Columbia University in the City of New York New York, N.Y. 10027

[Department of Applied Physics]{.smallcaps} Seeley W. Mudd Building

TEL (212)854-4457, FAX (212)854-8257 500 West 120th Street

20 December 1996

Dr. Ronald R. Davidson, Director

Princeton Plasma Physics Laboratory

P. O. Box 451

Princeton, NJ 08543

Subject: Report of the NSTX Program Advisory\
Committee -- November 1996

Dear Ron:

The newly chartered NSTX Program Advisory Committee (PAC) met for the
first time at the Princeton Plasma Physics Laboratory on 21-22 November
1996. In this letter we report on our activities at this meeting and on
our initial response to your charge to the committee (copy attached)
which asked us to provide comments and advice in 3 areas: (1) Initial
NSTX research priorities; (2) Physics requirements for the NSTX design;
(3) Preparation for NSTX research. In preparing this report, we were
provided with considerable background information by the NSTX Program
and Project (as indicated in the attached agenda) and we thank the NSTX
team for their well structured and clear presentations.

[NSTX Research Program Priorities]{.underline}

Proceeding with a Spherical Torus proof-of-principle program as part of
the US fusion effort was endorsed in recommendations by the Fusion
Energy Sciences Advisory Committee at its July 1996 meeting together
with the FESAC Scientific Issues Subcommittee (SciCom) report on
Alternative Concepts. In its report, the SciCom identified the following
six issues which should be addressed in a proof-of-principle spherical
torus program:

> 1\) Extension of the data base to determine the dependence of plasma
> confinement on aspect ratio
>
> 2\) Achievement of high beta by auxiliary heating
>
> 3\) Development of techniques for clean, efficient, non-inductive
> start-up
>
> 4\) Development of efficient current drive techniques for low aspect
> ratio
>
> 5\) Achievement of high bootstrap current fraction in advanced
> operation
>
> 6\) Long pulse, fully relaxed operation

The mission of the NSTX research program has been defined to be: "prove
the physics principles of the Spherical Torus." As described to the PAC
by the NSTX Program, the physics program objectives of NSTX which
support this proof-of-principle mission have been divided into the
following 4 areas, with more detailed physics goals identified for each:

> • Confinement and Transport
>
> \- characterize global scaling and local transport properties of
> plasmas at low aspect ratio
>
> \- identify and characterize microinstabilites which play a role in
> confinement
>
> \- explore turbulence suppression and transport barrier formation
>
> • MHD Stability and Mode Control
>
> \- identify β limiting processes in low aspect ratio plasmas
>
> \- develop methods to control beta limiting MHD modes and extend the
> operating β limit
>
> • Non-inductive current drive
>
> \- develop technique(s) for non-inductive start-up of an ST plasma
>
> \- create a near-steady state plasma sustained completely by
> non-inductive means
>
> • Divertor/SOL Physics
>
> \- effect of large mirror ratio on power handling and SOL
>
> \- characterize H-mode/SOL properties of ST plasma with a natural
> divertor

The committee concurs that these mission elements are the appropriate
primary objectives for the NSTX program, which if achieved would address
the 6 issues identified as critical in the FESAC SciCom report. However,
the PAC recommends that these detailed physics goals be further refined
to more clearly illustrate the unique Spherical Torus contribution to
these physics areas. As an example, we have taken the detailed physics
goal of "power handling" listed in the NSTX Program presentations under
Divertor/SOL Physics and expanded this to highlight 2 unique physical
effects expected in an ST.

A plan for the first few years of operation of NSTX was presented to the
PAC which is based on a strategy of advancing confinement/beta studies
and high harmonic fast wave (HHFW) heating and current drive using
inductive start-up techniques in parallel with an effort to develop
non-inductive start-up techniques. Machine performance milestones
(heating power, βN, Ip, and pulse length) were established at 3 points
during the five-year plan. We endorse this strategy proposed by the NSTX
Program and recognize that this research plan acknowledges important
fusion concept development issues. We also recommend that the NSTX
Program prepare a version of this plan which highlights the specific
physics issues being studied leading up to the achievement of the
identified machine performance milestones.

We were asked specifically in our charge to comment on the relative
emphasis among the ST physics areas in the first few years of the NSTX
research program. The PAC recommends the following priorities:

> 1\) Studies of Transport, Beta, Heating and sustaining current drive,
> using inductive start-up
>
> 2\) Non-inductive start-up studies
>
> 3\) Divertor & SOL studies

While we recognize that it is critically important to the development of
the ST concept to discover means for non-inductive start-up, obtaining
early results on the transport, beta limits, heating and current drive
using the inductive start-up capability of NSTX should get somewhat
higher priority. The PAC also recommends that the physics program effort
be tightly focused on pursuing the physics areas which are unique to the
ST.

