Report of the NCSX Review

Committee

Charles Baker,

Richard Hazeltine (Chair),

Chris Hegna,

Timothy Luce,

Gerald Navratil,

Shoichi Okamura,

Ronald Parker,

Max Tabak,

Joseph Talmadge,

Friedrich Wagner

Tuesday, October 16, 2007
Executive Summary

The NCSX Review Committee was appointed by the Fusion Energy Sciences Advisory

Committee to respond to a charge from Dr. Raymond Orbach, Under Secretary of Science, US

Department of Energy, that was issued on August 9. Details of the charge, and the procedures

used by the Committee in its deliberations, may be found in the text of this report. The present

summary provides a condensed version of the Committee's answersto each question (shown in

boldface) in the charge.

1. Critical issues for the US compact stellarator program

a. What unique toroidal fusion science and technology issues can a compact

stellarator program address independent of its potential for a reactor concept?

Stellarator research programs have the generic mission of studying a confinement system that

addresses two key issues in fusion energy research: disruption avoidance and steady-state

operation. This mission is approached by means of plasma shaping in three dimensions,

sacrificing the axial symmetry of the tokamak, and thus allowing scientific exploration of the

opportunities and penalties associated with three-dimensional geometry. The scientific issues to

be addressed in the program include transport, energetic particle confinement, equilibrium,

stability and density limits (including disruption avoidance), and particle and power handling.

Technology issues to be explored include simpler magnet coils and support structures,

metrology, correction coils and divertors.

The compact quasi-axisymmetric stellarator is distinctive in having relatively small aspect ratio

- typically less than five. In this respect its geometry approachesthat of a tokamak, and indeed

the scientific and technical similarity with the tokamak, along with improved cost-effectiveness,

provides the main scientific impetus for compactness: there is no strong motivation for low

aspect ratio ITomthe perspective of stellarator physics. One expects experiments on a compact

stellarator to provide particular insights into tokamak physics; and similarly one expects existing

knowledge of tokamak behavior to benefit particularly a compact stellarator research program.

The Committee finds these potential mutual rewards to be plausible and significant.

b. What are the advantages and disadvantages of the quasi-symmetric stellarator as

a potential fusion system concept? What unique features does the compact

stellarator offer in this regard?

Stellarators offer the important advantages of steady-state operation with relatively soft and

forgivingstabilitylimits- advantages that might become especially important as the

international fusion program begins to study the DEMO device that is expected to follow ITER.

Quasi-symmetry reduces the effective radial widths of particle drift-orbits, thereby ameliorating

the large neoclassical transport rates associated with more conventional stellarator designs.

Because it allows for decreased zonal-flow damping, quasi-symmetry may also reduce turbulent

transport rates, although this apparent advantage requires more study.
The key disadvantage of the stellarator is the complexity and cost of its field coils, whose precise

constructionandalignmentare essentialfor acceptableconfinement.Compactnessexacerbates -

this problem, in that the increased toroidicity for a given rotational transform makes the flux

surfaces more fragile. The Committee points out that present coil designs significantly

complicate external access to the plasma and to the plasma blanket, for maintenance or other

purposes, in a stellarator reactor.

A study by the ARIES team indicates somewhat smaller construction and operation costs for a

compact stellarator, compared to a stellarator with large aspect ratio. At the same time it is

recognized that, at this early stage of investigation, there remain many unanswered questions

about compact stellarator reactor performance. In particular the smaller surface to volume ratio

associated with compactness is disadvantageous with regard to heat removal and tritium selfsufficiency.

c. What scientific and technical issues need to be resolved to evaluate the compact

stellarator as a viable concept for a fusion energy system?

Many issues, detailed in the body of this report, will need to be addressed in the design of a

stellarator for fusion energy production. Three issues of particular note for compact stellarators

as a fusion concept are: determination of the size scaling of confinement; the required tolerances

for coil construction; and the magnitude of plasma current below which disruptions due to

plasma instabilities are avoided.

2. Role of NCSX in the international context:

a. What critical, unique contributions does NCSX offer for addressing the issues

identified in (I)?

NCSX is designed to address most of the critical physics and technology issues discussed in this

report, using a compact, quasi-axisymmetric configuration that is unique in the world stellarator

program. The Committee finds that, assuming successful construction and testing phases, the

NCSX device is likely to perform at a level sufficient to address its scientific and technical

missions. Therefore the Committee expects the NCSX experimental program to have a profound

impact on stellarator research worldwide.

There are several methods to ameliorate the large orbital excursions that can degrade

confinement in non-axisymmetric systems. Quasi-symmetry, the approach now favored in the

US, itself includes several varieties: quasi-poloidal symmetry (QPS), quasi-helical symmetry

(QHS) and quasi-axial symmetry or quasi-axisymmetry (QAS). NCSX is the only experimental

device in the world program that would employ the QAS concept.

By virtue of both QAS and compactness, NCSX offers a similarity to tokamak science that is

unmatched by any other stellarator device. In this regard, the hybrid nature ofNCSX

confinement- that is, dualoriginofitsrotationaltransform,whichis producedpartlyfrom

externalcoilsandpartlyfromplasmacurrents- should allow a particularly instructive research
program. Similarly its resemblance to the tokamak should allow NCSX to illuminate a number

of issues concerning symmetry and effects of symmetry-breaking on confinement.

NCSX is a Proof-of-Principle (PoP) class experiment, minimally sized to provide credible

integrated confinement and stability results. Thus it is larger than such stellarators as HSX at the

University of Wisconsin-Madison, but smaller than, for example, the DIII-D National Tokamak

Facility, and the stellarators W7-X (Germany) and LHD (Japan). Comparative studies ofthe

threemajorlinesin the internationalstellaratorprogram- LHD, W7-X, andNCSX- will

inform a decision on which system has the highest reactor potential, and thus influence

discussions on the continuation of the fusion program toward the DEMO reactor.

b. Given PPPL's [Princeton Plasma Physics Laboratory's] proposed plans for

operation of the National Spherical Torus Experiment [NSTX] and NCSX, what

would be the timetable for resolving relevant issues identified in (1) above?

NCSX plans to achieve its first plasma in 2012, two years before W7-X and four years before

ITER. For the following several years, the NCSX team plans to alternate its operation with that

ofNSTX; thus the key initial experimental results for NCSX would be obtained in FY2013 and

FY2015. The Committee is concerned about the practical realism of this plan; we find in

particular that the resolution of key experimental issues is likely to require five years of actual

operation.

