#  *Chapter 4*

# ![](media/image1.jpeg){width="3.029166666666667in" height="3.125in"}

# Waves and Energetic Particles

# 4.0 Overview

Spherical torus (ST) discharges, such as those in NSTX, provide a unique
opportunity for studying wave-particle interactions in plasma at high
and dielectric constant. Scenarios that use high harmonic fast wave
(HHFW) and electron Bernstein wave (EBW) heating and current drive are
being developed and explored in NSTX to assist non-inductive plasma
startup, plasma current ramp up and sustained plasma operation at high .
Fast ions produced during neutral beam injection in NSTX have velocities
that exceed the Alfvén speed and can consequently resonate strongly with
a wide variety of Alfvén waves. Controlling the distribution function of
energetic ions through phase-space engineering is an important topic for
spherical torus physics, as well as for ITER and tokamak research in
general.

The 30 MHz HHFW heating and current drive system has been operational on
NSTX since its inception. Recent HHFW experiments and modeling have
provided a deeper understanding of the HHFW coupling physics and have
led to significantly improved coupling efficiency, with and without
neutral beam heating, in L-mode discharges. These new results have led
to several proposed changes to the HHFW antenna that will support the
2009 - 2013 research program, and have important implications for future
tokamak devices, including ITER. Section 4.1 of this chapter discusses
the status of HHFW research and plans for the NSTX HHFW program.

Until recently the EBW research program on NSTX has been focused on
determining the EBW coupling efficiency for various plasma scenarios via
the measurement of thermal EBW emission, and the theoretical modeling of
EBW coupling, propagation, heating and current drive. By 2009 a 28 GHz,\
350 kW gyrotron will be installed and operational on NSTX. The
installation of additional gyrotrons is proposed for the 2009 - 2013
research program, with the potential for coupling 1 MW of EBW power by
2012. This 28 GHz heating system will allow the first high power EBW
coupling and heating tests on NSTX. Section 4.2 presents the status of
EBW research and plans for the NSTX EBW program.

Energetic particle research on NSTX has yielded important new results;
including the discovery of CAE/GAE modes, multi-mode transport and a new
understanding of chirping modes. These experimental results have helped
advance theoretical modeling and have significant implications for
future burning plasma devices. Included in the 2009-2013 research
program is a proposal to install a CAE stochastic ion heating system.
Section 4.3 presents the status of energetic particle research and plans
for the NSTX energetic particle program.

4.1 High Harmonic Fast Wave Physics

*4.1.1 HHFW Research Goals*

The ultimate goals of the HHFW program on NSTX are 1) to assist with
startup and ramp-up to improve the ST concept for very long pulse
operation, and 2) to support heating and current drive of H-mode plasmas
to optimize NSTX performance and stability behavior \[4.1.1\]. Research
to date has been directed toward understanding the HHFW coupling physics
and improving this efficiency for current drive phasing \[4.1.2, 4.1.3,
4.1.4\] and, more recently, beginning the exploration of HHFW current
drive in\
L-mode plasmas. Considerable improvement in coupling efficiency has been
obtained in keeping with fast wave modeling by setting conditions to
avoid fast wave propagation too close to the antenna/wall surfaces. It
turns out that HHFW propagation is rather strongly directed close to the
magnetic field direction, which favors propagation in the surface plasma
and thus surface damping. Also, GENRAY \[4.1.5\] and AORSA \[4.1.6\]
modeling show that the waves inside the plasma tend to be localized to
the outer half of the plasma, potentially again favoring surface
damping. This latter effect, which is particular to the HHFW regime, may
also afford a means of localizing the HHFW power deposition off the axis
of the plasma and therefore the potential to drive current off-axis if
desired.

The near term HHFW research goals for the 2008 experimental campaign and
for the period of the next NSTX five year research program are directed
toward advancing the present physics basis for L-mode plasmas to include
the H-mode regime and the startup/rampup regime. These include:

1.  Optimize heating and current drive in L-mode and H-mode regimes
    (2008-13)

-   for both startup/ramp-up and sustainment periods

-   using theory and modeling to guide experimental optimization

2.  Increase the plasma-antenna gap to provide for less interaction of
    NB ions with the antenna and to permit optimization of the
    heating/current drive of H-mode plasmas