At the next meeting of the NSTX PAC, we request an update on the
progress in developing a more detailed research plan for NSTX.

[Physics Requirements of the NSTX Design]{.underline}

The second element of our charge was to provide an assessment "on a
continuing basis" of the operational objectives and the design
requirements of NSTX. At each of our next few meetings, the PAC will
select a specific aspect of the NSTX design for a detailed examination,
beginning with the NSTX diagnostic systems at our next meeting. This
process of carrying out a "rolling review" will allow the PAC take a
close look at and provide recommendations on each of the major elements
of the NSTX device prior to start of experimental operation in FY99. In
addition, as part of our initial assessment of the NSTX design
requirements, a sub-committee of the PAC will review and provide
comments on the draft NSTX General Requirements Document (GRD) and
Project Requirements Document (PRD) at our next meeting. This
sub-committee will be chaired by Ed Lazarus and include Ken Gentle,
Gerald Navratil, Steve Scott, and Yuichi Takase.

Since this was the PAC's initial exposure to the NSTX design, a number
of questions and issues were raised. In many cases the NSTX Project is
aware of and working on resolving these, but due to limited resources
this work is still in progress. Below are summarized a list of these
issues, and where appropriate, our recommendations.

*Non-inductive start-up*

As mentioned previously, the development of techniques for the
non-inductive start-up of an ST is a critical issue. While the PAC
endorses the plan to test coaxial helicity injection (CHI) as the
primary approach to non-inductive start-up of NSTX, we view CHI as a
high risk/high pay-off approach. Given the great importance of this
issue, we believe it would be prudent for the NSTX Program to identify
and plan for the development of alternative approaches for non-inductive
start-up. The use of ECH for non-inductive start-up, as demonstrated on
CDX-U, has already been

identified by the Program as one possibility. We recommend that the NSTX
program actively seek out additional ideas and evaluate their
suitability and requirements to test these on NSTX.

With respect to CHI, experimental work on DIII-D using CHI was
unsuccessful largely because the strike point control was inadequate to
maintain contact with the biased electrode. We recommend that the
capability of the NSTX control system (equilibrium coil set and magnetic
diagnostics) be reviewed to assure adequate strike point control.

*Neutral Beam Heating and Associated Diagnostics*

There is a strong consensus by the PAC that adding neutral beam heating
and the associated diagnostics (notably MSE for *q*-profile
measurements) are essential for achieving the goals of the NSTX Program.
However, the required timing of adding these systems is still somewhat
unclear. Based on the information available, we believe that a delay of
6 months or less after start of operations in installing NBI/MSE would
have little impact on the program, while a delay beyond 2 years would
have severe program impact. In order to understand the impact of a delay
in the availability of NBI and associated diagnostics on NSTX, we
recommend that the NSTX Program more clearly identify when NBI and
associated diagnostics are critically needed in the proposed physics
program and also examine the option of use of a diagnostic neutral beam
for MSE measurements as an interim solution.

The MSE diagnostic is a particularly important diagnostic for the NSTX
and we recommend that because of the critical viewing access required
for this system, that it be carefully analyzed for expected spatial
resolution, signal-to-noise, neutral beam voltage requirements,
interference with operation of other heating beams, need for beam
modulation, *etc.* Since beam emission spectroscopy (BES) is also a
likely upgrade diagnostic, the access requirements (which differ from
MSE in general) should be assessed prior to fabrication of the vacuum
chamber.

*RF Heating and Performance Modeling*

A number of questions were raised relative to the plans to implement
high harmonic fast wave (HHFW) heating and current drive on NSTX. The
strong magnetic field strength gradients in an ST present special
challenges and the modeling of NSTX operation with these systems is only
beginning. In this context we recommend the Program consider the
following questions/issues:

> • What is the back up approach in the physics program plan if HHFWCD
> is unsuccessful?
>
> • If HHFW heating doesn't work in some configurations, what Te can be
> achieved with NBI?

We agree with the NSTX Program that carrying out additional modeling
studies is important during this next year and we highlight three areas
identified during our discussions:

> • Additional modeling is needed beyond the present Solovev equilibria
> studies for cases with the level of plasma diamagnetism expected in
> well-aligned/high bootstrap fraction equilibria.
>
> • Additional modeling of start-up scenarios with NBI only HHFWCD only,
> and NBI/HHFWCD is needed.
>
> • The current profile control capability of HHFWCD should also be
> investigated. In particular, to what degree can the driven current
> profile be controlled (including the effects of the anti-current drive
> component of the spectrum)?
>
> • For well-aligned/bootstrap sustained plasmas used as a design basis
> for NSTX, what values of H and χ(ρ) are required to sustain these
> profiles?

