On new reality in tokamak physics and fusion


Necessity (a) of initiating the LiWall research program and (b) of terminating the "compact" stellarator project (NCSX)

Several years ago,  when TFTR program and ITER project were both shut down and the funding was substantially reduced, the US fusion program faced a reality of dealing with a broad audience of experts in physics. There are still many misconceptions in our field on what did happen.

Nevertheless, an important lesson has been learned that stagnation in any part of the program may backfire. It has become clear to plasma physicists inside the fusion that the conventional way of dealing with one of the most severe problems (in quasi-stationary fusion), i.e., the power extraction from the plasma and reactor control, is incompatible with the most promising as well as realistic regimes in tokamaks.

Last year, one of such low recycling regimes was discovered and demonstrated in DIII-D experiments (GA, San Diego, CA. See, K.H.Burrell, http://fusion.gat.com/pubs/search.cgi, R. D. Stambaugh http://fusion.gat.com/presentations/).  From the plasma physics point of view, these regimes can potentially give a boost to the magnetic fusion program if made consistent with the technology at reactor level of power.

Imminent switching to these regimes created apearently new priorities for tokamaks and stellarators. E.g., the problem of power extraction has become what can be called a "stopper" from implementation of the next step for both of them.

Independent of DIII-D results, a LiWall ("Li" here stands for Li, not for "liquid Li") concept has been formulated theoretically and then developed in the US to a certain level  of maturity (since Dec. of 1998)   in close collaboration with the plasma physics boundary and technology groups (ALPS/APEX in the US, Kurchatov/TRINITI  in the RF, IPP in Germany). It was invented for solving the "first wall" problem in a consistent way with the low recyling regime. Now, LiWalls can be qualified as the most complete concept in quasi-stationary fusion, which lifted tokamaks  to a good level of consistency with the known plasma physics and potential reactor technology. This consistency now covers basic tendencies in plasma confinement and MHD stability as well as it  takes into account nuclear physics and material issues associated with the controlled fusion reactor. (For those, working in the area, the basic supporting materials can be found on web site http://w3.pppl.gov/~zakharov as well as on ALPS/APEX web sites.)

The way of dealing with the key issues, such as power extraction from the plasma, which are conceptually resolved for tokamaks by the LiWall concept, should reassess the position of stellarators inside the fusion program. At least for the moment, it is evident that intrinsic 3-dimentionality in the power deposition on the plasma facing structural components highlighted conceptual problems for stellarator based reactors.

In this regard,  "Compactness", which was exposed as a priority for the National Compact  Stellarator Experiment (NCSX) makes the problem for stellarators even worse by additional concentration of the power deposition. Thus, this project is evidently inconsistent with the tendencies in magnetic fusion science. It also does not serve the strategy of the potential US stellarator program.  It should be terminated as a funded project.  (I do not want to comment here more on this issue).

Recently, our collaboration on LiWalls in the US resulted in a new design approach  (I called it the "Yacht sail"-like design, which is dynamically balanced) for the first wall and the neutron energy absorbing layer in the fusion reactors. It lifts tokamaks even more with respect to other fusion approaches. This design approach also allows to drop the requirements of the stationary regime in tokamaks, thus, reducing importance of "stationarity" of the magnetic configuration.  It raised additional challenges for stellarators in a strategic "competition", if any, with tokamaks.

Going to a little more deeply into detail, the LiWall research path has been also elaborated this time.  Essentially, it consists of (a) plasma physics research with the lithium conditioned walls (e.g., Li coated copper) and (b)  development (plausibly shifted in time) of new technology of flowing metals and, finally, liquid  Li. Since its first disclosure in early 1999, liquid lithium was never considered or proposed for the plasma physics research part of the LiWall concept. Instead of this still existing misinterpretation,  the practicality of research, safety and reduction of activation in the fusion reactors was always in the scope in this concept.

These very positive aspects of new reality, all together, should be reflected in the strategy in the fusion energy science program in the US by making tokamaks our distinguished leader. This means that potential contributors (such as Reversed Field Pinches) for resolving the prominent issues in the  leading concept, independent of how they are called (I hate the word "alternatives"), should be given higher priorities than, e.g., for stellarators (which in terms of the plasma physics are very similar to tokamaks, but appear to be disadvantagous in all other aspects).

Now, let me switch to a different, but inherently related, programatic ussue. Existed during the past 2 years internal NCSX/LiWall standoff in PPPL revealed also a very dangerous tendency of the evident, at least to me, suppression of innovations in the Lab, as soon as they do not fit to administration "policy". This suppression of innovations simply cannot be tolerated under any circumstances. Creativity is the essence of physics and science. It is just the immune system of the program which allows to withstand any external perturbations. It is also the most fragile component of the government funded programs, well understood in DoE according to publications and reflected in the mission of PPPL. Ability to see the potential of innovations and to nurture the creativity is essentially the major reason of having scientific administration at the Lab level.  I do not want to elaborate anymore on this, except of pointing out that the way PPPL administration works right now against this my motion is just the same as it  was during the entire 2 years of LiWall development. It is empty promises, widespreding their own misinterpretation or desinformation of how "bad" is any initiative  (except of their own, middle men, "hard work") for the Lab, for PU, for people or whoever it is, and so on. These are the only things they acquired from the ongoing revolution in communications.