-   reduces edge density near the antenna to reduce edge power loss

```{=html}
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-   helps to avoid arcing & edge MHD

3.  Enhance coupled RF power by modifying the antenna to have two
    feedthroughs per strap and to have lower RF-imduced electric fields
    (2008-9)

4.  Modify the matching circuit to better couple to H-modes without
    arcing and use electronic feedback system to avoid coupling power
    during ELMs (2010-11)

5.  Further optimize the antenna structure (2011-13)

Efficient HHFW heating of L-mode plasmas has been demonstrated in 2006-7
for heating and current drive antenna phasings, and demonstrated in 2007
in the presence of neutral beam heating. Also, analysis of MSE
measurements of current drive is underway to provide as quantitative a
comparison to theoretical predictions of the current drive as possible
for the power levels launched to date. Optimization of heating and
current drive at higher power (goal 1 above) should make very clear the
appropriate conditions for applying HHFW to heat and drive current on
NSTX and for applying HHFW to future ST devices.

*4.1.2 HHFW Physics*

HHFW coupling, propagation and damping physics are important areas of
research on NSTX. Each must be understood to properly predict core vs.
surface heating as well as the location/profile of the heating and
current drive. Extensive theoretical/modeling studies have been and are
being carried out to address these three critical areas for NSTX as well
as for tokamaks generally. These studies, which are supported by the
NSTX Project and by other DOE grants to the PSFC-MIT, CompX Corporation,
and the RF SciDAC project, include the following efforts:

1.) Analysis of the global 3D structure of the HHFW electric fields,
energy flow, and power deposition is under way using both ray tracing
\[GENRAY\] and full wave codes \[AORSA, METS, and TORIC\]. At the
present time, these codes include both linear collisionless and
collisional damping mechanisms. However, since experimental results
indicate that significant damping may occur in the edge regions due to
surface wave propagation, generalized boundary conditions that can treat
edge dissipation due to the formation of RF sheaths near the vacuum
vessel wall and launcher are now being developed by the RF SciDAC group
for inclusion in the full wave modeling codes.

2.) Self-consistent simulations of the HHFW interactions with energetic
ions are needed to accurately determine the power deposition and current
drive that can be achieved in discharges with co-resonant neutral beam
injection (NBI). Experimental observations indicate that the fast ions
can interact strongly with HHFW, leading to significant modifications of
the fast ion velocity distributions as well as the partitioning of the
wave power among the various plasma species. To properly include the
effects of these interactions, the wave propagation codes must be
coupled with a Fokker-Planck treatment of the ion and electron
distribution functions in ST geometry that includes effects of
quasilinear diffusion at high cyclotron harmonics, collisions, trapped
particle effects, and particle losses. Previously, the main focus of
these studies was the GENRAY / CQL3D package, which was used to model
the early combined HHFW and NBI experiments on NSTX \[4.1.7\]. More
recently, the 1D and 2D full wave codes have all been generalized to
include non-Maxwellian particle distributions in the plasma dielectric
response modules. The CQL3D code has been integrated with the AORSA
code, and work is currently underway to integrate it with the TORIC
code. Since the power deposition and driven current profiles can change
significantly as the resonant ion distributions evolve in time, the
TORIC code has been integrated into the TRANSP analysis package, and
work is currently underway to include CQL3D as well.

3.) In general, wave propagation and deposition codes have simplified
models for the wave launcher: the codes assume that the amount of power
that is coupled into the plasma is known. However, as the recent
experiments on NSTX and previous results from other devices have shown,
the core heating efficiency is strongly dependent on the launched wave
spectrum as well as the other discharge characteristics. An analysis of
the power coupled into the plasma that self-consistently includes the
effect of the core absorption and wave propagation dynamics, rather than
assuming a radiation boundary condition, is needed to understand the
experimental observations on NSTX as well as on other tokamaks,
including ITER. As a first step towards the development of this modeling
capability, collaborators from the PSFC-MIT and the RF SciDAC group have
been developing an interface between the TORIC 3D antenna module and the
TORIC full wave code. This package, when complete, will be used to
analyze the heating profiles in NSTX for various strap phasings on the
NSTX launcher. Results from these studies should indicate regimes of
operation in which the driven currents can be optimized for plasma
startup, ramp-up, or high performance.

4\) The theoretical analysis tools for quantitative, or even
semi-quantitative analysis of the PDI phenomenon are not presently
available. The AORSA1D all-orders full wave code is being modified to
include resonant three-wave interactions. Because AORSA employs a
spectral basis set, in essence a complete set of pseudo modes, this
modification to account for so-called "extended pump" interactions
should be straight forward to implement on a 1D model. This analysis
should provide an understanding of PDI interactions on NSTX for HHFW, an
ability to evaluate planned HHFW experiments for susceptibility to
parasitic PDI losses, and approaches to minimizing these losses and
optimizing the current drive efficiency.

The HHFW research to date on NSTX has been focused on how to optimize
coupling of the HHFW power to the core plasma. It became clear in 2005
that heating for current drive phasing of the antenna

(-90^o^ between adjacent antenna element currents) was about ½ as
efficient as for heating phasing
(180^o^)![](media/image2.png){width="5.56875in"
height="2.4291666666666667in"}

\[4.1.8\]. The plasma dispersion relation pointed to the onset of
perpendicular propagation of the fast waves close to the antenna/wall
surface as being a possible contributor to the enhanced surface loss at
the lower phasing values (longer wavelength) as shown in Fig 4.1.1.a.
Indeed, increasing the magnetic field and lowering the edge density in
2006 resulted in a heating efficiency at -90^o^ being equal to that at
180^o^ \[4.1.3\]. Also, above onset the direction of the ray path in the
HHFW regime is about 20^o^ to the direction of the magnetic field, as
opposed to the low harmonic case for which the angle increases with
density into the plasma as shown in Fig 4.1.1.b. This results in surface
fast waves, especially for higher m modes, and for those waves entering
the core of the plasma tends to keep the waves confined to the outer
region of the plasma in major radius due to the up-shift in k~\|\|~
along the ray path. GENRAY analysis shows this effect for a single ray
in Fig 4.1.2 \[4.1.9\] and 3D AORSA modeling demonstrates this effect in
the volume of the plasma in Fig 4.1.3 \[4.1.10\].

The focus of the HHFW theory and modeling efforts during 2008 and the
five-year plan period will expand to include other areas of importance
including:![](media/image3.png){width="4.847222222222222in"
height="2.477777777777778in"}![](media/image4.png){width="4.919444444444444in"
height="2.05625in"}

1.  Mapping out propagation characteristics for L-Mode and H-mode
    plasmas:

    -   Density versus magnetic field along the ray path can affect the
        path and hence the location of power deposition.

2.  Understanding the fast wave damping mechanisms:

    -   Core: Landau/TTMP and short wavelength mode conversion effects.
        ![](media/image5.png){width="3.379861111111111in"
        height="2.8020833333333335in"}

    -   Edge: Surface wave damping via collisions, sheaths, wall/antenna
        currents, and parametric decay instability heating.

3.  Predicting conditions required to optimize plasma heating and
    current drive (CD):

    -   Co vs counter CD

-   Can heating and CD be tailored for on and off axis applications?

-   Can HHFW CD be used to help stabilize core MHD, e.g., by increasing
    reverse shear in the core with counter CD?

*4.1.3 HHFW Heating System*

The present HHFW system on NSTX \[4.1.11\] has 12 adjacent antenna
straps which cover \~ 90^o^ of the torus toroidally as shown in Fig
4.1.4. Pairs of the straps are powered with six sources via a
transmission/impedance matching system comprised of 6 long transmission
lines, 6 phase shifters, 6 stubs, and 6 decoupling loops (Figure 4.1.5).
The system uses digital control of power and phase, and automated
matching calculations to facilitate rapid changes in matching for
desired phases. To meet the need for more HHFW power with a larger
antenna-plasma gap to help avoid bombardment of the antenna, especially
for H-mode operation at full beam power, a series of HHFW system
upgrades are planned: ![](media/image6.png){width="3.01875in"
height="1.9243055555555555in"}![](media/image7.png){width="3.1993055555555556in"
height="2.06875in"}

*[2008-09:]{.underline}*

A major upgrade of the antenna will consist of providing two
feedthroughs per antenna strap, as shown in Fig. 4.1.6. This should
permit a \~ 3 × increase in power capability for the present antenna at
the same RF voltage. It is also planned to reduce the number of antenna
straps to 8 to make space for the ECH/EBW system. To compensate for
fewer straps, the antenna upgrade design will also minimize RF electric
fields to the extent possible for the same voltage to permit the voltage
to be increased as well.
![](media/image8.png){width="2.8986111111111112in"
height="2.765972222222222in"}

*[2010-11:]{.underline}*

A HHFW system upgrade for resilience to ELMs during H-mode is planned.

-   3 db hybrids are proposed to improve operation in H-mode

-   electronics will be provided to avoid feeding ELMs

*[2011-13:]{.underline}*

Further improvements to voltage standoff (to minimize the RF-induced
electric field) will be made.

Also, the antenna strap will be changed to have two discreet poloidal
segments (ITER-like) to permit reversal of exciting RF current in the
poloidal direction made if modeling shows that it could be important for
minimizing effects of surface waves and reactive fields on damping.

-   This change provides a poloidal spectrum with no m = 0 component
    which is peaked well away from m = 0

*[Diagnostics planned for HHFW Research:]{.underline}*

In addition to the diagnostics needed to operate and protect the HHFW
antenna array (such as acoustic arc detectors in the pressurized
transmission lines), information on the rf interaction with both the
edge and the core plasma is needed to improve the HHFW performance and
reliability. To make progress in modeling development, the codes need
both accurate input parameters for their analyses and detailed
measurements for output validation.

A wide array of edge diagnostics is presently employed in the study of
power coupling/propagation and more will be deployed in the future. A
fast ion gauge located behind the array is used to monitor the local
neutral pressure. It is observed that the pressure increases with both
RF power and with decreasing wavelength; this high neutral pressure can
