**Outstanding Opportunities for PPPL Leadership in Energetic Particle
Physics:**

> **EP-SFG Perspective & Recommendations**

Beginning with the seminal theoretical and experimental studies
establishing the importance of energetic-particle physics in DT plasmas
on TFTR and JET, PPPL has continued to lead the development and
experimental validation of fast ion theory for more than a decade.
Through our NSTX, DIII-D and JET research activities, a well-integrated
team of PPPL scientists continues to produce cutting edge research tools
and outstanding research results leading to new discoveries and
providing extensive validation of linear theory.

The challenge that PPPL must now address in order to maintain
international leadership, centers on the critical issues revolving
around the poorly-understood nature of fast ion transport in a system of
multiple (or even a single or few) Alfvén eigenmodes. While there will
continue to be a need for advances in the understanding of linear
physics to establish realistic onset conditions in burning plasmas, it
is now clear that the scientific frontier with the greatest potential
impact on the fusion program is in the area of non-linear phenomena
associated with fast ion transport. This document outlines the
challenges and opportunities in this emerging area of fusion science
together with priority recommendations for making major advances by
PPPL. The priorities and recommendations presented in this document have
been developed through extensive discussion with the PPPL's Energetic
Particle Science Focus Group (EP-SFG).

For more than a decade there is increasing evidence from experimental
results around the world that Advanced Tokamak (AT) regimes -- plasmas
with elevated values of the safety factor (q) and reversed central
magnetic shear -- exhibit a deficit in neutron emission relative to
classical predictions. This is very important because AT and hybrid
plasma regimes are considered candidate operating regimes for ITER and
are central to the development of Tokamak reactor scenarios. We
accordingly are calling special attention to a startling recent
development: *Experimental measurements in DIII-D suggest that Alfvénic
activity in the core of AT plasmas can lead to a complete flattening of
the radial fast ion distribution with up to a 50% reduction in the
expected neutron emission.* These results suggest that our current
*[predictive understanding of fast ion driven currents and associated
thermal plasma transport are seriously deficient in present and future
fusion devices]{.underline}*.

Observations on NSTX indicate that nonlinear interactions of multiple
Alfvén modes play a key role in the sudden redistribution and loss of
injected beam ions. Sudden drops in the neutron emission strongly
suggest that large beam ion loss/redistribution can be induced by the
appearance of several modes as seen on magnetic probes. Similarly, the
observations on DIII-D indicate that the flattening of the fast ion
distribution (according to Doppler D-alpha measurements) occurs in the
presence of only a few (3-5) Alfvén eigenmodes. Historically,
observations on TFTR in the early 90's indicated strong reductions in
the neutron emission with the presence of only a few modes (2-3) modes
as resolved on magnetic probes. Recent measurements on JET have
indicated a similar large flux of fast (ICRF heated minority) ions in
the presence of only a few modes.

The observations just described present a major challenge to
understanding from both an experimental and theoretical perspective.
Experimentally, the study of fast ion transport requires detailed
quantitative information on the internal mode amplitudes of Alfvénic
modes and on the losses and redistribution of the fast ions induced by
the modes. Qualitative analysis of the mode activity measured on
magnetic probes and the observed fast ion loss/redistribution serve as
useful input to theory. However, the quality of the data -- particularly
on the combined availability of internal mode amplitudes and bulk fast
ion distributions/loss -- is very poor from the US and international
facilities. On the theoretical side, the major deficiency is limited
capability of numerical tools that can simulate the self-consistent
interaction of several modes with the fast ion distribution. Results
from analysis using the guiding-center code (ORBIT) interfaced with
linear eigenmode structures calculated using the NOVA-K code yield
levels of transport inadequate to account for the level of
loss/redistribution inferred from experiments.

