Diagnostic Development on NSTX

D. Johnson, R. Kaita, A. L. Roquemore, and the NSTX Team

Princeton Plasma Physics Laboratory

P,O, Box 451, Princeton, NJ 08543

*Abstract*- Diagnostics are described which are currently installed or
under active development for the newly commissioned NSTX device. The low
aspect ratio (R/a≤1.3) and low toroidal field (0.1-0.3T) used in this
device dictate adaptations in many standard diagnostic techniques.
Technical summaries of each diagnostic are given, and adaptations, where
significant, are highlighted.

INTRODUCTION

The purpose of this paper is to describe the near term plans for
diagnosing the National Spherical Torus Experiment (NSTX) plasma. It
will briefly describe diagnostic systems currently installed or being
actively developed. It will give particular emphasis to new techniques,
or new twists on standard techniques. The spherical torus (ST) is a
close cousin to the tokamak, and benefits from a long history of tokamak
diagnostic development. ST specific diagnostic challenges will be
highlighted, as will other cases, where measurement difficulties are
eased for the ST.

As it's name implies, NSTX is a national team effort, with PPPL as the
host institution. Many of the diagnostics are being developed by
collaborators. A complete listing of the diagnostics being discussed is
given in Table 1, with team institutions identified.

NSTX DEVICE

The NSTX is an ultralow-aspect-ratio torus (R/a≤1.3) designed to produce
and study high β~Τ~  plasmas that are noninductively
sustained.[^1]^,^[^2] Heating and current drive will be done with a 3
source neutral beam (5 MW) and a 12 strap, high-harmonic-fast-wave
(HHFW) antenna (6 MW). The goal is to produce high-β~T~ (25-40%),
low-collisionality, high-bootstrap-fraction (≤70%) plasmas. In addition,
coaxial-helicity-injection (CHI) will be used for noninductive plasma
current initiation. Basic device capabilities are R = 0.85 m, a = 0.68
m, I~P~ = 1.0 MA, B~T~ = 0.3 T and κ = 2.0, with a maximum pulse length
of 5 sec. Central plasma parameters in the range of n~e~ = 4-5x10^19^
m^-3^ and T~e~,T~I~ = 3-4 keV are expected for high power heating
scenarios. Wall stabilization is achieved with a close fitting set of
conducting passive stabilizer plates. The device will operate in either
single or double null diverted configurations. The passive plates,
divertor plates and center stack (CS) are covered with carbon tiles.
These structures are designed to be baked to 350° C, and the vacuum
vessel itself to 150°C.

NSTX began as a project in late 1996, and achieved first plasma in
February, 1998. Following an outage to install the passive plates,
divertor plates, and protective tiles, initial research operations began
in September, 1999. The Phase 1 research goals are to characterize ohmic
plasmas, achieve plasma initiation with CHI, and obtain significant
plasma heating with HHFW. Installation of the neutral heating beam will
occur in late 2000.

DIAGNOSTIC ACCESS

As shown in Fig. 1, there is good diagnostic access in the midplane
region, with ports very close to the plasma edge, These ports exist in 5
of 12 toroidal bays between rather narrow TF coils. Smaller ports exist
in the upper and lower dome regions, although sightlines from these
areas are limited, due to the \~ 5 cm wide vertical cuts between passive
plate sections. Fig. 2 is a plan view of the machine layout showing many
diagnostic positions as well as that of the heating beam and RF
antennas. Since beam spectroscopy has become one of the most powerful
diagnostic tools, a midplane port was dedicated to viewing the beam with
optimum geometry. In addition, special ports were designed into the
domes above and below the beam trajectory, to provide vertical views of
the beam.

MAGNETICS

![](media/image1.pct){width="2.2243055555555555in" height="5.2in"} The
plasma current, shape and position is determined with inputs from a host
of magnetic sensors to an equilibrium code EFIT, which also contains the
geometry and currents of the NSTX coil set. Presently, there are 64 flux
loops, 27 B~Z~ coils, and 11 Rogowski coils installed.

> Fig. 1. Poloidal cross section of NSTX, showing internal components
> and diagnostic access port sizes.