One technical requirement that the Committee considers likely to affect the timeliness of physics

results is the quality of the magnetic flux surfaces in three-dimensional geometry. The NCSX

team has developed appropriate strategies, involving both assembly procedures and the use of an

array oftrim coils, for constructing and maintaining flux surfaces with relatively few magnetic

islands and relatively small chaotic regions. While finding these strategies to be well thought

out, the Committee recommends that attention to construction details that may affect flux-surface

quality, and the study oftheir effects and methods to counteract them, remain top priorities for

the project.

c. What are the technical differences of the current NCSX design compared to

other stellarators operating or being built abroad? What is the significance of these

differences? Does NCSX fill a critical void in the development of the stellarator

concept as a viable fusion energy system?

As pointed out earlier in this summary, NCSX will be unique in the world stellarator research

program, because of both its quasi-axisymmetry and its compactness. The W7-X device uses a

distinct configuration optimization, has a large aspect ratio, and moreover is designed to

minimize plasma currents (recall that NCSX will depend upon self-induced plasma current for a

fraction of its rotational transform). The LHD device achieves reduced orbital excursions by

means other than quasi-symmetry; it has a intermediate aspect ratio. Both W7-X and LHD

employ super-conducting magnets. The Committee finds that the comparison of these three

devices will be extremely useful in understanding the physics optimization of advanced

stellarator configurations.
3. Options for the US stellarator program

a. If the NCSX program were not continued, what options would exist or would be

possible to address the key issues of the quasi-symmetric stellarator in general and

the compact stellarator in particular?

The present US stellarator research program includes other devices, planned or in operation, that

could address at various levels a subset of the key issues listed in the response to question 1.

However no US experimental program, present or planned, could provide the breadth of

scientific and technical information that is expected to come from NCSX. The only PoP scale

device in the US repertoire that addresses quasi-symmetry is NCSX, and NCSX is the only such

device capable of examining the key issues in an integrated context. Therefore, ifNCSX were

abandoned, the US would have to reduce significantly its ambitions in stellarator research, or

begin constructing a new PoP stellarator experiment.

The Committee finds it important that the US have a significant stellarator presence as part of its

magnetic fusion energy research program. The Committee notes that at present about 75% of the

US stellarator effort is focused on the construction ofNCSX, so the loss ofNCSX would change

the basic character of the US program. The program would lose its integrated PoP facility and

the most relevant connections to tokamak research.

b. Assuming NCSX is not available, what program elements would be required to

maintain the US as a significant participant in the international stellarator

program?

i. Identify potential opportunities for US leadership.

ii. Include more international collaboration as appropriate.

The US has the only operating quasi-symmetric stellarator device in the world, the HSX at the

University of Wisconsin-Madison; it allows fundamental tests of quasi-symmetry and can span a

range of symmetry-breaking geometries. The CTH device is a low aspect-ratio stellarator at

Auburn University that is used to study passive disruption avoidance. Both ofthese relatively

small experiments provide valuable scientific information, and both could be upgraded, but

neither could provide the sort of integrated research program of a PoP device like NCSX.

The proposed QPS device at Oak Ridge National Laboratory is a low aspect ratio stellarator with

quasi-poloidal symmetry. It has some similarity with the (much higher aspect ratio) W7-X

device; furthermore it should allow strong poloidal flows, unlike those seen on existing toroidal

confinement experiments. Thus QPS could extend the stellarator data base in a useful way. But

it would not replace the scope of the NCSX program, in particular having much weaker links to

tokamakbehavior. .

The Committee recommends that the construction decision on QPS be expedited ifNCSX is

cancelled. However, we find it illogical to cancel a stellarator project that is nearing its final

construction phases only to begin a new stellarator with poloidal rather than toroidal quasisymmetry.
In the absence ofNCSX, a restructured US stellarator program could maintain scientific

leadership in selected research topics, but would have difficulty playing a significant role in the

direction of worldwide stellarator research. International collaboration is already a key element

of US stellarator research and would remain so in the absence ofNCSX. However, the benefits

gained from such collaboration would be diminished without a domestic stellarator experiment

on the PoP scale.

Quasi-symmetry is one of many ways to optimize 3-D configurations; other optimization

schemes could be pursued. IfNCSX were discontinued, the US stellarator program should

consider a variety of approaches to stellarator optimization in proposing a new PoP stellarator

project.

The US has been a leader in theory and computation on three-dimensional confinement, in large

part because of the impetus provided by the NCSX and QPS design programs. There are many

opportunities for useful theoretical and computational advance, and encouragement by the Office

of Fusion Energy Sciences of such research would help the US maintain its presence in the

international effort. However, the loss of a world-class experiment in the US would hurt the

recruitment of young scientists into stellarator theory.
Introduction

The NCSX Review Committee was formed in response to a charge to the Fusion Energy

Sciences Advisory Committee (FESAC) from Dr Raymond Orbach, Under Secretary for Science

at the Department of Energy (DOE), issued on August 9,2007. The full charge is appended to

this report (Appendix A); it requests FESAC to "conduct a scientific and programmatic review

focused on evaluating the NCSX program and its potential effect on the US fusion energy

sciences program." The charge lists four detailed questions concerning technical and scientific

aspects of (i) compact stellarator research, and (ii) the NCSX program specifically.

It was decided from the start, in discussion with Dr. Fonck, Associate Director ofthe Office of

Fusion Energy Sciences, that the Review Committee would focus its attention of the first three of

the four questions; these questions are repeated as headings of the three numbered Sections in the

Committee's report that follows.

The ten members of the Review Committee are listed in Appendix B. The membership includes

prominent scientists and engineers from laboratories, universities and private industry. Leading

stellarator researchers from the US, Europe and Japan are Committee members, as well as

scientists working outside the stellarator program.

The Review Committee began its work by reviewing a large body of technical literature:

technical articles on stellarators and NCSX from the published literature; unpublished reports

and presentations on the NCSX program, including the May 2001 FESAC Report ofthe NCSX

Physics Validation Review; and a special summary report, prepared as a service to the

Committee by the ARIES team. In addition, members of the NCSX team provided documents

addressing each question raised in the Review Committee's charge. The Committee takes this

opportunity to thank the ARIES and NCSX scientists for their time and effort in compiling this

extremely helpful material.

Most of the Review Committee's discussions were conducted by teleconference and email.