lead to increased collisional damping and breakdown in the antenna. The
heating of edge ions is measured with the Edge Rotation Diagnostic,
which shows increased heating of edge ions as the wavelength is
increased. Langmuir probes positioned between antenna boxes, in the
shadow of the local limiter, have seen increases in the local plasma
potential with decreasing wavelength. The characteristic spectrum of the
parametric decay instability (PDI), postulated as the mechanism for edge
ion heating, is observed with both a Langmuir probe and a microwave
reflectometer. B-dot probes that measure the rf fields near the wall
have seen increased signal strength when plasma conditions and antenna
phasing are expected to support surface wave propagation. A visible
light camera looking at the Faraday shield has been able to identify hot
spot formation as a precursor to some antenna arcs.

Light pipes should be added to the antenna enclosures during the
opportunity presented by the HHFW antenna upgrade, giving the ability to
look at impurity spectra and to identify internal arc locations.
Additional B-dot probes will be added to study and to minimize the power
loss associated with surface waves. The RF wave monitoring electronics
on the edge density reflectometer will be upgraded for improved
operation over a 2-50 MHz range, permitting observation of both the PDI
daughter wave spectra (low frequency quasimodes and higher frequency IBW
modes). A dedicated IR camera could be used to study the antenna/plasma
interaction on the FS.

HHFW power propagation and deposition in the core is inferred from
traditional diagnostics such as Thomson scattering, soft x-ray arrays,
and CHERS. Progress has been made in reducing the NBI perturbation on
the plasma during rf operation, permitting MSE measurements to be made
for HHFW current drive. More information is needed on the wave activity
in the plasma core with regard to propagation and mode conversion.
Direct observation of wave structure in the core, as could be provided
by the installation of a phase contrast imaging (PCI) array, and/or the
upgrade of the high-k scattering system for detecting at RF frequencies,
and/or possibly by extension of the FIReTIP system, is needed to make
detailed comparisons to code predictions.

*4.1.4 Status of HHFW Research*

*[HHFW Coupling Physics:]{.underline}*

It has been demonstrated that keeping the edge density near the antenna
low relative to the onset density for perpendicular propagation
(k~\|\|~^2^/) results in a substantial increase in coupling efficiency
for current drive phasing k~\|\|~ ≈ -7m^-1^ (-90^o^) \[4.1.3, 4.1.4\].
This is demonstrated in Fig. 4.1.7 where it is shown that the efficiency
at -7m^-1^ is equal to that for k~\|\|~ ≈ 14m^-1^ (180^o^) for the last
two RF power pulses, when the density is at or below the onset density 2
cm in front of the antenna. However, the efficiency for -7m^-1^ during
the first RF power pulse is half the 14m^-1^ value where the density is
\~ 3 × the onset value for\
-7m^-1^. The fact that the placement of the perpendicular propagation
onset is an important condition for determining surface wave losses is
further illustrated in Fig 4.1.8, where it is shown that at even longer
wavelengths (lower k~\|\|~ or phase) the stored energy falls off with
phase. Note that the onset density is proportional to k~\|\|~^2^ and is
pushed against the antenna/wall at the lower phases. Also, whereas the
wavelength plays a dominant role in RF surface losses, the data of Fig.
4.1.8 suggests that the cancellation of reactive currents from near
strap currents is not as
important.![](media/image9.png){width="2.826388888888889in"
height="3.5in"}![](media/image10.png){width="2.890277777777778in"
height="3.6319444444444446in"}

Finally, to further demonstrate the surface losses associated with
having the onset density above the edge density at the antenna/wall, the
core heating has been measured for a relatively long RF pulse during
which the phase was changed from -90^o^ to -30^o^. The results are
compared to cases without the phase change and to the no RF case in Fig.
4.1.9. Again, this data shows that the wavelength plays a dominant role
in RF surface losses and that these losses do not appear to be dependant
on core damping (the increase in central temperature during the -90^o^
period did not result in better heating during the -30^o^
period).![](media/image11.png){width="3.154166666666667in"
height="2.3465277777777778in"}![](media/image12.png){width="3.408333333333333in"
height="3.3722222222222222in"}

*[Initial HHFW Current Drive Studies:]{.underline}*

The improved coupling efficiency for current drive phasing (90^o^) that
has been obtained by lowering the edge density now allows current drive
measurements to be pursued. Significant HHFW electron heating has been
obtained with current drive phasing for L-mode helium discharges with
HHFW alone (Fig 4.1.10.a) and in conjunction with neutral beam heating
(Fig 4.1.10.b). A 90 keV beam pulse was applied to the case with HHFW
only to permit MSE measurements. The MSE measurements obtained for the
-90^o^, -60^o^ and no RF cases of Fig 4.1.8 are compared in Fig 4.1.11.
It is evident that current drive effects are present for the -90^o^
current drive case. However, it is not easy to quantify the HHFW current
drive effect since it is predicted to be relatively small, the electron
heating is large, the total current is held constant, and the
redistribution of current caused by the HHFW and that caused by the
heating are difficult to separate. MSE analysis is ongoing and
theoretical analysis is underway for comparison to permit an estimate of
the current drive to be made. For this effort, matched pairs of shots
taken with different phases, but having similar electron temperature
profiles, have been selected for several cases to permit the current
drive effects to be separated from the heating effects.
![](media/image13.png){width="3.6416666666666666in"
height="2.709722222222222in"}![](media/image14.png){width="5.399305555555555in"
height="2.402083333333333in"}

*4.1.5 HHFW Research Plan for 2009-2013*

The HHFW research in 2008 will be directed toward extending the coupling
and current drive studies to deuterium H-mode conditions. For the
five-year plan the HHFW research plan includes:

*[2009-10:]{.underline}*

-   Optimize heating and current drive operation with NB for L-mode and
    H-mode with the upgraded antenna and using guidance of modeling.

-   Use larger plasma-antenna gap permitting greater stability and at
    higher power (more voltage standoff and greater power for same
    voltage). The importance of using a larger gap is illustrated in Fig
    4.1.12. For a change of only 2 cm in the gap distance, the RF power
    coupled at the antenna is no longer affected by the "diagnostic"
    neutral beam at the end of the RF pulse. Also, faster feedback
    control of the plasma position would help to reduce the beam
    interaction with the antenna. An even larger gap will probably be
    required for operation with all three high-power neutral beam
    sources in the H-mode regime.

-   Begin startup/ramp-up optimization in conjunction with CHI, ohmic,
    and ECH/EBW systems

*[2011:]{.underline}*

-   Begin operations with ELM dump - further optimize H-mode heating and
    CD operation

*[2012-13:]{.underline}*

-   Improve antenna based on previous results and continue
    startup/rampup and H-mode optimization

4.2 Electron Bernstein Wave Physics

*4.2.1 EBW Research Goals*

Off-axis radiofrequency-driven current is expected to be critical for
stabilizing solenoid-free ST plasmas when \> 20% \[4.2.1\]. For example,
modeling predicts that adding 1 MA of off-axis EBW current drive (EBWCD)
to the ST-CTF \[4.2.2\] would significantly improve the performance of
the plasma (Fig. 4.2.1). The long-term research goal for the EBW
research program on NSTX is to assess the viability of EBWCD as a tool
for generating this critical stabilizing current. There are several
intermediate research goals for the 2009-2013 research program that
support solenoid-free ST research, these are:

1\. Develop ECH-assisted plasma startup in order to increase T~e~
sufficiently for effective HHFW\
coupling (2009).![](media/image15.png){width="2.557638888888889in"
height="2.682638888888889in"}

2\. Evaluate EBWH-assisted overdense plasma I~p~ ramp-up (2010-11).

3\. Study EBW coupling with 250 kW of 28 GHz power (2010-11).

4\. Assess the effect of ponderomotive force, heating & parametric decay
on EBW coupling efficiency\
(2010-13).

5\. Test EBW heating with 500 kW of EBW power and benchmark EBW code
predictions for heating\
efficiency and localization (2011-12).

6\. Test off-axis EBWH and core EBWCD with 750 kW of EBW power and
benchmark predictions from*\
*EBW codes for core current drive (2012-13).

*4.2.2 EBW Physics*

When ~pe~ \> 1.6 ~ce~ , where ~pe~ is the electron plasma frequency and
~ce~ is the electron cyclotron frequency, the electrostatic EBW
\[4.2.3\] can propagate across the magnetic field. This regime is
normally referred to as the overdense regime, and is a regime that is
not accessible to conventional ECRH or ECCD. Except during plasma
startup and the initial stages of plasma current ramp-up, NSTX
discharges operate well inside this overdense regime. EBWs are strongly
absorbed at electron cyclotron resonances and have the potential to
provide local electron heating (EBWH) and EBWCD. However, EBWs can only
propagate inside the upper hybrid resonance (UHR) that surrounds
overdense plasma, so EBWH and EBWCD systems that have microwave
launchers external to the plasma, must employ a mode conversion process
in the vicinity of the UHR to couple the microwave power to the EBWs.
Electromagnetic radiation can couple to EBWs either by tunneling from
the extraordinary mode to the EBW (X-B conversion) \[4.2.4\] or by mode
conversion from the ordinary mode, via the slow extraordinary mode, to
the EBW (O-X-B conversion) \[4.2.5\]. For X-B conversion X-mode
polarized microwave power is launched normal to the magnetic field (Fig.
4.2.2(a)). The left hand cutoff of the slow X-mode (SX), the UHR, and
the right hand cutoff of the fast X-mode (FX) form a
cutoff-resonance-cutoff triplet allowing the fast X-mode electromagnetic
wave to tunnel through the UHR to the slow X-mode \[4.2.4\]. For O-X-B
conversion (Fig. 4.2.2(b)) elliptically polarized microwave power is
launched at an oblique angle to the magnetic field that is chosen to
make the X-mode and O-mode cutoffs coincident \[4.2.6, 4.2.7\]. An O-X-B
antenna will be used for the EBW coupling and heating experiments
planned for NSTX.![](media/image16.png){width="3.897222222222222in"
height="1.5104166666666667in"}

EBWs can exhibit large changes in refractive index after being launched.
The change in the parallel refractive index, *N~//~*, is given by:

$N_{\text{//}}\text{µ-}\frac{1}{(k_{\hat{}\rho_{e})^{2}}}\left| \frac{B_{q}}{B} \right|n_{\hat{}\text{sin}q\left( \frac{Dr}{R} \right)}$
,

where $\text{ck}_{\frac{\hat{}}{w} = N_{\hat{}}}$, *c* is the speed of
light, is the wave angular frequency, *~e~* is the electron Larmor
radius, *B*is the poloidal component of the magnetic field, *\|B\|* is
the magnitude of the total magnetic field, is the poloidal angle
measured from the outboard side of the plasma midplane, *r* is the
radial distance of propagation of the ray, and *R* is the major radius.
For 0 the shift in *N~//~* is minimal. However, for EBWs launched above
or below the plasma midplane the change in *N~//~* can be large, since
