Magnetic fields are required for containment and control of the plasma in magnetic confinement fusion (MCF). Many configurations rely on both dc and pulsed magnetic fields for plasma initiation, confinement, ohmic heating, inductive current drive, plasma shaping, equilibrium and stability control. Inertial Confinement Fusion (ICF) may also utilize magnetic fields, for example, in beam focusing quadrupole fields in the Heavy Ion Driver (HID) concept or Helmholtz coils in KrF lasers. These magnets may use either resistive conductors or superconductors. The preponderance of past and present magnetic fusion devices use normal resistive magnets, but almost all of the large fusion machines being built outside the United States (e.g. LHD, Wendelstein VII-X, KSTAR, SST, and HT7-U) use superconducting magnets.
Most design concepts for power producing commercial power plants depend on superconducting magnets for efficient production of magnetic fields. The attraction of superconductivity is the ability to carry very high current density with zero dc power dissipation. Superconductors will dissipate energy in a changing magnetic field, but overall power losses, including refrigeration power required to maintain the magnets in the superconducting state (typically 4K-8K temperature range for Low Temperature Superconductors (LTS)), are extremely small compared with resistive magnets. This advantage grows with increasing magnetic fields and magnetic field volume, or where relatively long pulse or steady state operation is required.
There are also design concepts for power producing commercial power plants based on dc (steady) resistive magnets or combined resistive and superconducting magnets (e.g.- ARIES-ST), particularly for confinement concepts promising high beta. The attractions of dc resistive magnets over superconducting magnets include: 1) potentially increased compatibility with DT fusion neutron and gamma radiation environment, 2) the possibility of lower initial capital cost for the magnets, 3) elimination of capital expenses for superconducting magnet support systems such as quench protection energy dump systems, or such as cryogenic cooling systems if room temperature magnet operation is used, but partially offset by a water treatment/cooling system, 5) demountable joints, which simplify both the initial assembly and the maintenance of magnet components in a radiation environment, while also allowing the efficient centralized industrial manufacturing of magnet modules small enough to be shipped to the power plant site on roads. Although resistive magnets do consume part of the fusion power produced, their recirculating power fraction disadvantage is minimized by increasing magnetic field strength and increasing plasma volume. The desirability of this approach increases with increasing plasma beta.
Magnets require a high level of electrical, mechanical and structural engineering design and technology, advanced materials, as well as supporting cryogenic technology if they are superconducting or cooled with a cryogen (e.g.- liquid nitrogen). At present, only the Low Temperature Superconductors with critical temperatures of ~10K for the ductile alloy NbTi or ~18K for the brittle compound Nb3Sn are in use or planned for future devices. The newer High Temperature Superconductors (HTS) with critical temperatures of order 90K and above are expected to be used primarily for low-loss magnet current leads in the short term. Longer-term application of HTS depends on progress in development of materials in long lengths with significantly improved critical current densities. The primary area for use would be in magnetic fields of 0.5-20 Tesla and at higher operating temperatures than typical LTS. An essential issue is that the HTS must be available at similar cost (including refrigeration cost impact).
Some of the key technical issues for dc (steady) resistive magnets designed for use in commercial power plants are similar to and simplified from the case using superconducting magnets: 1) steady active cooling optimization within mechanical stress limits, 2) low cost, 3) radiation compatibility, 4) reliability/availability/maintainability. The relative importance of these issues depends on the concepts, such as plasma beta, size and field strength.
Significant success in development of magnet technology for fusion applications has been achieved over the past two decades, but no large fusion programs using superconductors or resistive conductors are now underway in the U.S. Countries that have been more aggressive in introducing superconducting technology with working tokamaks and stellerators include France, Russia, China and Japan. Germany (Wendelstein VII-X), Korea (KSTAR), India (SST-1) and China (HT-7U) all have significant programs underway for introducing new superconducting steady state stellerators and tokamaks in the next few years. The last large operating fusion experiments with resistive conductors were produced in Europe and Japan whereas smaller scale devices were built in the US (i.e.- D-IIID and Alcator C-MOD).
For the U.S. fusion program, the main issues for magnet technology revolve around cost and reliability. The United States should be developing the elements of magnet technology that are specifically focused on the experimental needs of the magnetic and inertial fusion physics programs and that will substantially lower the projected cost of the experiment or of fusion power in the future. There are primarily three ways in which magnet technology can lower the cost of experiments and fusion power production: 1) by lowering the cost of the magnet components and/or assembly processes, 2) by reducing the size of the magnet systems, so that the cost of other fusion subsystems may be reduced, and 3) by providing magnet performance which substantially increases or optimizes the physics performance, e.g. increased magnetic field or some special magnetic field configuration.