(Email correspondence was simplified by a Review Committee email reflector, set up with help

from the US Burning Plasma Organization, whom we thank.) The Committee met once at the

NCSX experimental site at the Princeton Plasma Physics Laboratory (PPPL). The site visit

included extensive presentations by members of the NCSX team addressed specifically to the

Committee's charge, as well as a brief tour the NCSX construction.

All members of the Review Committee participated in the discussion of each charge question,

and all members agreed to the conclusions presented in this report.

Before addressing the specific questions in the Charge, we survey the scientific and

programmatic context of the NCSX project.

The US stellarator program has chosen a research path that emphasizes simultaneously two

design principles: quasi-symmetry and compactness. A quasi-symmetric configuration is one in

which the magnitude of the magnetic field in a particular direction along the torus is roughly
constant even though the components of B are not. Such a device combines the inherent steadystatenatureofthe stellaratoras a magneticconfinementschemeforfusionwiththe goodsingle--

particle confinement of a tokamak. The main element of the program is the PoP experiment

under construction, NCSX at Princeton Plasma Physics Laboratory (PPPL), which is quasisymmetric in the toroidal direction (quasi-axisymmetric, or QAS). The other experimental

elements of the program are the quasi-helical HSX at the University of Wisconsin-Madison,

which is operating and the quasi-poloidal QPS at the Oak Ridge National Laboratory (ORNL)

which is under prototype development but not yet approved for construction. In addition, CTH at

Auburn University is a nonsymmetric stellarator with ohmic heating that is exploring equilibrium

and stability issues that are relevant to the operation of a compact stellarator with substantial

plasma current. The designation of "compact" within the framework of the US stellarator

program typically means that the aspect ratio is less than five. Only HSX has an aspect ratio

greater than this. The other key elements of the US compact stellarator program are theory and

computation.
1. Critical Scientific Issues For The US Compact Stellarator Program:

a. What unique toroidalfusion science and technology issues can a compact stellarator

program address, independent of itspotential for a reactor concept?

Tokamaks rely on plasma shaping in the poloidal and radial directions to improve plasma

stability and confinement. The compact stellarator program utilizes additional shaping of the

magnetic surfaces in the toroidal direction to explore the potential science, technology and

reactor benefits as well as possible challenges. The scientific issues to be addressed in the

program include disruptions, transport, energetic particle confinement, equilibrium, stability and

density limits, and particle and power handling. Technology issues to be explored include

simpler magnet coils and support structures, metrology, correction coils and divertors. While

operating stellarators at an aspect ratio comparable to tokamaks may help elucidate differences in

scaling laws, transport and stability, there is no strong motivation for low aspect ratio from the

perspective of stellarator physics. Rather, the incentives for compactness are a stronger tie to the

tokamak database and the prospect that lower aspect ratio will decrease capital costs and lead to

a more economical fusion reactor.

Disruptions: Quasi-axisymmetric stellarators like NCSX would have large bootstrap currents.

Disruptions in currentless stellarators are generally not observed, even when operated at betas

above the ideal stability limits. The key issue for these devices is to determine at what level of

free energy in the magnetic field due to plasma current do disruption-like effects begin to appear.

Stellarators with plasma current can also contribute to understanding and controlling tokamak

instabilities such as Neoclassical Tearing Modes (NTM) (which limit many tokamak operating

regimes) and Edge Localized Modes (ELMs) (which may limit the lifetime of plasma-facing

components in large-scale tokamaks).

Transport: While tokamaks have good particle confinement due to symmetry in the toroidal

direction, conventional stellarators have poor neoclassical transport properties at low

collisionality because of the breaking of that symmetry. The innovative concept of quasisymmetry bridges the gap between stellarators and tokamaks by restoring a direction of

symmetry in the magnetic field magnitude. Results from the HSX experiment with quasi-helical

symmetry at moderate aspect ratio have demonstrated reductions in plasma flow damping,

particle transport and electron thermal conductivity.

As neoclassical transport is reduced, it is anticipated that turbulent-driven transport (so-called

"anomalous" transport) as seen in tokamaks will become increasingly important for the compact

stellarator program. Tokamak transport is believed to be strongly influenced by the presence of

large-scale plasma flows. With quasi-symmetric stellarators, anomaloustransport could be

strongly affected in the same manner as in tokamaks because there is now a direction of

symmetry along which plasma can flow. Zonal flow damping in such configurations may also be

reduced. A stellarator with the capability to heat with or without applied torque in a magnetic

configuration with varying degrees of quasi-symmetry should be a powerful physics tool for

understanding flows and their role in plasma confinement. Once the character of the turbulence

in stellarators is understood, the design tools created to optimize quasi-symmetry might also be
applied to optimize the magnetic geometry for neoclassical and anomaloustransport

simultaneously.

Energetic Particle Confinement: Another issue related to transport is that of energetic particle

confinement. Ripple and stochastic transport are issues for high energy particles in tokamaks and

needs to be explored in the compact stellarator program as well. How susceptible energetic

particles are to Alfvenic instabilities needs to be addressed. Whether fast particle losses can be

mitigated by operation at higher densities in stellarators or by other means also needs to be

explored.

Equilibrium: There is some indication that equilibrium limits in stellarators may be set by the

degradation of magnetic surface quality. The response of the plasma to magnetic perturbations is

a cross-cutting issue with ties to the tokamak program. A key element of the compact stellarator

effort is to accurately reconstruct the 3D magnetic equilibrium from magnetic coils and other

diagnostics. Such capability would be beneficial to tokamaks which also exhibit signs of 3D

behavior. At present, the CTH stellarator is the primary test bed for this work. An important

addition to the 3D reconstruction effort would be the inclusion of magnetic islands in the

equilibrium.

Stability and Density Limits: Experimental evidence indicates that operational limits of

pressure and density in a stellarator are relatively benign and lead to degraded performance, in

contrast to tokamaks where encounters with the operational boundaries can lead to termination of

the plasma. A volume-averaged beta of 5% was recently achieved in LHD, which is above the

calculated limit for ballooning modes. Plasma pressure in conventional stellarators is not limited

to the linear ideal MHD stability threshold. A compact stellarator experiment with significant

bootstrap current needs sufficient auxiliary heating to reach the theoretical beta limit and a long

enough pulse length so that the total rotational transform remains steady. Data from such an

experiment would also allow correlation with the extensive database of tokamak

stability. Similar arguments also apply to density limits. The density limit in tokamaks and

stellarators may eventually be shown to arise from radiative cooling of the plasma edge;

however, the limit often leads to disruption in tokamaks, while in stellarators, the plasma simply

collapses from recombination. Experiments are needed to test the density limit in compact

stellarators with plasma current.