Summarily, recent results from both theoretical and experimental studies
indicate that breakthroughs in nonlinear physical models and
experimental observations are needed. Our current programs are limited
in two ways: on the one side we have inadequate experimental
observations of internal mode amplitudes and loss/redistribution of fast
ions and on the theoretical side, our numerical codes are limited in
computational speed and physics model for addressing multimode
interactions. The PPPL-developed nonlinear M3D-K code has only recently
been applied to analysis of multimode interactions. However, this code
needs significant additional development before it can be applied
routinely and more self-consistently to the interpretation of existing
experimental results and future burning plasma experiments.

**Recommendations for Improved Experimental Capabilities**

Future advances in understanding the nonlinear physics that would enable
a reliable predictive capability for the loss of fast ions will depend
critically on the simultaneous availability of reliable internal mode
and internal lost fast ion measurements. Given the degree of difficulty
in inferring properties of the fast ion distribution from a single
diagnostic, NSTX has wisely developed an array of diagnostics that each
measures some aspect of the fast ions. Taken together, the multiplicity
of measurements (NPA, scintillator, D-alpha-FIDA, etc.) collectively
produce a more reliable picture of the fast ion distribution and
transport than any single diagnostic. However, the greatest deficiency
in NSTX diagnostics is the absence of a means to carry out precision
measurements of internal mode amplitudes across the plasma radius -- an
increasingly standard diagnostic capability in present day experimental
studies. As a leader in the internal measurement of Alfvenic phenomena
since the days of TFTR, it is particularly important that PPPL maintain
leadership in this essential area of internal measurements.

The SFG strongly recommends that diagnostic upgrades/enhancements be
provided for addressing the nonlinear physics of Alfvénic modes on NSTX.
In particular, there is need for a **tangential interferometer** with
dual measurement capability for possibly achieving a 0.01% level of line
density sensitivity with the required bandwidth. Currently, the
tangential interferometer on NSTX is set up in a fringe counter mode
optimized for line density measurements that provides a roughly 1% line
integral density sensitivity. This is clearly inadequate for carrying
out the fluctuation measurements needed to support the
energetic-particle-relevant studies described earlier. We recommend that
a dual measurement system be developed where some of the signal is split
off and independently detected in order to optimize the fluctuation
measurement capability of the system. The target sensitivity should be
0.01% line density sensitivity with a 10 MHz bandwidth to encompass the
ion range of cyclotron frequencies. It should be pointed out that the
failure to have a high-sensitivity/high-bandwidth dual measurement
interferometer capability on TFTR likely led to many missed
opportunities for resolving beam ion driven modes in enhanced reverse
shear (ERS) plasmas. Note that although the reflectometer system on NSTX
measures some MHD signals, this capability is limited to only the L-mode
where the central density is peaked. However, it is the H-mode with its
higher density that is considered to be the main operating regime -- a
regime where this diagnostic cannot be reliably used for internal
measurements. In addition, a reflectometer is not as sensitive as an
interferometer for Alfvenic mode measurements when there are large
turbulent fluctuations in the background. The soft X-ray diagnostic has
greater capability in terms of measuring internal mode structure, but it
lacks the required sensitivity and frequency bandwidth. With significant
investments in a dual measurement capability for a tangential
interferometer, we believe that it is feasible to achieve 0.01% level of
line density sensitivity for the required bandwidth. While progress can
and will be made on fast ion redistribution and loss in NSTX, the
scientific value of the work will be greatly enhanced by these suggested
improved measurements.