Now, let me give a list of several "hot" issues related to this message and to the LiWall concept

    1. Choice of addresses of the message.
    2. Friday, Jan.12, 2001 at 2:00 - 4:00 pm - PPPL seminar (PS&T) at MBG.
    3. LiWalls and (hydrogen retention) & (Helium exhaust).
    4. LiWalls and tokamak-reactor control and its activation.
    5. LiWalls and their possible first research steps - relaunching TEXT and PBX-M machines.
    6. LiWalls and ongoing programs in fusion.
    7. Concluding remark.
which are briefly commented below.
 
 

1. Choice of addresses of the message.

I address this (e-mail) message, first of all, to leaders of several fusion institutions as well as experimental and technology groups inside the US in order to make efforts in redirecting our biggest asset, PPPL, to its mission. This mission relies on new approaches/innovations and which, at the same time, by no means suggested demolishing our most developed devices, the tokamaks, or suggests abandoning them (see, PPPL web-site http://www.pppl.gov/about_pppl/pages/mission_vision.html). In this regard,  it is our common mission and mutual responsibility to take care about the best Lab in magnetic fusion in the World.

I  included into the list of recipients  plasma physics and technology people who in one way or another contributed into the  remarkable development which I called the LiWall concept.  These people, their knowledge, expertise and dedication constitute the backbone of the powerful scentific team behind the entire concept.

I addressed this message to a couple of persons in Princeton University in order to keep them informed on important things happening in PPPL.

I  include also into the List of addressees the current staff of OFES of DoE. For the last 2 years, administration successfully covered up the very existence of a new tokamak-reactor concept and associated views on program for PPPL.

!!! <Please, if replying, be sure you do not bother all busy people listed as the addresses> !!!

2. Friday, Jan.12, 2001 at 2:00 - 4:00 pm - PPPL seminar (PS&T) at MBG (as it was announced).

                 Leonid E. Zakharov

"Power extraction from tokamaks and tokamak-reactors"

                        Abstract
{
   Only two aspests of the LiWall concept will be discussed in
presentation :

(a) power extraction from the plasma facing components in tokamaks and
(b) power extraction from the energy absorption layer in the neutron
    zone of the tokamak-reactor, which includes the  a new "Yacht sail"
    approach for the tokamak-reactor design,

(other issues will be discussed on Jan. 18 and on consequitive PPPL seminars).

For the first time in the quasi-stationary fusion, LiWall concept
provides a consistent approach for solving both  problems of
power extraction. It also opens for US fusion program practically

             unlimited RESEARCH capabilities

(presumably, up to ignition and burning plasmas)
on tokamaks with the Li coated copper walls.

The presentation makes obvious to broad audience of physicists the
progmatic and scientific meaningless of continuing the  "compact"
stellarator project in PPPL.
}

Key details of this talk are scattered in my several most recent presentations (e.g., at "Burning plasma workshop" at Austin, TX), which can be found on the same web-site http://w3.pppl.gov/~zakharov and will be assembled shortly in a separate file.

3. LiWalls and (hydrogen retention) & (Helium exhaust).

Problems of the particle control (including impurities) and of future He ash exhaust was the major reason of  abandoning (long time ago) development of the wall limited plasma  (in favor of the divertors). LiWall is making this reversed by suggesting a right solution to the power extraction problem.


Its solution to the particle control problem is based on gettering hydrogen (or D, T) ions by the Li surface layer, proved to be evident on T-11M experiments (TRINITI,  Rus.Fed.). Also a reliable chemical retention of D or T in Li was expected initiallly in the concept. T-11M results are unambigous for the lithium coating the walls. Not well-known yet, but already reported, T-11M indicates that even He gas is retained by the Li covered (cooper) shell if it maintained within the certain temperature range.

For the flowing plasma facing lithium coolant (considered in LiWall concept only for a future operational tokamak-reactor, not for research machines) chemical retention of T would translate sooner of later into a problem with regulations on trithium inventory. This was a big concern until recent PISCES (UCSD, CA) measurements. Chemical or other enhanced retention of D has been doubted (still measurements has been made for the divertor parameters, i.e., 3 order of magnitude larger particle fluxes than expected for LiWall regime), and, thus, additional contribution was made in maturing LiWall concept.
 

This PIECES results mean also that the problem of the hydrogen retention, if resolved without relying on chemical binding, will automatiically  resolve the unavoidable problem with the He exhaust, which make the physics of the flowing lithium more interesting. I do not want to elaborate more on this issue publically.