Particle and Power Handling: A key challenge for stellarator design is to deal with the heat and

particle fluxes in the complex 3-D magnetic configuration. It is important to assess these issues

in a plasma that is sufficiently opaque to neutrals and has sufficiently high heat fluxes so that

both the confined plasma and the edge plasma are in regimes interesting for fusion. In a compact

stellarator design, it should be possible to reach and study these conditions more easily (smaller

device with lower power) than with a conventional design. The sensitivity ofthe magnetic

configuration at low aspect ratio to applied fields that break the stellarator symmetry may

actually be advantageous to this area of research. Tokamaks, for example, are investigating

symmetry-breaking at the plasma edge to control ELMs.

Coils and Structures: As for technology issues, a main concern that could be addressed is that

of designing and building simpler magnetic coils and support structures. These are issues that are
presently causing difficulties in the W7-X and NCSX construction projects. Much depends on

understanding the scientific issues within an overall optimization scheme such as how Iowan

effective ripple is needed or what constraints are necessary to satisfy stability criteria. Relaxation

of such constraints might lead to coilsthat are easier to build. Also the issue of tolerance

requirements needs to be addressed for magnet coil manufacture as well as coil and field period

assembly. Field error control, mode locking and subsequent disruptions are concerns related to

tolerance requirements for tokamaks.

Metrology: A necessary adjunct to this effort is the need to accurately determine the as-built

structure of the magnetic coils and sensors. The development of metrology capabilities

significantly beyond that existing in the US fusion program is being driven by the stellarator

community. This is applied to the quality assurance in component fabrication and assembly.

Such capabilities might be transferred to other scientific and industrial applications.

Correction Coils: Once the coils are built and mounted within the support structure and the

metrology capabilities determine the tolerance that has been achieved, the next technology issue

to be addressed is that of correction coils. Inevitably there are deviations of the as-built device

from the design coils and correction coils may be helpful in relaxing tolerance requirements as

well. This is an area of concern for tokamaks as well as stellarators, so understanding the physics

of error correction (and shielding by rotation and plasma currents) and developing a technology

for minimizing the impact of imperfect design implementation should benefit all magnetic fusion

research.

Divertors: Another important technology issue to be addressed in the compact stellarator

program is that of particle and power handling. The 3D shaping capabilities of stellarators allow

for flexibility in edge geometry including local island divertors and ergodic regions. Control of

neutral recycling and impurities needs to be demonstrated. A key issue to be addressed is

whether the divertor in a lower aspect ratio device can handle the higher heat flux. Also to be

determined is what design constraints can be relaxed by operating at higher densities and with

radiative cooling.

b. What are the advantages and disadvantages of the quasi-symmetric stellarator as a

potentialfusion systemconcept?Whatuniquefeatures doesthecompactstellaratoroffer

in this regard?

The compact quasi-symmetric stellarator could address questions arising in the transition from

ITER to a DEMO fusion reactor. Specifically, first-wall lifetime and reactor availability require

disruption and ELM suppression. Stellarators have already demonstrated the ability to operate

disruption-free also at stability boundaries. They rely on external currents instead of current drive

to provide rotational transform and are inherently steady-state. High density operation is possible

in stellarators without regard for the rotational transform. This facilitates solutions for divertor

operation and allows the density to be chosen to minimize potential problems due to the fastparticle population and synchrotron radiation losses.

The quasi-symmetric approach to plasma shaping overcomesthe poor neoclassical confinement

of conventional stellarators at low collisionality. Without the resulting increased gain, a

stellarator reactor would have difficulty reaching ignition conditions.
From the science perspective, a disadvantage ofthe quasi-symmetric approach is that the

reduction in the effective ripple, while beneficial for transport, makes access to the electron root

of the ambipolarity constraint more difficult. This could imply that shear stabilization of

turbulence by the neoclassical electric field, through the proximity of electron and ion roots, may

no longer be possible. A quasi-symmetric stellarator would then have the same problems faced

by tokamaks: E x B shear stabilization may not scale to a reactor because of small p* (ratio of

gyroradius to minor radius); additional momentum input may be required. Another disadvantage

of the quasi-symmetric approach is the complex technology of the modular coils that are needed

for the plasma shaping.

Compared to a higher aspect ratio stellarator, a compact stellarator offers the possibility of lower

initial capital cost, lower cost of electricity (COE), lower fusion power output (for more flexible

application) and lower volumes of radioactive waste. These are the major arguments why the US

stellarator community advocates a stellarator reactor with a tighter aspect ratio. The ARIES-CS

(CS for compact stellarator) fusion reactor design study was completed in 2006 based on an

NCSX-class quasi-axisymmetric configuration. The device has a major radius of7.75 m and an

aspect ratio of 4.5. The size and mass were similar to an advanced tokamak power plant.

In a summary report to this Committee, the ARIES-CS team described some of problems

associated with compact stellarator reactor design. For a major radius less than 7.5 m, there was

not enough space for blankets to provide tritium self-sufficiency. There were concerns with high

heat flux to the first wall and divertors. More work was needed to reduce the energetic alpha loss

further. A complex support structure was designed which would be difficult to build with

conventional manufacturing. A new fabrication technology called "additive manufacturing" was

assumed. Access to the blanket was challenging because of the modular coils. The ARIES team

emphasized that in their estimates of the COE they did not include a penalty for the complexity

of the components or a possible lower reliability because of that complexity. Hence they stated

that cross-comparison with a tokamak is not meaningful, although comparisons with other

stellarators was valid.

The ARIES team did make a comparison to a larger aspect-ratio stellarator reactor design

conducted in 1996.This was the SPPS study which was based on a configuration similar to HSX.

The SPPS reactor has a major radius of 14 m and an aspect ratio of 8.5. At roughly half the

aspect ratio of SPPS, the ARIES-CS COE was only 20% lower at 78 mills/kWh. Furthermore,

they stated "System analysis, however, shows very little cost benefits in going to smaller size

devices."