Many of these sensors are mounted on the center stack. On the air side
of the CS, there are 2 plasma current Rogowskis[^3], 4 halo current
Rogowskis, and 17 flux loops. These sensors had to fit in a radial space
of 3.4 mm between the OH solenoid and the insulation between the
solenoid and the inconel CS vessel wall. On the vacuum side of the CS,
there are 4 halo current Rowgowskis, and 15 B~Z~ coils arranged in one
toroidal and one poloidal array. These coils are mounted in small
cutouts on the backs of the 1.3 cm thick CFC tiles. To protect the OH
solenoid, these tiles are thermally isolated, and during high power
operation, they are radiatively cooled. It is predicted that through the
course of a day of high power operation, the tile temperature will climb
to \>500°C at some locations. The B~Z~ coils are made with ceramic
insulated wire, wound on macor mandrels, and high temperature lead wire
runs through channels in the backs of the tiles[^4]. There are also
presently 32 thermocouples mounted in the CS tiles. Coaxial helicity
injection requires the centerstack to be electrically isolated (to 5 kV)
with large ceramic breaks between the CS and the outer vessel. Therfore,
CS sensor leads must be carefully segregated, and electronics racks must
be specially isolated.

The poloidal B~Z~ coil array described above is continued outward on the
passive plates, with 12 additional sensors. Space constraints are not as
severe on the outer plasma facing components, so the outer B~Z~ coils
are larger versions of the inner coils. The same materials were used,
since the divertor plates and passive plates are designed to be baked to
350°C. In conjunction with fast digitizers, the B~Z~ coil arrays are
also useful indicators of high frequency Mirnov activity.

To provide poloidal magnetic field measurements for equilibrium
reconstruction, and to derive the toroidal currents flowing in the
passive plates and the outer vacuum vessel, a large array of flux loops
has been installed. There are 16 loops mounted in tubes pressed into
grooves on the back of the copper passive plates. Ten similar tubes are
arrayed on the inside surface of the vacuum vessel and 14 loops are
mounted on the outside surface. Nine additional loops are mounted on the
poloidal field coils.

Many of these sensors also provide parallel input to a digital plasma
control system based on a Skybolt computer with fast array input
capability.[^5] Recently, this system has achieved closed loop control
of the plasma current, radial, and vertical position.

EDGE PROBES

Fixed Langmuir probes will be used to determine divertor inner and outer
strike point locations and plasma temperatures and densities in the
divertor region. Presently there are 14 flush-mount Langmuir probes
mounted on the center stack and inner divertor tiles. 10 additional
flush mount probes are mounted in tiles in the outer divertor region.

A pneumatically driven, fast scanning Langmuir probe array will be
installed to study the plasma edge, in a region extending a few
centimeters inboard and outboard of the outer separatrix. This probe is
based on designs used previously on TEXTOR[^6], PBX-M, and DIII-D to
study edge electron temperature, plasma density, and floating potential
profiles, as well as fluctuations in these quantities. The drive has a
maximum insertion speed of 3 m/s. Measurements with \~2 mm spatial
resolution and 200 kHz bandwidth are possible. This diagnostic will be
very useful in characterizing edge transport and turbulence in all
regimes, and for achieving conditions for optimal RF coupling in HHFW
heating and current drive experiments.

Two CS tiles have been modified and coupons have been placed at various
positions. These will undergo surface analysis after exposure to study
erosion and deposition. The CS tiles were implanted with silicon depth
markers. Half of the area of 2.5 cm square, SS coupons were coated with
\~300 nm of carbon. 16 such coupons along with 2 silicon coupons are
arrayed on the outer vessel and in gaps between the passive plates.

VISIBLE AND IR IMAGING CAMERAS

Because of the small size of the center column, and the proximity of the
midplane ports to the plasma, with a sufficiently wide angle view from a
midplane window, a large fraction (\>90%) of the plasma can be seen, in
contrast to similar views on standard tokamaks. An intensified, fast
framing CCD camera equipped with such a view has been extremely useful
during the commissioning of the plasma control system. This system
consists of a fisheye 2.8 mm FL lens imaging onto a 10' coherent fiber
bundle needed to bring the image out of the high magnetic field area.
The bundle output is reimaged through an interference filter onto a
Kodak EKTAPRO EM1012 camera, capable of 1000 fps and storage for \~1600
frames, and exposure times as short as 10μs. The fast camera is
complemented by a high resolution standard frame rate CCD camera with a
similar fisheye view mounted directly behind a low profile midplane
window. These cameras provide valuable operational feedback on plasma
shape and position, and the character of the plasma edge and wall
interaction.

