils (Cnare effect) have also been used to pro- duce fields in the 300-T region. To date, with the exception of explosive systems whose volumes can be 10-30 cm3, the volumes are small, ranging from about 1 cm3 to a fewhun- dredths of a cm3 at the higher fields. The duration times of the fields, more than 90 percent of peak, are usually short, ranging from a few tenths of a microsecond for some fields produced by electromagnetic implosion to a microsecond or so for fields produced by direct capacitor bank discharge into a coil. Again, these times are at present much larger for explosively produced fields. Some of these observations, however, can be misleading, because the time duration of the fields is usually larger when the coil radii are larger, and in implosions, when the liners are thicker. As larger capacitor banks become available, both final field radii and liner thicknesses can be increased, thus increases in volumes and pulse duration times can be expected. Generally speaking, samples and diagnostic equipment have a good chance of surviving in direct-driven-coil arrangements; samples and some equipment are normally lost in explosive or electromagnetic implosion-produced fields. Research should be encouraged with all systems, for each has specific advantages and the possibility of improvement. Many people believe that it will be difficult to greatly exceed 300 T without using flux compression techniques, at least in reasonable volumes and useful time scales. It is perhaps not unreasonable to anticipate useful laboratory-produced fields of more than 500 T with electromagnetic implosion techniques.

70 HIGH-MAGNETIC-FIELD RESEARCH AND FACILITIES Imploding foils or wires can be used to produce direct pressure by col- lapsing on the sample. Foils have produced pressures of about 150 kbar, and imploding wires, 300 kbar. The wire experiments represent the least energy, and, although the samples packed into the interior of the wire clearly are fragmented by the implosion, such experiments can safely be carried out in a small laboratory. They rely upon the well-known Z-pinch effect for compres- sion, with corresponding azimuthal magnetic fields. Examples will be men- tioned later where much higher azimuthal fields might be generated. Axial fields greater than about 300 T have only been produced so far by explosively driven cylindrical flux compression systems. The highest fields reported (and confirmed by other investigations) are in the range of 1000-1500 T, diameters of 5-6 mm, and lengths 20-50 mm (estimated). For a fixed explosive-liner system, a lower initial magnetic field (seed field) gener- ally results in a higher final field but at a smaller radius. The fields are limited to levels that develop sufficient pressures to stop the incoming liners (modi- fied somewhat by diffusion processes). The duration times for fields greater than 90 percent of peak are usually less than a microsecond, depending on the system geometry and the peak field developed. The maximum pressure available from chemical explosives, together with explosion symmetry available, has so far limited fields produced by this meth- od to about 1500T. Use of nuclear explosives allows higher fields, and 4000 T may have been achieved in the Soviet Union. Experiments with con- ventional explosives are often carried out in remote sites, but modest-scale experiments can be performed, as they often are in the Soviet Union, in laboratory confinement vessels. Small experiments generally involve less than 50 MJ of explosive energy. Some exploratory work has been done on localized azimuthal megagauss field generation by other techniques. It should be remarked here, however, that the magnetic fields are not uniform but instead vary inversely with distance from the current source. Use of these fields for high-field experi- mentation will therefore be very difficult but perhaps feasible for some situ- ations. In one case a very fast capacitor bank of the type used with high- power pulse generators was used to explode a fine wire. From the known current and current channel x-ray measurements, calculations indicate that fields in the 500-T range were generated for periods of 50 nsec. In related experiments, current has been passed through a thin (5000 A) aluminum shell, causing it to vaporize and implode and the resultant plasma to constrict. Calculations indicate that fields of 1000 to 2000 T should be achievable by this technique. Highly localized high fields are apparently found in certain intense plasma discharges. Laser-driven pellet implosions are accompanied by a filamentary current structure that produces local 100-T fields. The same filamentary

Magnet Design and Materials 71 structure in plasma-focus experiments gives rise to intense local fields, with speculation suggesting fields in the 10,000-T range. Destructive Coil Approaches-Future Possibilities Established high-field researchers believe that fields considerably above the 1000-T level are possible with experiments of sufficient scale. Some combina- tions of explosive-driven, transformer-coupled foils and compressing fields produced by large superconducting coils outside the blast shield might pro- duce fields in the 5000-T region. High-field flux compression calculations are uncertain, as they involve not only sophisticated MHD plasma stability considerations but also require equa- tions of state and conductivity information that is poorly known at the extreme conditions encountered. Some preliminary calculations have been made recently with the same computer simulation that predicted earlier 1000-1 500T shot results. Peak field values near 4000 T, diameters of 4-5 mm, and duration time for fields greater than 90 percent of peak of 0.2-0.25 Msec were calculated. Further calculations can probably project even more favorable results. These results must be treated with great caution, however, in view of the uncertainties mentioned previously. The importance of producing such fields merits further calculations, complemented by a number of actual experiments. The typical scale of a few millimeters can be increased if there is sufficient justification. Calculations for a 50-cm-diameter liner driven by hundreds of kilograms of TNT lead to fields in the 2000- to 3000-T range in final dia- meters of 20 mm. Single experiments would cost many thousands of dollars at this scale. Electromagnetically driven experiments, such as the axial current flow in a thin foil, driven by the large pulse generators built for E-M simulation, could be a promising direction. These very large units are found in several laboratories. Certain types of spectroscopy might possibly be carried out in the intense plasma discharges: although the environment is highly localized and virtually uncontrolled and nonrepeatable, it might be used if one could develop a local trustworthy spectroscopic probe. Experiments using the ultrahigh fields universally list experimental tech- niques as being as difficult as the generation of the fields. Field changes of 109 T/sec induce voltages of 10 kV in a probe of only 10 mm2, and ablation of surfaces from strong heating creates a damaging plasma environment. Ex- periments with spectroscopy, or perhaps chemical reactions, are undoubtedly better suited for ultrahigh fields than more traditional solid-state experi- ments. Such experiments do not involve expensive carefully prepared samples, nor are they troubled by sample thermal problems.