The ARIES team took special note of two points, which we quote:

a- "The ARIES-CS study was the first major study of compact stellarators. As such,

it was conducted in the 'problem finder' mode. For example, the design point was

pushed to the limit for a "compact" configuration with low aspect ratio to better

understand the constraints imposed by the "compactness" and the possible tradeoffs. In most areas, we find that increasing the machine size compared to ARIES-
CS reference design will provide more margins on space and engineering

constraints such as material stress, temperature limits, etc.

b- Most of the engineering research was performed on NCSX-class quasi-

[axi]symmetric configuration. Caution should be used in extrapolating to other

compact stellarator configurations."

c. Whatscientific and technical issues need to be resolved to evaluate the compact

stellarator as a viable conceptfor afusion energy system?

A number of scientific issues need to be resolved to evaluate the compact stellarator as a fusion

reactor. Some of these issues directly impact the problems tokamaks face in making the

transition from ITER to DEMO. For a quasi-axisymmetric (QAS) stellarator in particular,

because of the relatively large bootstrap current, it is important to understand how much external

rotational transform is needed to eliminate disruptions. Does the 3D shaping stabilize

neoclassical tearing modes? What magnetic structure at the plasma edge is effective in

eliminating large ELMs? Does the combination of internal rotational transform (from the

bootstrap current) and externally generated transform indeed lead to stable steady-state

operation?

Some of the scientific issues are generic to the quasi-symmetric concept. At this time, it is still

not clear how low the effective ripple should be. What is the relative role of neoclassical versus

anomalous transport? To what extent does zonal flow damping affect turbulent transport? Does

drift optimization reduce turbulent transport as well as neoclassical transport? Is it possible to

increase the E x B shearing rate in a quasi-symmetric stellarator to the extent that it scalesto a

reactor? How does quasi-symmetry affect energetic particle confinement and stability? Finally,

which form of quasi-symmetry makes the best reactor? Do we have to test them all or are there

distinguishing features that favor one form over another?

Other scientific issues are more generic to stellarators in general. This part of the assessment is

carried out within the world-wide stellarator community, of which the major devices are LHD (a

superconducting heliotron device in Japan that achieves some improvement in transport by

shifting the plasma inward), W7-X (a superconducting, large aspect ratio device in Germany that

relies on transport optimization that is not quasi-symmetric and also minimizes plasma currents)

and NCSX. Being a compact stellarator with a tokamak-like aspect ratio and quasi-symmetry,

NCSX could playa unique role in this assessment. What limits the plasma beta and density in a

stellarator? How can impurity transport be controlled? How does the plasma respond to magnetic

perturbations? How do equilibrium islands and flux surface robustness scale with increasing

pressure? Is it possible to do 3D equilibrium reconstruction that detectsthe presence of island

structures in a stellarator?

The scaling of energy confinement with size is also important to the viability of stellarators as a

fusion energy concept. The size scaling determines how large a reactor must be to produce

fusion power with sufficient gain. While most of the issues discussed previously can be studied

effectively in a single stellarator; determination ofthe size scaling needs data from an array of

stellarators. The two methods now employed to determine the size scaling of tokamaks, database
analysis and dimensionless parameter scaling, have also been applied to stellarators. From

database analysis, the confinement in stellarators appears to scale with volume, which is

favorable compared with tokamaks. It will be important to determine whether the scaling of

optimized stellarators also follows such scaling. It will be challenging to compare the many

varieties of stellarators in the absence of a validated fint-principles model for stellarator

confmement.

In addition to the issues of fusion science, a number of technical issues need to be resolved for

the compact stellarator to be properly evaluated. One issue that is very much dependent on a

resolution of the scientific issues is whether certain constraints such as stability or confinement

can be relaxed so that magnet coils can be designed that are easier to build. An issue that the

ARIES-CS team faced is whether a complex support structure for a compact stellarator reactor

could be built without inventing a new fabrication technology. If such a technology is needed,

how easy is it to develop? Does failure of a magnet coil compromise the power plant or can the

coils be replaced remotely? Is there a remote handling scheme that can deal with the complex

geometry of a compact stellarator? How reliable are the complex components needed for a

compact stellarator? How does that affect availability and the cost of electricity (CaE)? What is

the best way to control particle and heat fluxes at the plasma edge? Which stellarator geometries

best accommodate divertors?

One central question that needs to be addressed is raised by the report produced by the ARIESCS team. They found that for configurations of the NCSX~typethere was little benefit in terms

of CaE in going to even lower aspect ratio. Are there any compact stellarator configurations at

aspect ratio much lower than five that show a significant decrease in the cost of electricity?

2. The Role of NCSX in an International Context

(a) What critical, unique contributions does NCSX potentially offerfor addressing the

issues identified in (lj?

NCSX is designed to address most ofthe critical physics and technology issues discussed in this

report using a compact, quasi-axisymmetric configuration that is unique in the world stellarator

program. The Committee finds that, assuming successful construction and testing phases, the

NCSX device is likely to perform at a level sufficient to address its scientific and technical

missions. Thus the Committee expects the NCSX experimental program to have a profound

impact on stellarator research worldwide.

As described in Section 1,the fusion program's strong interest in the stellarator approach is that it

potentially offers solutions to the important problems of disruption avoidance and steady-state

operation facing tokamaks in going from ITER to a DEMO fusion reactor. Specifically, fust wall

lifetime and reactor availability require nearly complete disruption elimination and ELM

suppression. Stellarators have already demonstrated the ability to avoid disruptions and where

ELMs have been observed, they are not severe. Furthermore, by relying primarily on external

currents instead of current-drive or bootstrap currents to provide rotational transform in the

confming magnetic field, stellarators can more easily operate in steady state.
To explore the potential advantages of disruption avoidance and steady state operation in a more

compact (low aspect ratio) advanced stellarator employing the quasi-axisymmetric magnetic

configuration, the NCSX experiment was approved by FESAC in 2001 as a PoP class device in

the US fusion energy science program. This places it between smaller Concept Exploration

experiments, such as HSX at the University of Wisconsin-Madison, and larger Performance

Extension class experiments, such as the DIII-D National Tokamak Facility in the US, the LHD

stellarator in Japan, the W7-X stellarator under construction in Europe and the even larger

burning-plasma class experiment, ITER.

There are two other PoP class experiments in the US fusion program: the NSTX experiment at

PPPL, exploring the compact torus magnetic configuration, and the MST experiment at the

University of Wisconsin-Madison, exploring the reversed field pinch magnetic configuration.