The fast camera described above will also be used to study edge
turbulence, with a narrower field of view.[^7] An orifice has been
located at a position displaced appropriately from the camera to provide
a gas puff which can be viewed along an edge field line during nominal
plasma conditions. A puff of D or He gas will be used to illuminate 2-D
edge density turbulence, for example, during L to H confinement
transitions.

During high power operation, IR cameras will be used to view power
deposition on the CS and in the divertor regions. This is particularly
important for other components near the midplane, such as the RF
antennas and the neutral beam armor, since calculations indicate that
significant heating will occur in these areas due to prompt loss of fast
ions (see 'fast ion diagnostics' below). Current IR camera capability
includes two Inframetrics 525 cameras with HgCdTe detectors, operating
from 8-12 μ. These cameras have a temperature range from --20°C to
1500°C with a 30 Hz frame rate.

PASSIVE SPECTROSCOPY

Passive spectroscopy provides the tools, for example, to monitor wall
conditioning trends, to understand the causes of sudden impurity
influxes, to perform quantitative particle balances, and to assess
Z~eff~ and P~rad~ profiles.

Two survey spectrometers previously used on TFTR have been relocated to
NSTX. VIPS is a 0.64 m Czerny-Turner visible survey instrument with
spectral coverage from 185-750 nm, configured either with a midplane
view across the machine, or with a view of one of several optical fibers
with relocatable viewing telescopes. SPRED[^8] is a VUV survey
spectrometer tunable over a range 10-110 nm with midplane view. Both of
these instruments use 1024 element reticon array detectors with full
frame time of 17 ms.

A 16 channel bolometer array is installed with a midplane tangential fan
ranging from the edge of the CS to the outer plasma edge. The instrument
is similar to several at the C-Mod tokamak,[^9] consisting of pinhole
(\~0.2 mm) optics illuminating an International Radiation Detectors
(IRD) 16EL0 AXUV silicon photodiode array, with data acquired at 5 kHz.

Several fiberscopes are mounted on the machine for use with interference
filters to monitor D~α~, CII, CIII, and HeII. One of these is used as a
visible bremsstrahlung monitor at 523 nm, and views with a narrow beam
across the midplane into a long pumping duct, which serves as an
effective viewing dump.

ACTIVE SPECTROSCOPY

# Thomson Scattering

Detailed T~e~ and n~e~ profile measurements will be essential for many
NSTX experiments. It was recognized that conventional ECE techniques
would not be applicable at the low toroidal fields of NSTX (see further
discussion below in microwave diagnostics section), and so Thomson
scattering was chosen to provide time resolved T~e~(R) measurements. The
need for multipulse capability led to the choice of a Nd:YAG based
system, similar to many in use on tokamaks.

A system is currently being installed which uses a backscattering
geometry, which gives full radial coverage on the horizontal midplane,
with high sensitivity and high spatial resolution at the outer
edge.[^10] A feature of this geometry is that the viewing system does
not see the center stack tiles, or the top or bottom divertor tiles.
During high power, long pulse operation, these surfaces will get hot
enough to blind systems sensitive at 1 μm. The 30 Hz, 1.6 joule/pulse

  ------------------ ---------------------------- --------- --------------
  Type               Diagnostic                   Phase     Lead