in the outboard edge region of the ST plasma the poloidal and toroidal
magnetic field strengths are comparable. EBWs heat electrons and drive
current at the Døppler shifted cyclotron resonance, given by:

$\text{gw} = k_{\text{//}}\frac{p_{\text{//}}}{m_{e}} + {nw}_{\text{ce}}$
,

where is the Lorentz relativistic factor, *p~//~* is the parallel
momentum, *m~e~* is the electron rest mass, and *n* is the harmonic
number of the electron cyclotron resonance. So the location where
electron heating and current drive occurs depends sensitively on
$N_{\text{//}}( = \frac{\text{ck}_{\text{//}}}{w})$, and therefore on
where the EBW coupling occurs relative to the plasma midplane. Also, the
direction of the driven current depends on whether the EBWs are launched
above or below the midplane \[4.2.8\]. Current drive occurs via two
mechanisms; *Fisch-Boozer* current drive \[4.2.9\], where EBWs
predominantly heat electrons with one sign of v~\|\|~, creating an
asymmetric resistivity, and *Ohkawa* current drive \[4.2.10\], where
EBWs increase the v of passing electrons in one direction so that they
become trapped, resulting in a net current being driven in the opposite
direction. Normally in an ST both these current drive mechanisms are
present, driving current in opposite directions. Fisch-Boozer current
drive dominates near the magnetic axis and Ohkawa current drive
dominates off-axis on the low field side, where there is a large trapped
electron population in an ST.

*4.2.3 EBW Heating System*

A 28 GHz ECH/EBWH system will be installed on NSTX for the 2009 NSTX
experimental campaign (Fig. 4.2.3). This system will use a gyrotron with
an output power of up to 350 kW for \~ 500 ms. For the 2009 campaign,
the system will use a fixed horn antenna on the plasma midplane to
launch X-mode microwave power to support ECH-assisted plasma startup
experiments.![](media/image17.png){width="4.788194444444445in"
height="2.825in"}

The gyrotron for the ECH/EBWH system will be located in the NSTX test
cell, providing a relatively short, straight run of 6.35 cm diameter
circular waveguide to transport RF power to the antenna. Allowing for
transmission losses between the gyrotron and the antenna, the 28 GHz
system should provide about 250 kW of on-axis second harmonic ECH in
NSTX at B~t~(0) = 0.5 T. Tests are currently being conducted on a lower
power 28 GHz gyrotron, that like the 350 kW gyrotron normally operates
in TE02 mode, to assess the possibility of operating the gyrotron in
TE01 mode at 15.3 GHz. If these tests are successful, this frequency
would provide fundamental ECH/EBWH near the magnetic axis at B~t~(0) =
0.5 T. ![](media/image18.png){width="3.2930555555555556in"
height="2.125in"}

In 2009 the HHFW antenna, which currently has 12 antenna straps, with a
single antenna feed per strap, will be reconfigured to 8 antenna straps,
with two feeds per strap (see Section 4.1.3). Following reconfiguration
of the HHFW antenna, an O-X-B EBW antenna will be installed in the
location previously occupied by the four removed HHFW antenna straps
(Fig. 4.2.4). For EBW coupling and heating experiments in 2010 and 2011
this EBW antenna will use a locally steered mirror launcher. The 28 GHz
ECH/EBWH system installed in 2009 will have sufficient power supply
capability to support up to four gyrotrons. Installation of a second 350
kW, 28 GHz gyrotron is proposed for 2010-11 run campaign. With the
addition of this second gyrotron, and assuming transmission and mode
conversion losses are about 30%, the 28 GHz ECH/EBWH system will provide
up to 500 kW of EBW power for heating experiments. If the initial EBW
heating experiments are successful, it is proposed to upgrade the EBW
antenna so that the mirror launcher can be remotely steered in 2011-12,
providing greater shot-to-shot flexibility to heat a wider range of
plasma scenarios. If initial EBW heating experiments are successful, a
third 350 kW, 28 GHz gyrotron is proposed for the 2012 run campaign,
allowing the 28 GHz ECH/EBWH system to provide up to 750 kW of EBW power
for off-axis heating and on-axis EBWCD experiments.

*[Diagnostics planned for EBW Research:]{.underline}*

EBW coupling physics studies will require local measurements of the edge
density profile evolution near the EBW launcher. An edge density
reflectometer, capable of measuring density fluctuation frequencies up
to 100 kHz at densities around the EBW mode conversion density, will be
installed close to the EBW launcher port. Also, a lower hybrid probe,
capable of measuring both electrostatic and electromagnetic
fluctuations, will be installed near the EBW launcher to detect daughter
waves from EBW-driven parametric decay. EBW coupling physics studies
would also benefit from the additional multi-point Thomson scattering
system channels in the plasma scrap off and near the EBW mode conversion
layer.

*4.2.4 Status of EBW Research*

This section summarizes significant advances in experimental EBW
research on NSTX and other devices, and in theoretical EBW modeling
since the 2004-8 NSTX 5-year research plan was published in 2003.

High levels of EBW coupling efficiency have been measured via thermal
EBW emission from the magnetic axis of NSTX and Fokker-Planck modeling
of NSTX discharges has predicted that efficient off-axis Ohkawa current
drive is achievable in \>20% plasmas. There have also been recent
encouraging experimental results on MAST and TCV that show clear
indications of O-X-B EBW heating, and recently efficient EBW heating of
PF-only startup discharges in MAST has been measured at 28 GHz.

*[Status of NSTX Experimental EBW Research:]{.underline}*

Except for power dependent effects like parametric decay, the O-X-B mode
conversion process involved in coupling microwaves to EBWs for EBWH and
EBWCD is the reciprocal of the B-X-O mode conversion mechanism involved
in coupling thermal EBW emission out of the plasma. So it is possible to
study EBW coupling efficiency by measuring thermal EBW emission
\[4.2.11\]. Initial B-X-O coupling studies on NSTX were performed in
2004 with a dual-channel, 8-18 GHz radiometer connected to a fixed,
obliquely-viewing, quad-ridged antenna with its ridges aligned to
simultaneously measure the radiation temperature of EBE polarized
parallel (T~//~) and perpendicular (T) to the magnetic field at the UHR
\[4.2.12\]. An example of data from this antenna is shown in Fig. 4.2.5,
which shows the evolution of the EBW radiation temperature, T~rad~ =
(T~//~+T), for a L-mode plasma with B~T~(0) = 4 kG and a flattop
current. I~p~ = 800 kA between 0.27 and 0.46 s. The EBW coupling
efficiency, T~rad~/T~e~, where T~e~ is the electron temperature of the
EBW emitting layer, was measured to be 80 ± 20% at 16.5 GHz, in good
agreement with the \~ 65% EBW coupling efficiency predicted by a model
that includes a 1-D full wave calculation of the EBW mode conversion at
the UHR, radiometer antenna pattern modeling and 3-D EBW ray tracing and
deposition modeling \[4.2.13\]. Emission at 16.5 GHz was from the
fundamental electron cyclotron resonance at the plasma magnetic axis
during the I~p~ flattop. The measured EBE polarization ratio, T~//~/T,
was also measured to be in reasonable good agreement with theoretical
modeling.![](media/image19.wmf){width="2.7645833333333334in"
height="2.657638888888889in"}![](media/image20.wmf){width="1.3263888888888888in"
height="1.9625in"}

In 2005 a pair of remotely steered B-X-O antennas were installed on NSTX
\[4.2.14\], allowing the B-X-O emission process to be studied in more
detail. Results showed that the EBW coupling efficiency peaks at a
poloidal and toroidal angle that is close to the direction predicted by
theory \[4.2.15\]. Recent studies of B-X-O emission from H-mode plasmas
in NSTX show a wide range of behavior; sometimes the coupling efficiency
remains relatively high (\~ 50%) throughout the H-mode phase, sometimes
it gradually decays, and sometimes the coupling efficiency collapses
soon after the L to H transition. Measurements and modeling of EBW
emission on NSTX are supportive of collsional damping of the EBW, prior
to mode conversion, as being a possible major cause for the degradation
of EBW emission in H-modes. Collisional damping is expected to become
important when T~e~ near the EBW mode conversion layer is \< 15 eV. In
NSTX H-mode plasmas, measured T~e~ in the B-X-O mode conversion region,
particularly for f~ce~ emission, can be as low as 10 eV. There is also
evidence from recent experiments on NSTX that Li conditioning can
significantly enhance B-X-O emission from H-modes, either by changing
the density scale length or increasing T~e~ in the conversion region. It
is possible that EBW collisional damping may not be a problem for EBW
heating of H-modes if the incident microwave power can heat the EBW mode
conversion region sufficiently to avoid EBW collisional loss.\
*[Status of Experimental EBW Research on Other
Devices:]{.underline}*![](media/image21.png){width="2.877083333333333in"
height="3.8847222222222224in"}

Following the first demonstration of O-X-B EBWH in the W7-AS stellarator
in 1997 \[4.2.16\] there have been several O-X-B heating experiments on
other devices that show encouraging results. 60 GHz B-X-O EBW heating
experiments conducted on the MAST ST used three plasma scenarios
\[4.2.17\].

MAST was the first ST device to use O-X-B heating. The O-X-B coupling
efficiency was at least 50% in high-density ELM-free H-mode discharges.
In sawtoothing H-mode plasmas, a 10% increase of stored energy was
measured during EBWH, and a 10% increase in T~e~ was measured in small
ohmic H-mode plasmas \[Fig 4.2.6\]. Future plans on MAST call for the
installation of a 1 MW 18 GHz O-X-B EBWH system. Recently, there have
also been plasma startup experiments on MAST that employ a 28 GHz
heating system. 90 kW of RF power generated 32 kA of plasma current,
without using the central solenoid, in good agreement with modeling
\[4.2.18\]. The resulting plasma had a T~e~ \~ 500 eV and n~e~ \~ 10^18^
m^-3^. This result suggests the NSTX 350 kW, 28 GHz ECH system, that
will be operational in 2009, should be able to heat the plasma
sufficiently to allow effective HHFW
heating.![](media/image22.png){width="3.1131944444444444in"
height="2.372916666666667in"}

The first demonstration of EBWH via O-X-B mode conversion in an
overdense conventional aspect-ratio tokamak was achieved in the TCV
device \[4.2.19\]. The central electron temperature evolution during
82.7 GHz EBWH was measured to increase by 80 eV when the RF power was
increased from 1 to 2 MW \[Fig. 4.2.7\]. The optimum injection angle for
O-X-B conversion was obtained experimentally by measuring the reflected
microwave radiation during a two-dimensional angle scan. Ray tracing
simulations yielded an optimum launch angle that was in good agreement
with the measured angle. About 60% of the RF power was absorbed after
conversion to EBWs.

[*Status of EBW* *Theory and Numerical Modeling:*]{.underline}

The GENRAY numerical ray tracing code \[4.2.8, 4.2.20\] and the CQL3D
Fokker-Planck code \[4.2.21\] have been used to model EBW propagation,
heating and current drive in NSTX discharges with\
≥ 20%. Efficient off-axis Ohkawa current drive at normalized minor
radius, and on-axis Fisch-Boozer current drive is predicted \[4.2.22\]
for a range of plasma and EBW launch scenarios. An example of 28 GHz EBW
ray tracing results from the GENRAY code for a = 20% NSTX plasma with a
vacuum toroidal field, B~t~(0) = 0.55 T is shown in Fig. 4.2.8. EBW
power is near the magnetic axis. Figure 4.2.9 shows the EBW power
deposition and EBW-driven current profile calculated by the CQL3D