The characteristic of a PoP class experiment is that it has the size, magnetic field, and plasma

heating systems to allow study of key physics behaviour of the magnetic configuration concept

with collisionless ions and electrons for many energy confinement times. With these parameters,

the PoP class experiment is minimally sized to provide credible integrated confinement and

stability results that scale to the 10 keV fusion power plant regime of temperature and

collisionality, and hence prove the basic physics principles of the confinement configuration

under study. This normally requires temperatures of both ions and electrons at the keY level, 1 to

2 Tesla magnetic fields and several MW of external heating input to the plasma for several tenths

of a second. In terms of key dimensionless parameters, this requires collisional mean free paths

to be dominated by Coulomb collisions and to be an order of magnitude larger than toroidal or

poloidal connection length scales; a device size many times the characteristic size of the ion

gyroradius; and a plasma pressure normalized to the magnetic energy density ~of at least several

percent.

The NCSX experiment was designed to meet these conditions for the exploration ofthe quasiaxisymmetric stellarator (QAS) magnetic configuration. Using expectations of stellarator

confinement based on an extensive data base, the NCSX design projects central ion temperatures

of 1to 2 keY,magneticfieldvaluesof 1to 2 Tesla,dimensionlesscollisionality,v* ~ 0.1, ~~

5%, and at least 10poloidal ion gyroradii across the 0.33 m average minor radius using 6 MW of

neutral beam heating. Thus, NCSX is well positioned to carry out PoP experimental tests of the

QAS magnetic configuration.

Three principal concepts are possible for quasi-symmetry: quasi-helical, quasi-poloidal and

quasi-axisymmetry (QAS). Each of the three principal quasi-symmetric stellarator concepts is

being pursued in the world fusion program. Quasi-helical symmetry is being studied in HSX,

which has been operating for several years; quasi-poloidal symmetry is the guiding principle of

the configuration design of the QPS device proposal. There are also techniques for optimizing

particle orbits outside of quasi-symmetry. W7-X (under construction in Germany) and

Heliotron-J (operating in Japan) pursue such alternative approachesto advanced stellarator

design. There is some experimental confirmation that orbit optimization, using quasi-symmetry

or other means, significantly improves neoclassical confinement. For example HSX has

demonstrated reduced neoclassical losses.
As the only representative of quasi-axisymmetry in the world program, NCSX is intended to

demonstrate the performance of a quasi-axisymmetric system, and explore the performance

consequences of its mix of internal and external rotational transform. It will demonstrate whether

good confinement, sufficient equilibrium and stability beta limits can be achieved, specifically

with regard to current-driven modes, and whether the potential for steady-state operation can be

achieved simultaneously by the proper mix of internal and external transform. Because of the

complicated nonlinear interaction between plasma profiles, bootstrap current and confinement,

these are issues considerable scientific and fusion interest.

Among the advanced stellarators, QAS has two attractive and unique features. First, the QAS

design fits naturally into a low-aspect-ratio configuration because it requires relatively strong

toroidicity compared to other components of the magnetic field curvature. Thus, the QAS is a

good candidate for a 'compact' (low aspect ratio) stellarator configuration. Second, confinement

studies with QAS would have many common physics elements to tokamak research. From this

aspect, QAS should produce complementary physics results to tokamak research aimed at fusion

energy development. An important question is whether the tendency for impurity accumulation

is controlled in NCSX by inducing current-driven core instabilities to prevent it. Finally, when

allsystemshavebeenstudiedin detail,the threemajorlines- LHD(Heliotron),W7-X

(optimization outside of quasi-symmetry), and NCSX will allow us to decide which system has

the highest reactor potential. This information will influence the debate on the continuation of

the fusion program toward a DEMO reactor.

Because sufficient confinement requires a large volume, toroidal systems deliver fusion power at

a level that motivates steady-state operation from an engineering and grid-integration point of

view. Current-driven tokamaks resort to high bootstrap-current fractions, with the complications

of non-linear interactions between confinement, equilibrium and stability, NCSX aims to explore

these regimes with the help of external contributions to rotational transform. Thus it should allow

study of the criticality of bootstrap-current based equilibria in a unique way. For this assessment

the low-aspect ratio ofNCSX, which is comparable to that oftokamaks, makes the comparison

especially relevant.

(b) Given PPPL 's proposed plans for operation of the National Spherical Torus

Experiment and NCSX, what would be the timetable for resolving relevant issues

identified in (1) above?

The time scale of fusion development is determined to a large degree by that ofITER. Major

decisions on the direction of DEMO are unlikely to be made before clear answers are obtained

on the critical burning plasma issues that can only be addressed by ITER, such as stability with

a-particles, burn control and ash removal. When these burning plasma studies are completed, the

stellarator research findings will be needed to enter into the general debate on how to structure

the next steps beyond ITER. The time scale set by NCSX projects the achievement of first

plasma in 2012 and initial results in 2013. W7-X is scheduled for first plasma in 2014 and ITER

is presently scheduled for operation in 2016. LHD and HSX are operating now and will continue

to contribute.

The NCSX team proposed a timetable for the scientific exploration during the first years
including operation in alternate years with NSTX. Since this schedule extends to a time beyond

even the proposed next 5 year plan for NSTX, it cannot be considered certain. The planned

phases ofNCSX device commissioning, preparation of the necessary diagnostics in line with the

major topics to be addressed and the development of the divertor and the heating periphery are

well thought out. The planning reflects the considerable experience at PPPL on commissioning

such experiments.

The question whether NCSX can indeed meet its proposed resolution of critical issues with only

FY13 and FYl5 was discussed in detail by the committee. Based on the PPPL experience with

the comparably sized NSTX experiment which has a comparable operation budget level as

NCSX, a more likely time frame for resolving the initial set of critical issues is about 5 years of

actual operation, with detailed scientific understanding and an extrapolable basis for optimization

of follow-on devices requiring about 10 years of experimental operation.

Technical requirements that may extend the time scale for resolving the initial set of critical

issues include achieving (and maintaining) sufficient quality ofthe confining flux surfaces. The

quality of flux surfaces depends on symmetry breaking field errors introduced by tolerance

deviations of single components or by errors introduced by assembly. The project has shown that

it follows the development of the components in detail by calculating the resulting island widths

from resulting mechanical deviations. The project has also developed strategies how to cope with

errors of the components (by compensating it by slight changes to the geometry of the device

during assembly) and by installation of specific trim coils. Trim coils are also the selected

compensation method on ITER and other stellarators (LHD, W7-X). It is important that these

studies are continued during NCSX assembly. In addition the Committee recommends that

attention to construction details that may affect flux-surface quality, and the study of their effects

and methods to counteract them, remain top priorities for the project.