  Equilibrium        Flux loops                   1         PPPL

                     Rogowski coils               1         PPPL

                     Bz coils                     1         PPPL

                     Slow plasma TV               1         PPPL

                     Fast plasma TV               1         LANL

                     Locked mode coils            1         PPPL

  Basic              2 mm interferom.             1         PPPL

                     X-ray PHA                    1         PPPL

                     X-ray crystal spect.         1         PPPL

  Impurity           VIPS visible spect.          1         PPPL

                     SPRED VUV spect.             1         PPPL

                     Single channel VB            1         PPPL

                     Filtered fiberscopes         1         ORNL

                     VB array                     2         PPPL

  Profile            EBW radiometer               1         PPPL

                     Thomson scattering           2         PPPL

                     Reflectometer                2         UCLA

                     FIR tang interf/polar        2         UCD

                     Toroidal CHERS               2         PPPL

                     MSE (CIF)                    2         FP&T

                     Poloidal CHERS               3         PPPL

                     MSE (LIF)                    3         FP&T

  MHD                USXR arrays                  1         JHU

                     SXR arrays                   2         PPPL

                     Fast tang. XR camera         2         PSI

  Edge/Divertor      IR Cameras                   1         PPPL

                     Fast scanning probe          1         UCSD

                     SOL reflectometer            1         ORNL

                     Edge reflectometer           1         UCLA

                     Langmuir probes              1         PPPL

                     Edge coupons                 1         SNL

                     Fast Edge Imaging            1         LANL

  Fast Ions          Fast ion loss probe          2         PPPL

                     Scanning NPA                 2         PPPL
  ------------------ ---------------------------- --------- --------------

> Table 1. NSTX Diagnostics currently installed or under development,
> the phasing of this development, and the lead institutions.

Spectra Physics laser is directed through the machine on the horizontal
midplane at R~TAN~ = 28 cm. It exits the machine along a long pumping
duct and out a window to an absorbing dump. The viewing system consists
of a 16 cm diameter window and circular aperture with a 68 cm radius
spherical mirror, which images the laser beam onto a curved array of 36
fiber optic bundles. The bundles transport the imaged light to filter
polychromators housed in an accessible, shielded room near the laser.
Spatial resolution will be \~ 1.0 cm at the plasma outer edge, 3-4 cm at
the magnetic axis, and 8-10 cm at the inner edge.

Initially, there will be a single 30 Hz laser and 10 polychromators
(spatial positions). As resources permit, the system will be upgraded to
utilize three lasers and the full 36 spatial channels.

Part of the motivation to install the heating neutral beam on NSTX was
to allow beam spectroscopy, initially for charge exchange recombination
spectroscopy for measuring impurity ion temperature and flow velocity,
and for motional stark effect polarimetry to measure B~θ~ and E~r~.
Another powerful fluctuation measurement technique, beam emission
spectroscopy, (BES) was also considered. However, the BES technique
relies on being able to view the beam nearly along field lines. Due to
the very large magnetic field pitch on the ST (\~50° at outer midplane
separatrix for 1.0 MA), and its large variation in space and time, this
technique is problematic with a single view on NSTX.

*Charge exchange recombination spectroscopy (CHERS)*

A tangentially viewing CHERS system is currently being designed for
installation at the time of initial NBI operation, to provide profiles
of T~i~ and v~φ~. This system will share the same viewing optics with
the motional-stark-effect system, described below. The geometry is
chosen to optimize the spatial resolution at the plasma edge, where it
will be \~0.5 cm, increasing to \~ 3.0 cm at the plasma core. The system
will have \~ 75 channels with a concentration of channels in the outer
10 cm, and will reuse the high throughput Kaiser spectrometers and
Princeton Instruments Pentamax CCD cameras used formerly on the TFTR
poloidal rotation diagnostic.[^11]

Design work is beginning on a vertically viewing CHERS system with
matched top and bottom views to provide v~θ~ measurements. This system
will be very similar to that described in ref. 9. Using the data from
both of the CHERS systems along with the radial force balance equation,
one can calculate the radial electric field, E~r~(R), an important
parameter affecting plasma confinement in tokamaks through ExB shear
flow suppression of turbulence. This phenomenon is expected to be even
stronger in lower aspect ratio ST devices. (see ref. 1)

*Motional Stark Effect Polarimetry*

Since noninductive current initiation and sustainment are top level NSTX
research goals, measurements of the current profile J(R) are essential
to many planned experiments. On tokamaks with neutral beams, the
standard technique for this task is motional-stark-effect polarimetry
(MSE),[^12]^,^[^13] which recently has also been critical in
measurements of E~r~(R).

The MSE technique relies upon the spectral splitting of the Balmer-α
line into othogonally polarized components. This Stark splitting is a
result of the strong electric field in the rest frame of the injected
deuterium atoms, induced by their motion through the confining magnetic
field (**E**=**v**~B~x**B**). Detecting the polarization angle of one of
these components provides a measure of the local magnetic field
direction. To date, MSE measurements using conventional techniques are
limited to B ≥ 0.8 T, due to the difficulty of spectrally isolating the
polarization components.