The ‘goodness’ parameters for a magnet system need to be defined for specific applications, but are obvious for some requirements. For instance, a strong benefit results from higher magnetic field, since fusion power is proportional to B4. Another benefit would result from the ability to absorb higher nuclear flux and fluence in insulation systems or to have increased superconductor stability in order to reduce the size of the radiation shield protecting the magnet system and consequently reduce the machine size and cost as a whole. Such shield size reduction might be limited in a DT fusion power plant because most neutrons are needed for power production and tritium breeding and so must be intercepted in any case by the blanket.
In general, assembly and maintenance access is less constrained if the magnets can operate at higher current density and/or higher stress levels, thus reducing the size of the hardware producing the field and the surrounding interface systems, or if resistive magnets are used that are jointed and demountable. In the longer term, the cost of superconducting magnets and their supporting systems may be reduced if their operating temperature can also be increased (e.g.-higher temperature superconductors) or if they can be constructed using less expensive materials. Therefore, magnet technology should strive toward operating at higher fields, higher current densities, higher stress levels and higher temperature levels. Each of these improvements implies smaller devices for lower cost field production. In parallel, design margins must be sufficiently understood and applied so as not to sacrifice reliability.
Thus, besides reducing cost, the thrust of a superconducting magnet technology improvement should be towards increasing the limitations on design margin (e.g.-high stability. low AC losses, and rapid quench detection in superconductors) and improving materials (e.g.-higher superconductor critical properties, and high shear strength/high radiation resistance insulation) to enhance performance or reduce size. Steady dc resistive magnet technology should also strive to operate at the highest fields and stress levels that are compatible with demountability and maintainability and require improved materials properties for high strength/high conductivity alloys or laminates of copper/steel for pulsed coils, and also high shear strength/high radiation resistance insulation.
Key questions and issues:
Issues: Lower cost, low temperature superconductors (LTS). Higher performance/cost ratio. Less expensive conductor and magnet (joints, cryostat) fabrication methods; Reduced refrigeration plant by introducing HTS leads; QA development for Incoloy 908 jackets; weld development for jackets; Optimization of heat treatment for a high field LTS like Nb3Sn to be more compatible with the magnet fabrication process; Insulation development to be compatible with heat treatment (e.g.- spray on or bulk ceramic),
Issues NbTi, APC, Fullerenes, A15’s - Nb3Sn, Nb3Al, other.
Issues: Critical current density, AC losses, ramp-rate limitations, thermal isolation, low-loss leads and bus.
Issues: Expected performance improvements from high field and high temperature operation. Present status of HTS technology and prospects for development.
Issues: Critical current density, AC losses, protection, piece length, structural reinforcement.
Issues: Designs for low cost manufacturing, transport and assembly.
Issues: Conductivity, stress, fatigue, practical limits on plate size, j2*t.
Issues: Higher field, higher current density, higher operating temperature, recirculating power, ability to absorb radiation, demountability.
Issues: Insulation, structure, thermal isolation, electrical isolation.
Issues: Developing force reduced winding topologies, sliding joints, or active presses for triaxial stress reduction.
Issues: Quench detection, electrical integrity, self-protection of HTS coils, instrumentation accuracy and reliability, system radiation resistance, active cooling.
Issues: Joints, demountable leads, elimination of trapped coils, quench detection, electrical isolation, feed-throughs.
Joe Minervini (MIT)
Bob Woolley (PPPL)
Joel Schultz (MIT)
Phil Heitzenroeder (PPPL)
Nicolai Martovetsky (LLNL)
Dick Thome (MIT)
Working Plan for Snowmass:
There is a total of 6 hours allocated for official working sub-group discussions during the first week plus a two-hour summary session during the second week. This provides only limited time to cover the wide range of questions and issues. Therefore, these sessions need to be structured to allow for coverage of the issues, but also allow as much time as possible for discussion and community input. We propose that the discussion be structured around the major areas listed below. We also propose that white papers be written in each of these technical areas to provide the focus and background for discussion. These white papers will be posted on the website and be available for distribution at the start of the workshop. The proposed white papers and authors should cover the following major areas:
Paper to web site by Minervini June 25
Paper to web site by Heitzenroeder June 25
Paper to web site by Schultz June 25