Fokker-Planck code. 85% of the EBW power is absorbed by at the
Doppler-shifted 2f~ce~ resonance, near ρ = 0.2, the remainder is
absorbed at the Doppler-shifted 3f~ce~ resonance, also near the axis.
The total EBW-driven current, generated in this case via the
Fisch-Boozer current drive mechanism is 29 kA, corresponding to a
current drive efficiency of 39
kA/MW.![](media/image23.png){width="4.209027777777778in"
height="2.9715277777777778in"}

Synergy between EBWCD and the current driven by the electron bootstrap
effect has been calculated to be relatively weak, typically \<10%.
However, the local bootstrap current density increases in proportion to
increased plasma pressure, and this effect can significantly affect the
radial profile of the RF-driven current density \[4.2.23\]. Benchmarking
between GENRAY/CQL3D and the European code BANDIT-3D \[4.2.24\] for a
NSTX EBWCD case shows good agreement for the magnitude, direction and
location of the EBW-driven current. A recent review of the most
important theoretical and numerical modeling results for EBWs in ST's
can be found in references 4.2.25 and
4.2.26.![](media/image24.png){width="4.163194444444445in"
height="2.529166666666667in"}

*4.2.5 EBW Research Plan for 2009-2013*

A 350 kW 28 GHz heating system will be used for ECH-assisted plasma
startup experiments in 2009. Starting in 2010 EBW research on NSTX will
be directed towards evaluating EBW heating in NSTX, initially with EBW
coupling experiments at power levels up to a few hundred killowatts, and
later with experiments to study localization and efficiency of second
harmonic EBWH and EBWCD at EBW powers up to 750 kW. If gyrotron
operation at reasonable power levels proves possible at 15.3 GHz,
fundamental heating experiments will also be conducted.

*[2009:]{.underline}*

A 350 kW, 500 ms, 28 GHz gyrotron will be installed in the NSTX test
cell. This system will initially be used for second harmonic X-mode ECH
to heat CHI and PF-only startup plasmas and increase the electron
temperature sufficiently for effective HHFW coupling. A fixed horn
antenna will be installed for the ECH-assisted plasma startup
experiments in the locations previously occupied by four of the HHFW
antenna straps. Experiments will explore the transition from
ECH-assisted startup to HHFW-assisted plasma current ramp up. Recent
modeling for a CHI target plasma, with T~e~ = 25 eV and n~e~ =
3.5x10^18^m^-3^, indicates about 25% first pass absorption near the
magnetic axis for second harmonic, X-mode ECH launched into this plasma.

*[2010-11:]{.underline}*

In 2010 a second 350 kW 28 GHz gyrotron will be installed in the NSTX
test cell, bringing the total available 28 GHz power to 700 kW. Assuming
coupling and transition losses of about 30%. This level of RF power will
allow EBW coupling and heating experiments at EBW powers up to 500 kW.
By 2010 the HHFW antenna will have been upgraded to have two antenna
feeds per strap, significantly increasing the RF power coupled per strap
and allowing more HHFW power to be coupled with eight straps with double
feeds to each strap than was previously coupled with 12 straps with
single feeds to each strap. This will also allow four HHFW antenna
straps to be removed to provide space for an O-X-B mirror launcher. In
2010 the EBW launcher will have only limited local steering, however in
2011 it is proposed upgrade the launcher so it can be steered remotely.

EBW heating experiments in 2010 will study EBW coupling and core heating
at EBW powers of about 250 kW. A microwave edge reflectometer will be
installed in the vicinity of the EBW launcher to monitor the density
profile near the EBW mode conversion layer. A reciprocating antenna
probe will be installed inside NSTX to detect parametric decay to lower
hybrid waves during the EBW coupling and heating experiments. High
resolution edge T~e~ measurements will be made in the mode conversion
region with the multi-point Thomson scattering. A fixed O-X-B horn
launcher will also be installed for EBW-assisted startup and I~p~
ramp-up experiments in 2010-11.

500 kW EBW core and off-axis heating experiments in 2011 will be
conducted with two gyrotrons. These experiments will study the
localization and steering capability of the EBW heating system.

*[2012-13:]{.underline}*

If the EBW heating experiments in 2010-11 are successful, a third 350
kW, 28 GHz gyrotron will be installed on NSTX in 2011-12. The additional
gyrotron will increase the 28 GHz source power available to 1 MW,
providing up to 750 kW of EBW power for experiments during the 2012 run
campaign. Experiments in 2012 will study both EBWH and EBWCD.

4.3 Energetic Particle Physics

*4.3.1 Energetic Particle Research Goals*

An important goal of the present proof-of-principle spherical tokamaks
(ST) is to evaluate the role of fast ion driven instabilities in fast
ion transport and confinement. The results will be used to benchmark
numerical codes such as NOVA/ORBIT and M3D which will be used to predict
the effect of fast ion driven instabilities on fast ion transport. The
codes may then be used with greater confidence to predict fast ion
confinement in either an advanced ST such as NHTX/CTF/ARIES-ST or for
tokamak reactors such as ITER and, ultimately, DEMO. Spherical tokamaks
have relatively low toroidal field which means that the fast-ion Larmor
radius is relatively large (~fi~ ≈ 0.1 -- 0.2 a~p~) and the fast ion
velocity is much greater than the Alfvén speed (V~fi~ ≥ 2 V~Alfvén~),
and overlaps the expected fast ion (fusion-alpha) parameters for either
tokamak reactors (ITER) or advanced STs. This regime of large Larmor
radius and low Alfvén speed is a regime in which fast ion driven
instabilities are potentially virulent. Considerable progress was made
during the last five year period of operation on NSTX in identifying the
broad spectrum of observed fast ion driven modes, as well as the drive
and damping mechanisms. Recently added diagnostics (MSE/fast
NPA/ssNPA/multi-channel reflectometers) as well as diagnostics planned
for the near future (FIDA, PCI, fast reflectometers), will allow for
more detailed studies of the saturation amplitude, internal structure,
and the effects on fast ion transport. The redistribution or loss of
fast ions will impact the interpretation of thermal transport data, beam
driven currents and momentum transport, as well as having a potentially
direct impact on the design of plasma-facing components in a reactor
(due to the potential for localized heating or ablation from lost fast
ions).

The presence of weakly damped plasma modes below the cyclotron
frequency, revealed through excitation by beam fast ions, has suggested
a unique heating method for spherical tokamaks. A large amplitude wave
at a fraction of the ion cyclotron frequency can stochastically heat
thermal ions. The presence of weakly damped plasma modes in this
frequency range suggests excitation of these modes, with a driven
antenna, to the threshold amplitude necessary for stochastic heating.
The fundamental principles of stochastic heating were developed in the
late 1970's and early 1980's \[4.3.1-4.3.5\], and experiments validating
the concept were done from the late 1980's to mid 1990's
\[4.3.6-4.3.10\]. In the coming five years it is proposed to explore the
possibility of using such a stochastic ion heating system on NSTX.

The key to developing the capability to predict the affect of fast-ion
driven instabilities on transport of fast ions, are good experimental
measurements to benchmark codes developed for this purpose. It is not
only necessary to have good measurements of the equilibrium parameters,
but the stability thresholds and saturation amplitudes of the fast ion
modes must be measured. The impact of these modes on the fast ion
distribution must be measured, as well. NSTX has accumulated a
comprehensive suite of diagnostics for these studies. Redistribution of
fast ions will soon be measured by the existing scanning Neutral
Particle Analyzer (NPA), solid-state Neutral Particle Analyzer (ssNPA)
and the Fast-Ion D-Alpha emission diagnostic, which is under
development. Losses of fast ions are detected with the scintillator Fast
Lost Ion Probe (sFLIP), an array of Faraday cups for detecting lost
fast-ions (iFLIP), as well as the fast neutron rate diagnostics. The
effect of loss or redistribution of fast ions on beam-driven currents,
and interpretation of momentum and thermal transport can be documented
with current profile evolution measurements based on Motional Stark
Effect (MSE) diagnostic data, plasma rotation measurements with the
toroidal and poloidal CHarge-Exchange-Recombination (CHERS) diagnostic
and complete complement of kinetic profile diagnostics.

Excitation of Alfvén eigenmodes with a coil inside the vacuum vessel and
a low power amplifier (≈ 5 kW) is a valuable tool for the study of mode
stability. The spectrum of stable modes can be detected and the linear
damping rate measured directly. This is very helpful information as
stability codes must calculate both the sum of the damping and drive
terms, the difference of which is the linear drive for the mode. Thus a
direct measurement of the linear damping term also helps to constrain
the resonant drive term.

Analysis of the data collected on fast ion instabilities will be
supported through an active theoretical program. The NOVA code
calculates the linear eigenmode structure, as well as most of the
damping and drive terms. While NOVA is a linear code, the saturated
amplitudes of the individual modes will be directly measured and can be
used to normalize the eigenfunctions calculated with NOVA. The resulting
spectrum of eigenmodes, together with the unperturbed fast ion
distribution calculated with the TRANSP code, is used by the ORBIT code.
The ORBIT code follows the trajectories of many fast ions, in the
presence of modes, and can predict the resulting loss or redistribution
of fast ions. A fully non-linear code, M3D, can be used to predict
saturation amplitudes for comparison with the measured values. M3D is a
"f" code, where only the perturbed component of the fast ion
distribution receives full kinetic treatment.

The final goal of this five year proposal will be to evaluate the
feasibility of stochastic ion heating of thermal ions with a
sub-cyclotron frequency RF system. Necessary input to evaluate the
potential of this approach would be a measurement of the spectrum of
stable and unstable eigenmodes which might potentially be excited by an
antenna. The threshold for stochastic heating will be estimated using
NOVA and ORBIT. The linear damping rate can be measured as discussed
above. Most of the linear damping can be assumed to be on the electrons,
thus with this information the heating power lost through the electron
channel at the stochastic heating threshold can be estimated. It will be
particularly valuable to measure the coupling efficiency of a prototype
antenna to help in the design of a full power system.

*4.3.2 Energetic Particle Physics*

The transport of fast ions can be greatly enhanced in the presence of
multiple modes. This can mean either multiple modes of a similar type,
for example several Toroidal Alfvén Eigenmodes (TAE), or potentially
modes as disparate from TAE as Compressional Alfvén Eigenmodes (CAE) or
fishbones . As the diffusive "step size" from interaction with fast ion
instabilities tends to be proportional to the ion gyro-radius, fast ion
losses in large, high field devices such as ITER are expected to be
through the interaction of multiple modes \[4.3.11\]. The study of such