4. LiWalls and tokamak-reactor control and its activation.

This is a very sensitive issue in advocating any fusion reactor to the physics (e.g., fission) experts.

 
Without going into details, I would say that, from the plasma physics point of view, LiWalls
- create the best envirionment for the plasma stabilization and for feedback control of the tokamak MHD,
- make machine not sensitive to disruptions and thermal perturbations, thus, eliminating practically unrealistic requirements for tokamaks to be stationary,
- make acceptable the cycle regime for the plasma discharge in the tokamak-reactor (e.g., with hours fo the cycle period).


Now, the issue is to get numbers.

At the nuclear physics side, LiWalls

- led to the Yacht sail-like approach, where most of forces in the structure elements in the neutron zone are absorbed by the wire ropes, insensitive to displacement and replacable on the fly,
- led to new design ideas on efficient power extraction from the neutron zone by the low pressure flow of the non-activaitable (on the long run) FLiBe coolant,
- minimize the ammoung of high-Z activatable structural elements in the neutron zone.

5. LiWalls and their possible first research steps  (relaunching TEXT and PBX-M machines).

 It is crucial, on my opinion,


1. To relaunch two still available machines in the US (redesigned to accomodate the Li coated copper shell walls), i.e.,

(a) TEXT tokamak (FRC, UT in Austin, TX) in the reactor-relevant "hot electron" mode with the ECRH heating and the pellet injection, and
(b) PBX-M facility (PPPL, Princeton University, NJ) in the "hot ion" mode with the NBI heating , first, for necessary continuation of the Li pellets and DOLLOP studies, interrupted by the TFTR shutdown (and then, as possible extention of the TEXT research toward order of magnitude higher heating power).

 
Both of facilities will provide not only important new scientific data but also the proper training for future steps in controlling good alighnement of the plasma and it stability in the low recycling regime with the walls in the very proximity of the plasma edge.


2.  Start the technology and design research for a proposal to DoE on relaunching these machine and LiWall program facilities. In this regard, it is important

(a) to extend the ALSP research toward high energy of incident particles and lower particle fluxes and well as toward all chemistry and maintanence issues associated with Li/copper pair.
(b) make intensive reseach of feedback stabilization (Columbia U, GA, IFS of UT, PPPL) in presence of the copper shell and expected flattened temperature profiles inside the plasma
(c) calibrate as much as possible the theory and numerical codes of FLR effects in the high-temperature plasma against already performed (TFTR, PBX-M) or ongoing experiments on tokamaks (GA, T-10, etc).


Because of simple magnetic configuration of the propose experiments (classical tokamaks with a circular cross-sections),  the design/technology research projects can be done (presumably)  at 1 year for TEXT and 1.5-2 years for PBX-M with most of the work done by PPPL engineers.

These steps, from my point of view, seems to be consistent with the pace of the ongoing fusion science program.

6. LiWalls and ongoing programs in fusion.

It will be a difficult to repair mistake, if made, to make any not necessary or unprepared jumps into new area of research. Instead, it is necessary to see how much can be taken from the ongoing programs and how much LiWall idea can contribute to the development of the ongoing reseach.

Thus, importance of ALPS/APEX technology programs become more evident for success in plasma physics.

At the  physics side,
1. DIII-D program is simply out of my comments. It is already on a free way. Its strategic advantage is the large available power of NBI and ECRH as well as tremendous scientific and technical potential of the team.

2. Alcator-C Mod program is also out of my comments as the program which provides a fundamental background for the entire program on the plasma/PFC interaction. Similar words on NSTX concerning the high-betas.

3. What I would like to emphasize is my conviction in the necessity to update the MST (UW) Reversed Field Pinch in order to obtain the tokamak relevant collisionless-regime. This facility can be considered as an intrinsic part of the LiWall program as a provider of science/technology data and of training for dealing with the MHD activity in the wall bounded plasma in the extreme (from the tokamak point of view) plasma edge environment. Other contributing to tokamak concepts may be naturally placed in this category.

4. What is traditionally considered as "alternatives" (wrong word, which deminishes the role of the most creative part of the fusion community and put  people in a position of a sort of rivals to tokamaks) are not really sensitive to categorizing them in terms of priorities.  These "alternatives" are our indispensible assets of expertise and contributions, creativity as well as interaction with the external World outside the fusion.

5. I would put a new possible stellarator facility, which might be necessary to keep us consistent with the stellarator program in other countries (as well as to keep alive all experience accumulated in the  NCSX project) at this place of priorities (from its present exaggerated one).

7. Concluding remark.


OK, at the end, my russian English text seems to be good enough.

Concluding, I would like to emphasize again that the recent development in tokamak research in the US did change the priorities in the plasma physics and technology.  It has changed them in the positive way by openning new opportunities and new vision on our  common path. My mission was relatively easy in assembling a consistent strategic concept. The real job is to be done by all of us, all of us together, independent of what place we've been honored in the program.