Other technical or managerial risks affecting the timetable are not part of the charge and were not

addressed.

(c) What are the differences of the current NCSX design compared to other stellarators

operating or being built abroad? What is the significance of these differences? Does

NCSXfill a critical void in the development of the stellarator concept as a viablefusion

energy system?

NCSX is designed as quasi-axisymmetric stellarator with low aspect ratio. In both properties,

NCSX is unique. IfNCSX were not completed, both stellarator and tokamak research will suffer.

W7-X is optimized using a different scheme than quasi-symmetry (with superconducting coils)

and all currents in W7-X, apart from the diamagnetic current, are strongly reduced. In NCSX,

with its compact, quasi-axisymmetric configuration, the bootstrap current is maximal. The

comparison between these two devices will be extremely useful in understanding the physics

optimization of these advanced stellarator configurations. Beyond obvious criteria

(confmement), the stability against current driven modes, the known differences between

tokamaks and stellarators (e.g. the isotopic effect in anomalous confinement), the different flow

pattern and the level of damping will be of extraordinary importance.
With respect to compactness or small aspect ratio, W7-X has a large aspect ratio, LHD a middle

aspectratio,andNCSXthe smallestaspectratio.In developingloweraspectratioapproachesin -

the stellarator configuration, NCSX clearly plays a significant role. In demonstration of the

potential for steady-state operation, LHD and W7-X are significant because they employ

superconducting magnets. In terms of the tokamak program benefitting from 3D magnetic

configuration physics, NCSX is also significant because of its quasi-axisymmetry and large

bootstrap driven currents, as discussed above.

3. Options for the US stellarator program

a. If the NCSXprogram were not continued, what options would exist or would bepossible

to address the key issues of the quasi-symmetric stellarator in general and the compact

stellarator inparticular?

In the absence ofNCSX, the Committee recommends that the DOE enhance the remaining US

stellarator program to enable the US to continue as a participant in the worldwide stellarator

research program. The present US stellarator research program includes other devices, planned

or in operation, that could address at various levels a subset of the key issues listed in Section 1.

However, such studies would have to be done at lower plasma parameters, and at some

somewhat less relevant values of the dimensionless parameters. Examples of such studies are :

Effects of strongly reduced effective ripple on energy confinement.

Determination of pressure limits in 3D configurations.

Disruption stabilization and avoidance.

Reduction of turbulent transport by flows and 3D shaping optimization.

Stabilization of equilibrium islands and tearing modes.

The Committee emphasizes that no US experimental program, present or planned, could provide

the breadth of scientific and technical information that is expected to come from NCSX. The

only PoP scale device in the US repertoire that addresses quasi-symmetry is NCSX, and NCSX

is the only such device capable of examining the key issues in an integrated context. Thus,

without NCSX there would be no experiment to address the issues of quasi-axisymmetry and the

potential of lower-aspect ratio stellarators. Furthermore, there would be no way to study hot-ion

and high beta physics in a low-aspect ratio quasi-symmetric 3D system. NCSX occupies a

unique niche in the complex stellarator design space.

Therefore, ifNCSX were abandoned, the US would have to significantly reduce its ambitions in

the quasi-symmetric stellarator research area, or begin constructing a new PoP stellarator

experiment.

The Committee finds it important that the US have a significant stellarator presence as part of its

magnetic fusion energy research program. The loss ofNCSX would have serious negative

consequences for the US and worldwide stellarator research programs. In particular, the US

would lose its world leading position in developing the compact, quasi-axisymmetric approach to

stellarator confinement research. The Committee notes that at present about 75% of the US
stellarator effort is focused on the construction ofNCSX so the loss ofNCSX would change the

basic character of the US program. The program would lose its integrated PoP facility and the

most relevant connections to tokamak research through quasi-axisymmetry.

b. Assuming NCSX is not available, what program elements would

be required to maintain the US as a significant participant in the

international stellarator program?

i. Identify potential opportunitiesfor us leadership

ii. Include more international collaboration as appropriate.

The US stellarator program has several existing components that make important contributions to

the worldwide stellarator effort. These activities are addressing selected key issues. In the event

that NCSX is not available, these activities should be strengthened and new stellarator research

opportunities should be explored.

In the absence ofNCSX, a restructured US stellarator program could maintain scientific

leadership in selected research topics, but would have difficulty in playing a significant role in

the direction of worldwide stellarator research. Furthermore, the lack of a domestic PoP program

would diminish the benefits the US could gain from international collaboration.

Nonetheless international collaboration is a key element to the US stellarator research effort, as it

is to every element of the US fusion energy science program. With the loss ofNCSX, it can be

expected that international collaboration will occupy a larger proportion of the US effort. In

particular the US has the opportunity to participate in experiments on the LHD device in Japan

(currently operating) and the W7-X device currently under construction in Germany. Although

these devices at not low-aspect ratio, they will provide critical information on stellarator plasmas

at higher density and plasma temperatures, higher beta and steady-state operation. The US

would seek to expand its opportunities to study 3D physics on LHD and W7-X.

Theory and computation research in the US has developed significantly in recent years and

developed and employed a number of world-class numerical tools. To a large degree, this

resurgence in US stellarator theory is due to the NCSX and QPS design programs. There have

been a number of recent advances in the theory of magnetic surface fragility, 3D MHD

equilibrium and stability, transport, concept improvement and reactor optimization.

Opportunities exist to strengthen this work in areas such as high-beta and high-density operation

of stellarators, micro instabilities in 3D configurations, zonal flows and flow damping and

simplification of future devices and reactor concepts. However, the loss of a world-class

stellarator experiment in the US would make it more difficult to attract young theorists to

specialize in the area of 3D magnetic confinement research.

The US has the only operating quasi-symmetric stellarator in the world, the HSX at the

University of Wisconsin-Madison. Because of its quasi-helical symmetry and very low ripple,

HSX has a very high effective transform that is responsible for very narrow banana widths,

reduced Pfirsch-Schliiter and bootstrap currents and low neoclassical transport. With additional

diagnostics and heating capabilities, HSX can address issues of turbulent transport and zonal

flows, confinement scaling, plasma stability, low collisionality ion transport, energetic particle
confinement and impurity transport.