multimode interactions will be a high priority in the next five years on
NSTX. Strongly enhanced transport has been seen when multiple TAE modes
interact, a process known as TAE avalanches \[4.3.12-4.3.16\].
Three-wave coupling of TAE, fishbones (kinks) and higher frequency
Alfvén eigenmodes has also been seen \[4.3.17\], with some data
suggesting that these modes may also interact through perturbations of
the fast ion distribution.

The impact of these studies on future devices such as NHTX or ITER will
be primarily through the numerical simulation code improvements needed
to accurately model the NSTX data. The capability to predict stability
thresholds for fast-ion driven instabilities in future fusion reactors
is an important component of the design and identification of attractive
operational regimes. The M3D-K code is a non-linear, f code which can
model the saturation amplitude of modes, frequency chirping and some
forms of multimode coupling. An important part of the five year plan
will be to utilize this capability to model well documented examples of
Alfvénic instability induced fast ion losses. The NOVA-K code calculates
the linear mode eigenfunction, as well as drive an damping terms. The
eigenfunctions calculated with NOVA can be normalized with experimental
data and used in the particle tracking code, ORBIT, to calculate the
affect on fast ion transport. These numerical simulations will then be
compared to enhancements of fast ion transport as measured with FLIP,
NPA and FIDA diagnostics.

The theory of stochastic heating of thermal ions by a large amplitude
wave was first described by Smith and Kaufman \[4.3.1\]. The threshold
amplitude for stochastic heating can be expressed in many ways, one of
which is that the motion of thermal ions in the wave field becomes
stochastic when the bounce-frequency of trapped ions is approximately
the cyclotron frequency. The analytical work has been done largely for
electrostatic modes, but numerical simulations have been used to predict
the stochastic heating threshold for Compressional Alfvén Eigenmodes in
an NSTX equilibrium \[4.3.18, 4.3.19\]. Experiments with large
amplitude, current driven Drift-Alfvén mode have found direct evidence
for stochastic thermal ion heating. A broad spectrum of weakly damped
Alfvén Eigenmodes is seen between ~ci~/4 \<  \< ~ci~ which could be
excited to the stochastic heating threshold.

*4.3.3 CAE Heating System*

Stochastic heating of magnetized ions or electrons with large amplitude
waves at frequencies just below the cyclotron frequency was proposed in
the mid '70's to the early '80's for a variety of modes \[4.3.1-4.3.5\].
This work was, perhaps, an outgrowth of earlier experiments and
theoretical work on heating of plasmas with stochastic waves
\[4.3.20-4.3.24\]. An extensive experimental study of stochastic ion
heating by a large amplitude wave was done on the ENCORE tokamak by Paul
Bellan's group between 1986 and 1998. Stochastic heating of thermal ions
was measured with a laser-induced fluorescence diagnostic. The wave was
a large amplitude drift-Alfvén wave \[4.3.25\], probably driven by the
plasma current. In a fairly extensive set of experiments, good agreement
was found with the theory of stochastic heating.

The broad spectrum of CAE and GAE modes in the proper frequency range on
NSTX offers an opportunity for stochastic heating. These natural modes
are weakly damped, as evidenced by the ease with which they are excited
by the neutral beam fast ion population. Based on measurements of the
mode damping rate and wave amplitude, and assumptions about the wave
localization, only a few 10's of kWs of power would be required to
excite waves to the amplitudes that are seen. Numerical simulations have
estimated that wave amplitudes 5 to 10 times higher than the peak so far
observed would be necessary for stochastic ion heating. This power heats
the electrons, as the damping is primarily electron Landau or continuum
damping. A low linear damping rate means that relatively little power
would be lost to the electrons when the wave amplitude is excited to the
stochastic threshold, and most of the heating power would be deposited
in the thermal ion population.

The waves excited by the beams are measured to have about one-half
wavelength over the length of an existing HHFW antenna strap, as
measured with a poloidal array of four coils. This array will be
expanded to provide a more accurate measure of the local poloidal
wavelength. Coils also exist to measure the toroidal and poloidal
components of the wave field, that is the pitch of the magnetic
fluctuations. The antenna will most likely be oriented to couple to both
shear and compressional waves.

The first goal is to measure the efficiency with which the existing HHFW
antenna couples to the waves. The antenna feed needs to be disassembled,
and a voltage divider used to match to a cable to a high-frequency
digitizer. This measurement would verify that the antenna matches well
enough to pick-up the waves. Following this, low power antenna loading
experiments would be done. This would require temporarily modifying the
HHFW antenna by taking one strap out of service during the period of the
experiment. This might be done as part of the planned upgrade to the
HHFW antenna, which includes removal of four of the existing straps.
These coupling and loading experiments will be done to aid in the design
of a dedicated Alfvén antenna. There were several experimental studies
where antennas were used to launch Alfvén waves in the early 1980s
\[4.3.26-4.3.28\].

The second phase of experiments will be low-power antenna loading tests
with a 5-10 kW, 0.5 - 1.5 MHz source. A matching network to couple the
source to the antenna strap would to be built and control and safety
hardware would be devised. Adequate diagnostics to measure antenna
loading would have to be installed. Some work might be done in trying to
measure loading into generic beam heated plasmas, but the presence of
modes excited by the beam ions might complicate this type of experiment.
It would probably be best to do the antenna loading experiments into an
ohmic or otherwise quiescent plasma. With 5 -- 10 kW, it might also be
possible to excite waves to the detection threshold of the microwave
reflectometer systems on NSTX.

The experiments would be supplemented with numerical calculations, as is
done for other forms of RF heating. An effort has begun to develop the
full-wave TORIC code for simulations of wave excitation with the HHFW
antenna at frequencies below or near the ion-cyclotron frequency. The
TORIC full wave code can complement traditional stability codes, such as
HYMN, M3D or NOVA-K. TORIC includes finite Larmor radius effects, finite
/~ci~, cyclotron resonances and a fast ion population. Further, TORIC
can attain higher spatial resolution, although in a linear model.
Initial simulations have found three waves near the cyclotron frequency,
one of which may be a core-localized GAE.

Following design and installation of a dedicated Alfvén wave antenna,
higher power experiments would begin. Several RF transmitters exist at
PPPL that might be modified to provide 100 -- 200 kW of power in the
required frequency range. This would easily be enough power to excite
waves detectable with the reflectometers, and good estimates of the
power necessary to reach the stochastic heating threshold could be made.

*4.3.4 Status of Energetic Particle Research*

*[Fishbones and EPMs:]{.underline}*

Bursting, chirping modes are seen in NSTX with frequencies starting at
over 100 kHz and which can chirp down 10's of kHz on a timescale of msec
\[4.3.29-4.3.37\]. These modes can be present with q~min~ ≥ 1 and with,
sometimes multiple, toroidal mode numbers 1 ≤ n ≤ 4. In plasmas where
q~min~ ≈ 1, the modes typically have characteristics resembling the
fishbone mode seen on conventional aspect ratio tokamaks. The modes are
core-localized with a kink-like structure (no or small islands). Under
conditions where q~min~ is believed to be well above unity, the modes
often have toroidal mode numbers greater than one. The modes still
appear to be core localized and kink-like in nature. Some of these modes
have been identified as bounce-precession resonance fishbone modes. They
are expected to be unstable in high current, low shear discharges where
the conventional precession-resonance drive is weak (due to low
precession drift frequency) \[4.3.30\]. These plasmas have a significant
population of trapped particles with a large mean bounce angle, produced
by near tangential beam injection into a small aspect ratio device.

![](media/image25.png){width="2.1729166666666666in"
height="2.408333333333333in"}

The strength of neutron drops (fast ion losses or transport) varies
widely for these bursting, chirping modes. Those resembling the
classical fishbone modes (n = 1, q~min~ ≈ 1) are most reliably
correlated with neutron rate drops. However, many of the bursts involve
multiple modes, and in some cases transient losses of fast ions
(directly measured with the scintillator Fast Lost Ion Probe, sFLIP) are
correlated with periods when multiple modes are present \[4.3.37\] (see
Fig. 4.3.1). ![](media/image26.png){width="2.5104166666666665in"
height="2.185416666666667in"}

Additional experimental studies of the mode structure, stability
boundaries and resonant drive are necessary to develop the ability to
predict their appearance and affect on fast ion confinement. At present,
even though the mode eigenfunction is relatively well known, simulation
of the fast ion losses has not been done.

*[Toroidal Alfvén Eigenmodes and Avalanches:]{.underline}*

> Multiple Toroidal Alfvén Eigenmodes (TAE) are commonly observed in
> beam heated NSTX discharges. However, evidence for significant fast
> ion transport or loss appears confined to instances of strong,
> multimode bursts (*e.g.,* Fig. 4.3.2). Fast ion orbits are modified as
> they interact with the TAE, with those fast ions resonant with the
> wave becoming trapped in the wavefield, *i.e.,* forming "phase-space
> islands". As the mode amplitude increases, more fast ions will be
> trapped and the islands will become larger. It has been proposed that
> when the resonances from multiple modes are nearby in phase space,
> sufficiently large amplitude modes will cause the phase space islands
> to overlap. The orbits of the formerly trapped ions become stochastic
> and very rapid transport of fast ions may occur. Additionally, the
> ensuing modifications to the fast ion distribution function may result
> in otherwise stable modes being excited, ultimately leading to an
> "avalanche" effect which further enhances the transport of fast ions
> \[4.3.12-4.3.16\]. This mechanism of fast ion transport through an
> interaction of many modes could be important in ITER \[4.3.11\].
>
> TAE avalanches, correlated with drops in the neutron rate suggesting
> fast ion losses, have been studied in NSTX. Recent experiments have
> documented the internal mode amplitudes, measured with the
> five-channel reflectometer system, together with MSE documentation of
> the current profile. It is hoped that these measurements will provide
> the basis for numerical modeling of the island overlap threshold and
> enhanced fast ion transport. These experiments were necessarily
> carried out in L-mode plasmas, necessary to get the reflectometer
> measurements of the internal structure. As with many of the studies of
> fast ion driven instabilities, it is important to extend these studies
> to the high performance, H-mode regimes, even at the expense of