On NSTX several modifications are planned to adapt the technique to
lower field.[^14] First, the Doppler broadening of the individual
components will be minimized, by reducing the spread in angles between
the beam and the viewing aperture. This will be done by narrowing the
viewing aperture in the horizontal dimension, and to compensate,
vertically elongating the aperture. Second, a narrower bandwidth
birefringent filter is being developed, based on a Lyot design currently
utilized in solar spectroscopy and dye laser tuning. The goal of this
development is an effective passband width of 0.075 nm, compared to 0.7
nm width of standard interference filters. Modeling the performance of a
system, with the new aperture shape and a 0.075 nm filter passband, one
obtains polarization fractions of Pf=0.2-0.4 for the central σ~0~
component. Typical MSE systems operate with a Pf\~0.8, and, to
compensate, the optical throughput on the NSTX system will be much
larger than on previous systems. The optics is designed to image the
beam onto 20 fiber optic bundles, providing spatial resolution of 2.5 cm
at the edge and 3.0 cm in the core.

The emission used in MSE systems results from
collisionally-induced-fluorescence (CIF) arising from collisions between
injected D atoms and the background plasma. Using a tunable laser to
induce this emission can have significant advantages. (see ref. 12) On a
parallel but somewhat delayed development path to the MSE work described
above, development is underway on laser-induced-fluorescence (LIF) MSE.
The plan is to inject a hydrogen diagnostic neutral beam with a coaxial
tunable laser resonant with the Doppler shifted H~α~ transition. With a
narrow spectral width, polarization fractions near unity are predicted
even at the low fields of NSTX. Lab tests are underway with a beam
designed to operate at 40 keV and 30 mA, with a 1.2 cm diameter.

The intrisic electric field, E~r~, can also affect the direction of the
polarization, and this fact has been recently exploited (see refs. 9,
10) to measure both the magnetic field angle and the radial electric
field. This requires using multiple MSE views or viewing different beam
components. If the CIF and LIF MSE systems described above are
successful on NSTX, determination of both parameters will be possible.

It is interesting to note that the design of the collection optics for
all of the active spectroscopy systems described above were severely
constrained by the presence of the TF coils. In these cases and several
others, a more optimum port configuration would have placed the ports
directly inboard of the TF coils, more easily permitting tangential
views.

X-RAY DIAGNOSTICS

X-ray survey spectra in the energy range 1-50 keV are measured using a
x-ray pulse height analyzer (PHA) previously used on TFTR.[^15] Six
different Si(Li) detectors measure different ranges of photon energy
with a resolution of 250 ev. The horizontal sightline is at the midplane
along the axis of the pumping duct. The electron temperature is derived
from the slope of the x-ray continuum radiation, and Z~eff~ due to
metals can be estimated from x-ray line emission.

At each of two toroidal locations, 90° apart, a set of three arrays of
discrete, large area IRD AXUV silicon photodiodes will be installed, to
study MHD activity and plasma turbulence. In each set, two of the arrays
are mounted on the large midplane ports viewing the upper and lower
halves of the plasma, and a third is mounted on a top flange, viewing
the plasma from above. The array enclosures are mounted behind gate
valves, readily permitting modification of the detector configurations.
Each enclosure will house 16 large area AXUV diodes, arrayed on an arc
around a viewing slot. A moveable slide behind the slot houses several
thin foil filters.

The first set of arrays, just installed, will be configurable with
multilayer mirrors, for high spectral resolution (≤2Å) imaging.[^16]
These will be particularly useful for rejection of the bright core
emission, when trying to study MHD and turbulence in the periphery. One
could, for example, image the C IV Lyα emission at 33.7Å with a Ti foil
filter-Ti/Cr mirror-diode arrangement, with an estimated background
rejection factor of \~100.

A second set of arrays, without multilayer mirrors, but with an
identical viewing geometry, will be used to study core MHD, and will be
installed before the NBI run. When configured with the same detector
arrangement, the two array sets would be useful in mode identification
of core MHD. Equipped with different filters, they could also provide a
fast estimate of T~e~(R) from the two color technique.