The CTH device at the University of Auburn investigates the stability of current-driven

discharges, from tokamak-like ranges of vacuum transform near zero to values oftransform

characteristic ofNCSX. Furthermore, CTH evaluates the effects on equilibrium and stability of

magnetic islands and error fields in helical configurations in which the rotational transform is not

tightly prescribed, as in NCSX. Improved support for reliable electron temperature profile and

current profile diagnostics would directly add to the effectiveness of this research program.

The proposed QPS device at the Oak Ridge National Laboratory is a low-aspect ratio quasipoloidal symmetric configuration that approximates linear linked mirrors with end losses in a

toroidal geometry. It may allow large poloidal flows and can potentially study the effects of flow

and flow shear on plasma turbulence. The physics program ofQPS would connect to higheraspect ratio stellarator devices like W7-X. QPS would extend the stellarator database to very

low-aspect ratio, similar to what the ST does for the tokamak database. However, while QPS

would have an important role in the world stellarator program, it would not be able to replace the

scope of the NCSX research program. In particular QPS could not address the assessment of

quasi-axisymmetry or the close link to tokamak behavior.

The Committee did not evaluate in detail the merits ofthe QPS device, which is currently in an

R&D and prototype-fabrication stage. Thus the Committee cannotjudge whether the DOE

should proceed with the construction of QPS. The Committee notes that it would seem illogical

to cancel a stellarator project nearing the final stages of construction only to begin a new

compact stellarator with poloidal rather than toroidal quasi-symmetry. However, in the situation

where NCSX is not available, the Committee believes that the DOE should expedite the decision

on the construction of QPS.

The Committee also recognizes that it might be possible to consider adding concept-exploration

level stellarator experiments to the US program. Potentially, such devices could pursue the

quasi-axisymmetric optimization path. However the Committee did not receive any specific

suggestions for such an undertaking nor did the Committee have time to carefully explore such

possibilities.

In the absence ofNCSX, the US stellarator program could pursue stellarator concepts outside of

the compact quasi-symmetric path. Quasi-symmetry is one of many ways to optimize 3-D

configurations; other optimization schemes could be pursued. Without the presence ofNCSX,

the US stellarator program should consider a variety of approachesto stellarator optimization in

proposing a new PoP stellarator project.
Appendix A: Charge to FESAC

August 9, 2007

Professor Stewart C. Prager, Chair

Fusion Energy Sciences Advisory Committee

Department of Physics

University of Wisconsin

1150 University Avenue

Madison, Wisconsin 53706

Dear Professor Prager:

The National Compact Stellarator Experiment (NCSX) project, which is being built at the

Princeton Plasma Physics Laboratory (PPPL), is projecting substantial cost (~$40 million) and

schedule (~2 year delay) overruns. These overruns are large enough to add new burdens on the

limited resources of the U.S. fusion energy sciences program, as well as undermine confidence

of the Administration and Congress in the ability of the Office of Fusion Energy Sciences and

the Office of Science to manage large and technically challenging construction projects. Given

the magnitude ofthe increases projected for NCSX, all options, including termination of the

project, must be considered. In that context, we would like the Fusion Energy Sciences Advisory

Committee (FESAC) to conduct a scientific and programmatic review focused on evaluating the

NCSX program and its potential effect on the U.S. fusion energy sciences program. Below is a

list of questions that we believe must be answered in order to allow us to make a decision on the

best course of action for the U.S fusion energy sciences program. This review will comprise part

of the set of reviews that will be conducted to inform a decision.

Questions for Scientific and Programmatic Review of NCSX:

1. Critical scientific issues for the U.S. compact stellarator program:

a. What unique toroidal fusion science and technology issues can a compact

stellarator program address, independent of its potential for a reactor concept?

b. What are the advantages and disadvantagesofthe quasi-symmetric stellarator

as a potential fusion system concept? What unique features does the compact

stellarator offer in this regard?

c. What scientific and technical issues need to be resolved to evaluate the

compact stellarator as a viable concept for a fusion energy system?

2. Role ofNCSX in the international context:

a. What critical, unique contributions does NCSX potentially offer for

addressing the issues identified in (I)?

b. Given PPPL's proposed plans for operation of the National Spherical Torus

Experiment and NCSX, what would be the timetable for resolving relevant
issues identified in (1) above?

c. What are the differences of the current NCSX design compared to other

stellarators operating or being built abroad? What is the significance of these

differences? Does NCSX fill a critical void in the development ofthe

stellarator concept as a viable fusion energy system?

3. Options for the U.S. stellarator program:

a. Ifthe NCSX program were not continued, what options would exist or would

be possible to address the key issues of the quasi-symmetric stellarator in

general and the compact stellarator in particular?

b. Assuming NCSX is not available, what program elements would be required

to maintain the U.S. as a significant participant in the international stellarator

program?

i. Identify potential opportunities for U.S. leadership

ii. Include more international collaboration as appropriate.

4. Role of the stellarator and NCSX in the long-term U.S. fusion energy sciences

program:

a. For a compact stellarator to be a viable reactor concept, what other

experimental facilities would be required to develop the required knowledge base?

b. For the cases with and without NCSX in the program, how can results from

the U.S. stellarator program impact the direction and/or risk level of the

development of the knowledge-base needed for a fusion energy system:

i. On the timescale for a first-generation DEMO after ITER?

ii. Longer-term, beyond a first-generation DEMO?

In summary, FESAC should answer all of these questions and provide their responses so we can

evaluate the situation and choose the most appropriate course of action. Given the urgency ofthe

situation, we would appreciate it if your answers could be provided by the scheduled FESAC

meeting in October 2007. I very much appreciate your assistance in addressing these questions

on such an expedited basis.

Sincerely,

/s/

Raymond L. Orbach

Under Secretary for Science
Appendix B: Members of the NCSX Review Committee

Dr. Charles Baker, Sandia National Laboratories (consultant)

Prof. Richard Hazeltine, University of Texas at Austin (Chair)

Prof. Chris Hegna, University of Wisconsin

Dr. Timothy Luce, General Atomics

Prof. Gerald Navratil, Columbia University

Prof. Shoichi Okamura, National Institute for Fusion Science (Japan)

Prof. Ron Parker, Massachusetts Institute for Technology

Dr. Max Tabak, Lawrence Livermore National Laboratory

Dr. Joseph Talmadge, University of Wisconsin

Prof. Dr. Friedrich Wagner, Max-Planck-Institut fUrPlasmaphysik (Germany)