Thrust 16 ChitID 16.001 David Hill 6/09/2009 16:06 Thrust 16 All research elements outlined, except #6, are those of larger aspect ratio tokamaks. Some issues (such as divertor heat flux) are not unique to the ST, but just reflect the consequences of design choice in going to a smaller radius tokamak. Proposed high-beta is only modestly higher than conventional tokamaks. Most importantly, it is limited by the same physics effects. Present ST-CTF proposals move to A~2, which is very close to "conventional" tokamak experience. So, rather than unique physics that must be investigated separately from the "conventional" tokamak, shouldn't the work here be folded into that of Thrust 5 or 8? ChitID 16.002 David Hill 6/09/2009 16:11 Thrust 16 The ST has very high divertor heat flux because of its small major radius. Are liquid metal divertors unique to the ST? That is, does the ST uniquely allow use of liquid metal divertors? Is the divertor physics of the ST unique as compared to conventional tokamaks? If so, how and at what aspect ratio do these difference appear? ChitID 16.003 Dennis Whyte 6/09/2009 16:12 Thrust 16 Why is relative increase in pulse length limited to 3 orders of magnitude as a goal? For ST-CTF as demonstrated goal this would suggest from today's ~ 1 s --> 1e5 (day) to 1e6 seconds (~weeks). This issue could have implications to heating/CD choices at low-A. ChitID 16.004 Richard Buttery 6/09/2009 16:28 Thrust 16 It undersells the CTF to pull it out into its own path and thrust - it has much wider relevance and value. So this thrust should be more explicitly about a full package to establish the physics basis for the CTF (including design studies), rather than just the ST specific research lines - though these would form a major part of this. Title should therefore change as well. ChitID 16.005 Tony Taylor 6/09/2009 16:29 Thrust 16 As presented, the physics motivation for this thrust does not seem well founded. Clarification on the title What does "reduced aspect ratio" mean in the title? On the last page, you showed a figure that goes from NSTX to NSTX-U to CTF. It is my understanding that the aspect ratio incrases with each. What I expected from the title was an effort that explored unique physics that might be obtained as the aspact ratio became smaller and smaller -- say like 1.2 - 1.3. Buttery's comment is worth considering -- physics basis for CTF. The physics is less unique as you move toward aspect ratio of 2, but the motivation and the representation of what is planned is much clearer. ChitID 16.006 david hill 6/09/2009 16:29 Thrust 16 The idea that we need special, untested high-current connections and new high current, low-voltage generators to produce the toroidal field in the ST, suggests significant technological risk. So too do the high divertor heat loads and startup and current drive requirements in overdense plasmas. Do these challenges negate the advantages of the ST for the CTF mission? Does the R&D cost to address these issues offset proposed cost savings with ST CTF devices? ChitID 16.007 Martin Greenwald 6/09/2009 16:29 Thrust 16 The research program described in this thrust is important and interesting, however I wonder why it is not integrated with other thrusts that are looking at the same issues. Maintaining the fiction that an ST is anything other than a low aspect ratio tokamak needlessly multiplies categories violating the principle of Occam's razor. Specifically it makes the overall MFE program less rather more clear by obscuring common themes. ChitID 16.008 Roger Raman 6/09/2009 17:57 Thrust 16 For the ST to realize its potential (so that it is simply not a low aspect ratio tokamak), the end result should be a simpler machine with much less auxiliary systems than a tokamak. This means: 1) complete elimination of the solenoid so that this space could be used to improve the reliability of the TF center leg, and 2) operation at very high levels of the bootstrap current fraction. At very high levels of the plasma self driven current, only a small amount of external RF power is needed to make up for the balance of current not produced by the plasma. This leads to a simpler system with the auxiliary input power reduced to low levels. This also means that the internal pressure profile still needs to be controlled to maintain the high bootstrap current fraction. A precision fueling system can do this by controlling the density profile (A system based on neutral beams are not suitable for a reactor because of neutron streaming through the large NB port openings and the higher recircul ChitID 16.009 John Sheffield 6/10/2009 08:33 Thrust 16 In the TAP committee we agreed that the goal of ST research was to prepare for the possibility to a CTF (component test facility). As a group we did not support the notion that the ST was a good DEMO candidate. The title of Thrust 16 should be changed to reflect this. ChitID 16.010 Richard Hazeltine16 6/10/2009 08:58 Thrust 16 I would find it difficult to explain to an outsider why the exploration of one parameter, among the many describing a tokamak, deserves its own thrust. Do we need another thrust about triangularity? The ST program would make much more sense if it identified a clear, special mission, as it did in the TAP report. ChitID 16.011 rajesh maingi 6/10/2009 11:50 Thrust 16 The action elements of the 1 pager have too much information for target audience. Suggest first two elements wording be streamlined: • Extend studies of STs to much lower collision frequency and smaller gyroradius in upgraded facilities • Extend ST experiments to near burning-plasma conditions, in a new long-pulse facility with higher BT (1.5 – 2.5T) and non-inductive startup capability (following chit addresses next 2 bullets) ChitID 16.012 rajesh maingi 6/10/2009 12:02 Thrust 16 Please clarify and streamline last 2 bullets of 1 pager: Does this entail a high Bt, short pulse facility with a test stand for magnet development? • A potentially advantageous option divides studies based on pulse length and BT, addressing high field magnet technology on test stands, or in a relatively short-pulse device studying current initiation and sustainment techniques only available at higher BT, disruption physics and impact, and impulsive heat loads. Long pulse studies would be conducted in a second, lower field (1.5T < BT< 2T) device generating continuous, high temperature plasmas. How would you do the bullet below: design study? Build a range of facilities? • Evaluate whether reduced aspect ratio can improve fusion characteristics of stellarators and RFPs at high pressure. Connect near-unity A research in STs with compact torus research. ChitID 16.013 Martin Peng 6/21/2009 16:19 Thrust 16 This one-page reads very well. Thanks for providing another opportunity to improve the final draft. I recommend dropping the word "aggressively". I agree with the earlier chits on the need to focus more on the TAP Tier-1 issues, and would like to suggest a further improvement to the additional goal, which is "to aggressively pursue improvements to advance the ST for energy production", by dropping this word. This is because that a "fusion nuclear science and technology component testing device" already aimes at establishing the knowledge needed to harness fusion power, ie, to produce energy. The two elements of the goal are therefore at least equally important. This can be conveyed simply by dropping "aggressively."