> detailed measurements of the internal structure.

*[Beta-induced Alfvén Acoustic Eigenmodes:]{.underline}*

New global MHD eigenmode solutions have been found numerically below the
geodesic acoustic mode (GAM) frequency in gaps in the low frequency
Alfvén -acoustic continuum \[4.3.38, 4.3.39\]. These new eigenmodes can
explain observations of modes with frequencies well below the TAE
frequency (Fig. 4.3.3). These global eigenmodes, referred to here as
Beta-induced Alfvén-Acoustic Eigenmodes (BAAE), exist in the low
magnetic safety factor region near the extrema of the Alfvén-acoustic
continuum. In accordance with the linear dispersion relation, the BAAE
frequency increases as the safety factor, q, decreases. We show that
BAAEs can be responsible for the observations of low frequency modes in
relatively high beta \> 20% NSTX plasmas. In contrast to the mostly
electrostatic character of GAMs, the new global modes also contain an
electromagnetic (magnetic field line bending) component due to the
Alfvén coupling, leading to wave phase velocities along the field line
that are large compared to the sonic speed. Initial measurements with
the high-k scattering diagnostics have found evidence of mode conversion
to short wavelength kinetic Alfvén waves as the BAAE enters the
continuum. The ability to make these measurements will be valuable in
the study of other Alfvén waves as well. Qualitative agreement between
theoretical predictions and observations are found.

*[Beta Dependence of Alfvén Cascade Modes:]{.underline}*

Alfvén Cascade Modes have been found in low density, low plasmas on the
National Spherical Torus Experiment (Fig. 4.3.4). An extension of the
theory of Cascade modes which includes the coupling to Geodesic Acoustic
Modes (GAM) \[4.3.40-4.3.42\] is shown to imply their absence for
typical spherical tokamak theratio of thermal to magnetic energy). A
scan in confirmed a threshold for suppression of Cascade modes in accord
with theoretical predictions. Further, good agreement was found between
the observed onset frequency and the frequency estimated from a simple
dispersion relation for GAM.
![](media/image27.png){width="2.8833333333333333in" height="2.71875in"}

![](media/image28.png){width="2.5277777777777777in"
height="3.966666666666667in"}

The time evolution of q~min~ as calculated from the observed mode
frequency sweeping was also found to be in good agreement with that
found from equilibrium reconstructions using measurement of the
equilibrium magnetic field pitch. This measurement was made with a
Motional Stark Effect (MSE) diagnostic.

Some limited measurements have been made of the internal structure of
these modes. Future experiments will try to improve on these
measurements, with a view, in particular, of detecting changes in the
mode structure as the Alfvén Cascade modes saturate at the TAE gap.

These studies have been primarily done in L-mode plasmas. It remains to
be seen whether a sufficiently low H-mode plasma can be made such that
AC modes would be present. Finally, no strong evidence has been found to
connect these modes to enhanced fast ion transport as has been reported
elsewhere. Future experiments could take a closer look at this issue.

*[CAE (Hole-Clumps):]{.underline}*

#### Neutral Beam Driven Compressional Alfvén Eigenmodes (CAE) at frequencies in the range 0.2 ~ci~ ≤  ≤ ~ci~ are commonly seen in NSTX plasmas \[4.3.43-4.3.45\]. The modes typically have a spectrum with multiple frequency peaks, sometimes equally spaced and with sequential toroidal mode numbers. The modes are counter-propagating for moderate mode numbers, 3 ≤ n ≤ 6, and co-propagating for high mode numbers 9 ≤ n ≤ 13. The frequency has a scaling with toroidal field and plasma density consistent with Alfvén waves. The modes have been observed with high bandwidth magnetic pick-up coils and with a reflectometer. The high-k scattering system has again, in some instances, detected coherent modes with frequencies matching those seen on the Mirnov coils, suggesting continuum damping through mode-conversion to kinetic Alfvén waves. Neutral Beam Driven Compressional Alfvén Eigenmodes (CAE) at frequencies in the range 0.2 ~ci~ ≤  ≤ ~ci~ are commonly seen in NSTX plasmas. The modes typically have a spectrum with multiple frequency peaks, sometimes equally spaced and with sequential toroidal mode numbers. The modes are counter-propagating for moderate mode numbers, 3 ≤ n ≤ 6, and co-propagating for high mode numbers 9 ≤ n ≤ 13. The frequency has a scaling with toroidal field and plasma density consistent with Alfvén waves. The modes have been observed with high bandwidth magnetic pick-up coils and with a reflectometer. The high-k scattering system has again, in some instances, detected coherent modes with frequencies matching those seen on the Mirnov coils, suggesting continuum damping through mode-conversion to kinetic Alfvén waves.

Alfvénic bursts (AB) of either compressional or global Alfvén
eigenmodes, with strong frequency chirps have been observed in the early
neutral beam-heating phase of discharges (Fig. 4.3.5). The bursts last
for \< 0.3 -- 0.7 ms, and the mode frequency chirps simultaneously up
and down. The frequency evolution of the bursts is well described by the
"Hole-clump pair" theory of Berk, Breizman, and Petviashvili,
\[4.3.46\]*.* The AB arises from the generation of a hole in the
particle distribution function, paired with a clump, and the non-linear
interaction of the instability with the distribution function results in
the propagation of the holes and clumps in phase space, which together
with background damping, results in the simultaneous upward and downward
frequency chirps of the mode. The hole-clump theory relates the rate of
the frequency chirp to the linear damping rate of the mode. The damping
rate determined by the frequency chirping is found to be in good
agreement with predictions of the NOVA-k code.

There still remains ambiguity regarding the identification of these
modes as GAE or CAE. Further numerical simulations and improvements in
the measurement of the mode polarization, as well as internal structure
with the multi-channel reflectometer system might help resolve this
question. Likewise, additional work needs to be done to understand the
extension of the hole-clump theoretical model to Doppler-shifted
cyclotron resonance driven modes as these are believed to
be.![](media/image29.png){width="2.4118055555555555in"
height="3.564583333333333in"}

#### [Global Alfvén Eigenmodes]{.underline}:

Initial observations of sub-cyclotron frequency modes, 0.2
~ci~ ≤  ≤ ~ci~, were interpreted as CAE. Simulations with the HYM code,
however, found that both CAE and Global Alfvén Eigenmodes (GAE) should
be expected for the parameters of typical NSTX plasmas. Evidence for
this second type of mode was soon found. The peaks in the frequency
spectrum for these new instabilities intersect each other as plasma
parameters evolve, whereas the CAE spectral peaks, which depend only
upon density, tend to evolve parallel to each other. The shear Alfvén
dispersion,  ≈ *k~\|\|~V~A~* , *k~\|\|~* = (*m *- *nq*)/*qR* allows for
more complex behavior as the *q* profile and density are evolving
independently during the discharge. A theory of localized compressional
Alfvén eigenmodes (CAE) and global shear Alfvén eigenmodes (GAE) in low
aspect ratio plasmas explains the observed high frequency instabilities
\[4.3.47-4.3.58\]. Both classes of modes may be excited through the
velocity space gradient of energetic super-Alfvénic beam ions via
Doppler shifted cyclotron resonances (or, in the case of CAE, directly
through a parallel bump-on-tail). CAE modes were also found on DIII-D in
an aspect ratio scaling experiment motivated by the NSTX observations
\[4.3.59\].

Non-linear, three-dimensional 3D hybrid simulations using the HYM code
have been employed to study the excitation of both CAE and GAE by
energetic ions in NSTX. The HYM simulations for typical NSTX parameters
find that GAE are the most unstable modes, for low toroidal mode
numbers, 2 ≤ *n* ≤ 7 at the large injection velocities of beam ions,
V*~b~* ≈ 3V*~A~*. The GAE are localized near the magnetic axis, and have
large *k*~\|\|~, so that  ≈ *k*~\|\|~*V*~b\|\|~ ≈ ~ci~/2. The perturbed
plasma pressure in the poloidal cross-section of an NSTX plasma due to a
GAE with *n* = 4 and *m* = 2 is shown in Fig. 4.3.6. These modes also
have a significant compressional component, *B*~\|\|~ ≈ *B*/3, due to
strong coupling to the compressional Alfvén wave. In the HYM simulations
for *n~b~*/*n~p~* ≈ 3 % , the growth rates of the unstable GAEs are
found to be of the order /~ci~ ≈ 0.002 -- 0.01 with frequencies
/~ci~ ≈ 0.3 -- 0.5, where ~ci~ is evaluated at the magnetic axis.
![](media/image30.wmf){width="2.2993055555555557in"
height="3.8208333333333333in"}

*[Stochastic Ion Heating:]{.underline}*

#### With strong neutral beam heating in NSTX a broad spectrum of fluctuations is observed in the frequency range from about 0.2 to 1.2 times the ion cyclotron frequency, demonstrating that there are many weakly damped eigenmodes present for stochastic ion heating. The modes have been identified as Compressional Alfvén Eigenmodes (CAE) or Global Alfvén Eigenmodes. The linear damping rate has been determined from non-linear behavior of the mode frequency under some conditions, and calculated with NOVA-K and HYM. 

We simulated this effect in a more complex toroidal geometry (see figure
4) than was reported previously \[4.3.58,4.3.60\]. Stochastic domains at
low energy corresponding to the plasma background ions occurs at a lower
amplitude than in the slab plasma. The initial temperature profile of
thermal ions is shown as a solid parabolic curve in figure 4. We used 21
CAEs to simulate the measured CAE spectrum by edge Mirnov probes in
NSTX. Internal measurements of the rms amplitude of the perturbations is
not available and was fixed at δB / B = 0.5 × 10^−3^ in calculations
with the collisional frequency ν = 0.01ωc . Mode frequencies were in the
range 0.2 \< ω/ω~ci~ \< 0.8. Calculations indicate that the heating is
proportional to the mode numbers if the amplitude is fixed. Stochastic
multimode heating by CAEs could provide a way of direct energy
channeling from beam ions (fusion alphas) to the thermal plasma ions.

*[Non-linear Multi-mode Coupling:]{.underline}*

Simultaneous bursts of energetic particle mode (EPM) and
toroidicity-induced Alfvén eigenmode (TAE) activity that correlate with
significant fast-ion loss are observed in beam-heated plasmas.
Three-wave interactions between these modes have been conclusively
identified, indicating fixed phase relationships \[4.3.17\]. This
nonlinear coupling concentrates the energy of the TAEs into a toroidally
localized perturbation frozen in the frame of a rigid, toroidally
rotating structure formed by the EPMs. This redistribution of energy is
significant because it will modify the effect of the TAEs on fast-ion
loss. ![](media/image31.wmf){width="2.9131944444444446in"
height="4.165972222222222in"}

*[Measurements of MHD-induced Energetic Ion Transport or
Loss:]{.underline}*

In quiescent or MHD-benign NSTX discharges, NPA measurements of the
energetic beam ion distribution are consistent with classical behavior
\[4.3.61\]. The appearance of strong, bursting or continuous MHD or
Alfvénic activity can have significant effects on energetic (E \~ 10 --
100 keV) deuterium ion populations. Evidence of MHD-induced energetic
ion transport or loss in NSTX relies primarily on neutron rate and