An x-ray crystal spectrometer is being installed to measure core ion
temperature, using non-perturbing puffs of Ar gas.[^17] The design uses
a Johann geometry and a horizontal diffraction plane. It will
accommodate either a cylindrically-bent or a spherically-bent crystal.
With the latter, and a 2-D imaging detector, many vertically displaced
sightlines can be simultaneously analyzed. The dielectric satellite
spectrum of Helium-like ArXVII will be used to derive values of the Ar
ion temperature and the electron temperature. This diagnostic will be
particularly useful in studies where T~I~ information from CHERS are not
available, such as ohmic and RF-only experiments.

Design work has begun on a fast tangential 2-D x-ray camera system,
similar to that recently developed for TEXTOR.[^18] X-rays would be
imaged by a pinhole onto a CsI scintillator faceplate, which is coupled
to a 100 mm diameter Hamamatsu electrostatic image intensifier. A fiber
optic reducer couples the P47 phosphor intensifier output to a fast
frame rate CCD. The PSI CCD[^19] is designed with 64x64 superpixels,
each 200μ square, with 40% active area. Each superpixel has a storage
register to store 300 frames. The effective frame rate will be up to
\~500 kHz. This camera will be used in a tangential viewing geometry,
which should allow high m-modes (m\~20) to be resolved with good
accuracy. It is predicted that MHD events which change the x-ray
emissivity of order 1% over a few cm could be resolved with 100 kHz time
resolution with this system.

MICROWAVE and FIR DIAGNOSTICS

Line density measurements are made with a 170 GHz interferometer system
adapted from part of the TFTR correlation reflectometer[^20]. The beam
is launched radially through a midplane window, reflects off the carbon
CS tiles, and is received through the same window.

A millimeter wave reflectometer system is being installed with a goal to
measure density profiles in swept FM mode, and fluctuations in fixed
frequency mode. An array of 4 pairs of transmit/receive horns has been
installed on a midplane port covering the range 12 - 50 GHz, accessing
densities in the range 2x10^12^ -- 3x10^13^ cm^-3^. The horns are
capable of launching arbitrary plane polarized waves. Because of the
large field pitch and field shear in the ST, particularly for high
β operation, mode mixing could have a significant impact on data
interpretation. Experimental data with both X- and O-mode launch will be
compared with numerical simulations to improve understanding in this
area. The reflectometer will be a valuable tool for studying phenomena
involving fast changes in the edge density profile, such as L-H
transitions and ELMS as well as edge turbulence.

A lower frequency reflectometer for studying density profiles in the
scrapeoff layer has horns integrated into the HHFW antennas. This system
has a frequency range 3-36 GHz, sweepable in 100-850 μs. The fast sweep
time will aid in avoiding problems with large density fluctuations in
this region. Once again, cylindrical waveguides are used near the
machine, and the polarization can be adjusted to match to the edge pitch
angle. This system will be used to optimize RF antenna coupling to the
plasma.

![](media/image2.pct){width="6.446527777777778in" height="7.5in"}
Electron cyclotron emission (ECE) radiometry has been a powerful
diagnostic tool in tokamaks, providing temporally and spatially resolved
measurements of electron temperature. On NSTX, the toroidal field is low
enough that Ω~ce~ \<\< ω~pe~, and electromagnetic radiation is not
generated for the first several cyclotron harmonics. Radiation at higher
harmonics will propagate, however, it does not satisfy the blackbody
condition necessary to make it a good monitor of electron temperature. A
promising new development for diagnosing electron temperature in low
field plasma confinement devices is electron Bernstein wave (EBW)
radiometry.[^21] For NSTX conditions, EBW waves propagate in the plasma
and meet the blackbody criterion. This concept has recently been tested
on the CDX-U device operating at 0.2 T, where a manually tunable device
operating between 8-10 GHz, has obtained initial T~e~ profile data, with
an averaging time of 0.5 ms. An antenna is mounted outside a quartz
window, viewing second harmonic EBW

Fig. 2 Plan view of diagnostic layout and sightlines on NSTX

emission, mode converted to X-mode electromagnetic radiation at the
upper hybrid layer near the plasma edge. This system will soon be
installed to make similar measurements on NSTX.