*Inductive Start-up*

The inductive start-up scenarios presented to us assume a dIp/dt rate of
5 MA/sec. While 2 to 4 MA/sec has been achieved in smaller tokamaks
(*e.g.* C-Mod), conventional large tokamak experience (TFTR, JET,
DIII-D) is more typically about 1 to 2 MA/sec. Frequently, higher rates
of current rise lead to MHD activity and poor confinement. What is the
consequence to the physics program if dIp/dt in NSTX is limited to a
lower value?

*Equilibrium Control*

We note that the expected values of the plasma internal inductance, *li*
, in NSTX are much lower than for conventional tokamaks. The design base
cases at high beta with 90% well-aligned bootstrap current have *li* \~
0.2 and CHI start-up cases at low-beta have *li* \~ 0.1. In the
equilibrium control space of κ and δ versus *li* which are possible in
the NSTX design, these should be plotted and the 'headroom' and control
flexibility quantitatively assessed.

Some reactor studies for low aspect ratio tokamaks suggest that higher
elongation, κ, may be needed to achieve the necessary plasma current
with high bootstrap fraction. What maximum value of κ can the baseline
NSTX equilibrium coil set support?

There has been considerable experience in the design of plasma control
systems in the DIII-D and C-Mod programs and we support the NSTX
Project's plan to use input from these groups in the design of the NSTX
control systems.

[Preparation for NSTX Research]{.underline}

A plan to form a set of ST Working Groups (STWGs) was described to us
for comment. Based on these presentations and the discussion which
followed we identified 4 objectives that the NSTX Program indicated it
wished to accomplish through the activities of these STWGs:

> 1\) NSTX national team selection and management
>
> 2\) development of a detailed physics program for NSTX
>
> 3\) outreach to the fusion community for input into the NSTX program
>
> 4\) coordination of proposals to DOE for involvement in NSTX

Because of the very project specific nature of the input desired from
these STWGs in support of the NSTX Program, we recommend that these be
renamed NSTX Working Groups (NSTXWGs) since they will not be acting to
coordinate the broader national effort in ST physics which involves
Pegasus, CDX-U, HIT, and related theory and modeling activity.

The use of a set of working groups tasked to accomplish objectives 2 and
3 listed above (which involve community outreach and development of a
detailed physics program for NSTX), we believe will be very helpful for
the NSTX Program and we encourage the Program to proceed with the
formation of these NSTXWGs. However, the PAC believes that combining
these activities with objectives 1 and 4 would be very inappropriate.
Attempts to use this mechanism for team selection and proposal
coordination would generate conflicts of interest. The appropriate
method for dealing with these issues is either through the NSTX PAC as
described in its charter or through a Research Planning Council
established within the NSTX Program. We recommend that the NSTX Program
obtain information from the TFTR, DIII-D, and C-Mod programs on how they
select and manage their multi-institutional teams, as well as review the
plans developed for the formation and management of the TPX National
Research Team, as input into the definition of a process for NSTX.

[Initial Terms of Membership]{.underline}

As described in the NSTX PAC Charter, the 9 regular members of the
committee were assigned initial terms of office with 4 members assigned
18 month terms and 5 members assigned 3 year terms at random, so as to
provide for an orderly rotation in PAC membership. The term for each
member of the PAC is listed below with terms ending on 31 December 1997
for 18 month terms and 30 June 1999 for 3 year terms:

Ken Gentle Univ. of Texas (3 years)

Ed Lazarus ORNL (3 years)

Farrokh Najmabadi UCSD (3 years)

Gerald Navratil, Chairman Columbia Univ. (3 years)

William Nevins LLNL (18 months)

Stewart Prager Univ. of Wisconsin (18 months)

Steve Scott PPPL (18 months)

Ron Stambaugh General Atomics (18 months)

Alan Sykes UKAEA/Culham (3 years)

Yuichi Takase MIT (3 years)

[Next Meeting of the TPX Program Advisory Committee]{.underline}

The next regular meeting of the NSTX PAC will be in about 6 months in
May 1997, as specified in our charter. At this meeting, we expect to
include the following activities as part of our agenda:

• NSTX PAC Comments on GRD and PRD documents

• Report on the initial activities of the NSTXWGs

• Update on the NSTX Physics Research Plan

• Review of Diagnostics Plans for NSTX

In closing, the PAC would like to compliment the NSTX Program again for
the excellent overview of the NSTX design and physics program they
provided to us at our first meeting, and we recognize they are working
hard and are committed to making this project a success. We all look
forward to continuing to work together with the leadership of NSTX to
develop and carry out a scientifically exciting and successful program
of research.

Sincerely yours,

Gerald A. Navratil, Chairman

for the NSTX Program\
Advisory Committee

cc: M. Ono

M. Peng

S. Kaye

R. Goldston

W. Dove

J. Willis

J. Hoy