charge exchange Neutral Particle Analyzer (NPA) \[4.3.62\] measurements.
Neither the volume integrated neutron rate nor the line-integrated NPA
effux, however, can distinguish between transport or loss so additional
diagnostics, such as sFLIP, FIReTIP and USXR are used to augment these
measurements. sFLIP is able to identify ion loss while the others serve
to spatially localize MHD-activity for the purpose of analysis using the
time, space and energy dependent Anomalous Fast Ion Diffusion (AFID)
model in TRANSP that is used to emulate MHD-driven transport or loss of
energetic ions. Comparison of MSE-reconstructed toroidal current
profiles with calculations taking into account AFID redistribution of
NBI driven current helps to identify fast ion redistribution \[4.3.63\].
The AFID model is iterated to simultaneously match measurements of the
neutron yield, NPA change exchange efflux and MSE reconstructed current
profiles with the TRANSP simulation.

Although redistribution or loss of energetic ions due to low-frequency
(f \~ 10 kHz) kink-type MHD activity has been reported previously
\[4.3.64\], the primary goal of recent work \[4.3.65\] has been to
contrast redistribution or loss due to continuous Energetic Particle
Modes (EPM) and Toroidal Alfvén Eigenmode (TAE) activity (f \~ 20 -- 150
kHz) modes with the low-f MHD modes, a topic that heretofore has not
been investigated in detail for NSTX
plasmas.![](media/image32.wmf){width="3.0236111111111112in"
height="4.271527777777778in"}

A NPA vertical scan was performed for a series of reproducible H-mode
discharges, some characteristics of which are shown in Fig. 4.3.7. In
panel (a), the Mirnov spectrogram shows robust activity in the low
frequency range (f \~ 0 -- 150 kHz) where continuous f \< 20 kHz
kink-type activity and bursting fishbone-like activity are intermingled
with a 'sea of Alfvénic modes'. A tentative classification of the mode
activity is presented in the header. Panel (b) illustrates the typical
disparity in neutron yield for H-modes having strong MHD activity
wherein the normal TRANSP-calculated rate (blue curve exceeds
measurement (black curve), often by as much as 50±20%. The
TRANSP-calculated rate using the AFID model (red curve) provides
agreement with the measurement. Panel (c) shows a contour plot of the
NPA efflux where it can be seen that energetic ion depletion above E \>
E~b~/2 starts around t \~ 150 ms (coincident with onset of strong MHD
activity) and grows throughout the discharge.

Selected NPA vertically scanning results are shown in Fig. 4.3.8 where
the vertical minor radius axis is the distance along a vertical line
centered at the intersection of the NPA sightline with the NB footprint
(where the NPA efflux is localized by charge exchange on the beam
primary and halo neutrals). The NPA flux is displayed on a linear z-axis
and the larger efflux at lower energy is the E/2 component of the
neutral beam. The spectrum in the top panel corresponding to a
MHD-quiescent period (dB \~ 10^-2^ Gauss) at t \~ 100 ms shows a
monotonically decreasing energetic ion distribution from the core
outwards. By contrast, during strong MHD activity (dB \~ 10^-1^ Gauss)
at t \~ 200 ms the energetic ion distribution in the core region is
depleted and the core ions are redistributed radially outwards to form a
'shoulder' on the measured distribution as evident in the bottom panel.
Following this initial redistribution, the NPA spectra continue to
exhibit depletion throughout the duration of the \~ 800 ms discharge.
This depletion is due in part to attenuation of the charge exchange
neutral efflux with increasing plasma density and in part to MHD-induced
energetic ion redistribution. TRANSP analysis is used to separate these
contributions.

The sFLIP and FIReTIP signal activities (not shown) correlate with the
Mirnov coil continuous low frequency (f \< 20 kHz) MHD activity (along
with sporadic bursting fishbone-like activity). sFLIP energetic ion loss
and FIReTIP n~e~/n~e~ core fluctuations are conspicuously absent during
higher-frequency continuous TAE and CAE/GAE activity in the time
interval t \~ 0.43 -- 0.9 s except possibly for MHD bursting activity at
t \~ 0.78 s and t \~ 0.83 s. The leads to a tentative conclusion that
both bursting fishbone-like and low-f kink-type MHD modes drive
energetic ion loss whereas continuous TAE-like Alfvénic modes at most
cause redistribution and the energetic ions remain confined \[4.3.66\].

So far, the role regarding energetic ion redistribution or loss of
higher-frequency (f \~ 200 -- 2000 kHz) CAE/GAE activity that is
omnipresent during NSTX H-mode discharges cannot be separated from the
lower-frequency activity (f \~ 0 -- 100 kHz). Ignoring the higher
frequency activity on the grounds that typical amplitudes of the
magnetic fluctuations are 1 -- 2 orders of magnitude below those for the
lower frequency fluctuations remains to be verified.

*4.3.5 Energetic Particle Research Plan for 2009-2013*

The research plan for energetic particle physics is largely limited by
available run time and personnel available to analyze the results of
experiments. Analysis of experimental results is relatively labor
intensive. However, there will be additional diagnostic capabilities
which will improve our understanding of the nature and impact of fast
ion driven instabilities. These include improvements to fast ion
diagnostics such as the addition of the FIDA diagnostic to measure
confined fast ion distributions and higher speed sFLIP data, either
through a fast camera, or as a finite number of fast photomultiplier
channels to improve correlation of losses with fast ion instabilities.
Improvements are planned for the reflectometer system, which is
presently the basic diagnostic for internal measurements of mode
structures and amplitudes. An upgrade to higher bandwidth is planned for
the tangential interferometers, which could provide information on mode
amplitudes in H-modes, something not possible with the reflectometers.
And there are plans for a PCI diagnostic, which could complement the
reflectometer measurements. The high-k scattering diagnostic has not
been fully exploited for studies of KAW, a result of continuum damping
of Alfvén modes. Active excitation of Alfvén waves is a powerful tool
for studying stability thresholds, linear damping rates and the first
steps in evaluating the potential for stochastic ion heating. In the
following schedule a list of some twenty experiments are apportioned to
the five campaigns in 2009 through 2013, although in many cases the
ordering is somewhat arbitrary. Undoubtedly, as we learn more,
additional ideas for experiments will present themselves.

*[2009:]{.underline}*

In the 2009 campaign we propose six experiments to continue studies of
the wide range of energetic article driven instabilities. The use of
non-resonant magnetic field braking is a capability of NSTX that hasn't
been fully exploited and could contribute to these experiments. The PCI
diagnostic may be available for some experiments. The newly identified,
beta-induced Alfvén Acoustic mode (BAAE) will have one day to complete
documentation of internal mode structure and begin scaling of threshold
parameters (high-k scattering and PCI?). There will be an experiment
dedicated to the study of three-wave coupling between fishbone-like
modes, TAE and the higher frequency GAE/CAE. Three wave coupling between
these modes could affect instability thresholds as well as particle
transport. Again, the PCI system would contribute to this experiment. An
additional day to complete documentation of the Alfvén Cascade eigenmode
evolution to TAE modes. The neutral beam voltage and source dependence
of TAE avalanches, a high priority in terms of impact on fast ion
transport, would be investigated. An experiment will be dedicated to
producing TAE modes and a search for continuum damping via coupling to
KAW will be made using the high-k scattering diagnostic. Finally, the
first experiments to measure CAE/GAE coupling to an existing HHFW
antenna strap will be done. This will provide useful information on the
design of a dedicated CAE antenna.

*[2010:]{.underline}*

The fast Mirnov arrays will be upgraded for the 2010 campaign.
Additionally, the FIDA diagnostic should be routinely available. The
FIDA diagnostic, together with sFLIP and NPA, will be used to look for
fast ion redistributions correlated with three-wave coupling. The impact
of fishbones on beam driven currents will be revisited with the improved
diagnostic capability. Experiments will start the study of Alfvén
Cascade modes and TAE avalanches in H-mode plasmas. This is an important
regime in which to understand the modes, but it is difficult to study
them due to lack of good internal mode diagnostics in H-mode.
Additionally, the sensitivity of TAE avalanches to He-D concentration
ratios will be studied. This may provide some insight into damping
mechanisms for TAE modes. The first low power coupling experiments to
Alfvén waves will be done using an existing HHFW strap and a low power
amplifier.

*[2011:]{.underline}*

The high frequency upgrade to Firetip should provide some internal mode
amplitude and localization data for CAE/GAE frequency modes in H-mode
plasmas. An experiment dedicated to the study of CAE/GAE hole-clumps,
which provides important understanding of fast ion phase space
transport, as well as tests present understanding of the non-linear
behavior of energetic particle modes, will document the internal
structure, as well as for the first time provide information on the
poloidal localization of the modes and on the outboard local wavelength
(important for CAE antenna design). Then an experiment will study the
suppression of hole-clumps during HHFW heating to understand whether
this reflects changes in the equilibrium plasma, or direct changes to
the fast ion distribution. The BAAE modes will be revisited with the
improved diagnostics to get better mode structure measurements and
measurements of fast ion transport. Also, parameter scaling of mode
stability will be done. An extensive documentation of fast ion
redistribution during TAE avalanches will be performed with the now
mature FIDA diagnostic. A five kW amplifier will be used with and
internal antenna (either left over HHFW strap or new CAE antenna) for
intial Alfvén spectroscopy studies. This would include direct
measurements of the TAE linear damping rates and intial attempts to
excite CAE/GAE to the level detectable with internal diagnostics
(reflectometer/firetip/PCI).

*[2012:]{.underline}*

The dedicated CAE antenna will be installed for this run and the high
priority experiments will address coupling, excitation and power loading
experiments, still at relatively low power ≈5 kW. Other experiments
could address bounce-resonance fishbones and TAE avalanches.

*[2013:]{.underline}*

The 100-200 kW RF source from CDX-U will have been converted to 1-2 MHz
and installed for this campaign. At these power levels it is possible
that CAE/GAE could be excited to the stochastic heating threshold.
Certainly direct measurements of the excited wave structures should be
possible with the fast internal diagnostics. A number of the previous
experiments, notably those on BAAEs and Alfvén Cascades could benefit
from the use of HHFW to control the electron temperature, thus altering
the role of the Acoustic mode coupling for these modes.

*2009 -- 2013 Timeline for Waves and Energetic Particles Research*

![](media/image33.png){width="6.495833333333334in"
height="4.229166666666667in"}

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