Design is underway for a 7 channel, tangential, FIR laser
interferometer/polarimeter for NSTX.[^22] Fully implemented, the
diagnostic will utilize two probing lasers with opposite circular
polarizations, and a third reference laser, all operating at 119
μ.  Phase shiftsbetween the two probing lasers are due to the Faraday
effect, and provide information on the toroidal field. The average phase
shift relative to the reference beam is a measure of the refractive
index, providing information on the electron density. With spatial
inversion, n~e~(R,t) and B~T~(R,t) profiles can be obtained with time
resolution up to 100 kHz. Core sightlines will typically see 10-20
fringes of density signal and 5-15° of Faraday rotation. The system
should have excellent S/N, since minimum detectable signals are
predicted to be .03 fringes and .03 degrees. Because of the low toroidal
field on NSTX, the deviation in the vacuum toroidal field in the core
due to plasma diamagnetism is predicted to be \~ 100 gauss for high β
discharges. This deviation represents a few degrees of Faraday rotation,
and should be readily observable. This measurement will provide valuable
constraints on the equilibrium reconstruction of NSTX plasmas.
Initially, two sightlines will be used with retroreflectors located
outside the vacuum, on the stable Thomson scattering viewing platform.

FAST ION DIAGNOSTICS

The low value of the magnetic field in NSTX leads to the existence of a
rich variety of energetic ion phenomena, such as high energetic particle
beta, large population of super Alfvénic ions, large Larmor radii and
orbit widths, large population of trapped ions, anisotropy induced by
prompt loss of edge or counter-going beam and/or RF-driven ions and
possiblity of significant loss of energetic particles due to MHD
interactions. Measurements of the ion distribution in energy, space and
pitch angle using a charge exchange neutral particle analyzer (NPA) can
provide essential information to support evaluation of these effects.

The passive charge exchange diagnostic for NSTX uses an E\|\|B-type NPA
originally designed for TFTR[^23]. The E\|\|B spectrometer has an energy
range of 0.5 ≤ A(amu)E(keV) ≤ 600 and for the NSTX application provides
mass-resolved energy spectra of H+ and D+ ion species simultaneously
during a single discharge. The detector consists of a large-area
microchannel plate which is provided with two rectangular,
semi-continuous active area strips, one coinciding with each of the mass
rows for detection of H+ and D+. Each mass row has 39 energy channels
and has an energy dynamic range of E~max~/E~min~ = 30. The energy
resolution is 3%≤E/E≤7%. Time resolution, is expected to be \~ 5 ms. To
facilitate the measurements noted above, the design of the support
structure for the NPA provides horizontal and vertical scanning
capability to map the ion energy distribution in space and pitch angle.

Predictions indicate that 10-30% of the neutral beam heating power will
be lost in moderate-to-high β, 1.0 MA NSTX plasmas, due to the large
orbit width and Larmor radius of these ions. Modeling of prompt 80 keV
beam ion loss rates indicate that peak fluxes in the range of \~10
MW/m^2^ (\~10 mA/cm^2^) occurs on surfaces near the outer plasma edge,
such as the neutral beam armor and the RF antennas. IR camera views of
these regions will map out resulting surface heating. A fast ion loss
probe is being designed to measure the time behavior of the fast ion
edge flux. Fast ion currents to thin metal films in an alternating
metal/insulator film collector will be measured to provide energy
resolution. For example, a structure consisting of vacuum-deposited Al
and SiO~2~ layers, each layer several hundred Å thick is being
considered. The collector will be mounted behind a slit in a probe tip
with a drive that is radially scannable at the midplane between shots.

CONCLUSION

A national team is assembling a broad array of diagnostics for the newly
commissioned NSTX device, which have been summarized. Most techniques
familiar to tokamak research are also adaptable to spherical tori.
However, the low toroidal field and the low aspect ratio of the ST
dictate changes to some standard techniques and development of some new
techniques, examples of which have been highlighted. As the NSTX
research program continues to mature, ongoing and future diagnostic
development will be a key component of the program, leading to many new
physics insights.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the support of the NSTX Program
Director, M. Peng, the NSTX Project Director, M. Ono, and the NSTX
Experimental Operations Head, J. Hosea. This work funded by U.S. DoE
contract DE-AC02-76-CH0-3073.

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[^19]: Under development by Princeton Scientific Instruments, 7 Deer
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