The relative strengths and weaknesses of the current diagnostic
capabilities on **DIII-D** can be viewed as being opposite to that of
the NSTX situation. In particular, in contrast to NSTX, an excellent set
of diagnostics for internal measurement of Alfvén eigenmodes are either
already operational or are scheduled to be in place for the near future.
These diagnostics have produced for the first time detailed radial
temperature structures with the expectation that the density structures
will be obtained with equal resolution in the near future. On the other
hand, unlike NSTX, DIII-D is deficient with respect to diagnostics for
lost/confined fast ions. Doppler D-alpha measurements show a dramatic
flattening of the radial fast ion distribution in the presence of only a
few (up to 5) Alfvén eigenmodes. However, this diagnostic operates
poorly at high density with no corroboration of its observations with
other fast ion measurements. *The importance of these observations to
the development of the AT concept provides an excellent opportunity for
PPPL to lead the effort to dramatically improve the fast ion measurement
capability on DIII-D using the proven technology developed on NSTX.* The
NSTX expertise should enable the timely implementation of new fast ion
measurements on DIII-D in an economically efficient way. In order to
complement the Doppler D-alpha measurements, the SFG recommends that a
**multi-channel Neutral Particle Analyzer Camera** be installed on
DIII-D using the solid-state silicon detector technology implemented on
NSTX. In addition, it is recommended that a **scintillator detector** be
installed for the measurement of the losses of beam ions. These three
diagnostics taken together can provide the definitive data set on AT
fast ion transport that is essential for the validation of theory. In
the interim period, progress can and will be made by coupling the NOVA-K
and ORBIT code simulations to compare with Doppler D-alpha measurements
and internal mode measurements on DIII-D. However, in the absence of any
additional information that is required to properly test/validate theory
and observations, it is quite likely that a large discrepancy will
persist between the results from ORBIT simulations and D-alpha
measurements.

Finally, PPPL has a strong collaboration with JET that has led to the
installation of a high frequency reflectometer system and Faraday cup
detectors that have resolved strong losses of fast ions in the presence
of a few Alfvén eigenmodes. These systems, together with the high
sensitivity interferometer array on JET, provide valuable quantitative
information for the modeling of the fast ion loss. The impediment to
advances in understanding here is the lack of theoretical tools. This
topic will be addressed in the following section.

**Recommendations for Improved Theory/Modeling Capabilities:**

PPPL leadership in energetic particle physics obviously cannot be
established by improvements in experimental measurement capabilities
alone. Significantly improved theoretical tools are essential to help
guide experiment, interpret observations, and validate models for
extrapolation to ITER and beyond. PPPL is internationally recognized as
a leading institution for the linear analysis of fast ion physics, and
our flagship numerical tool (the NOVA-K code) has been exported
world-wide. Similarly, the ORBIT code is internationally established as
the definitive tool for addressing the transport of fast ions in the
presence of static fields. On the nonlinear front, M3D-K is one of
leading nonlinear hybrid codes for simulating energetic particle-driven
modes. Recent highlights from M3D-K simulations include n=1 mode
stability in ITER, strong fishbone frequency chirping in tokamaks and
weak TAE frequency chirping in NSTX in agreement with measurements. Work
has also started to use M3D-K to simulate multimode interaction in NSTX.
However, M3D-K code is limited both in speed and in its physics modeling
capability. New advances are now required in order to produce a code
capable of routinely computing the self-consistent evolution of multiple
modes. This is needed to bring theory and experiment together on the
NSTX, DIII-D, and JET facilities. The SFG accordingly recommends that
the highest priority be placed on model development activities for
significantly upgrading the M3D-K code (or possibly developing a new
hybrid code) including parallel computing scalability and more mode
damping physics. Results from such advanced simulation capabilities can
be expected to accelerate progress in experiment/theory comparisons. At
present, the level of effort invested in this area cannot realistically
be expected to make the rapid advances in fast ion simulation
capabilities that are required for significant impact in the area of
energetic particle studies.

The recommendations made here by the EP-SFG identify specific action
items and investment of resources, which would enable PPPL to take a
dynamic leadership role in energetic particle physics in the world
fusion program. We believe that these recommendations should be
carefully considered in the planning of the programs and research goals
at PPPL.

**Summary of Recommendations:**

[Experiment: NSTX]{.underline}

-   FIReTIP

-   improve sensitivity to 0.01%

-   increase bandwidth up to 10 Mhz

-   SXR

-   increase sensitivity to 0.01%

-   increase bandwidth up to 0.5 Mhz

[Experiment: DIII-D]{.underline}

-   NPA camera

-   beam ion loss scintilator detectors

-   sufficient time resolution to observe loss dynamics

[Theory/Modeling]{.underline}

-   enhance efforts to improve the scalability and physics model of
    M3D-K on modern supercomputing platforms; e.g., support for a
    post-doc