> **Engineering Overview of the National Spherical Torus Experiment
> (NSTX)\***

C. Neumeyera, P. Heitzenroedera, J. Chrzanowskia, B. Nelsonb, J.
Spitzera, R. Wilsona,

L. Dudeka, R. Kaitaa, S. Ramakrishnana, D. Bashorea, E. Perrya, and the
NSTX Team

aPrinceton University Plasma Physics Laboratory, Princeton, NJ 08540

bOak Ridge National Laboratory, Oak Ridge, TN

**ABSTRACT**[^1]\*

**The NSTX Project will provide a national facility for the study of
plasma confinement, heating, and current drive in a low aspect ratio,
spherical torus (ST) configuration. The ST configuration is an alternate
confinement concept which is characterized by high β, high elongation,
high bootstrap fraction, and low BT compared to conventional tokamaks.
NSTX is the next step ST experiment following smaller experiments such
as the PPPL CDX-U (Current Drive Experiment, Upgrade), the START (Small
Tight Aspect Ratio Tokamak) at Culham, and the HIT (Helicity Injected
Tokamak) at University of Washington, and is similar in scale to the
MAST (Meg-Amp Spherical Tokamak) machine now under construction at
Culham.**

**This paper provides a description of the NSTX mission and gives an
overview of the main engineering features of the design of the machine
and facility and discusses some of the key design solutions.**

**I.** **INTRODUCTION**

*A. Overview*

The NSTX machine is shown in isometric view in figure 1, and in cross
section in figure 2.

The core of the NSTX machine consists of a narrow center stack (CS)
bundle which contains the inner legs of the Toroidal Field (TF) coil, an
Ohmic Heating (OH) solenoid coil and associated tension cylinder, a pair
of inner Poloidal Field (PF) coils, thermal insulation, and a center
stack casing which forms the inner wall vacuum vessel boundary. The CS
Casing is electrically isolated from the remainder of the machine via
ceramic insulator assemblies which permit the use of Coaxial Helicity
Injection (CHI) as one of the means of

advanced current drive. The CS bundle presents one of the main
engineering challenges of NSTX and is described in more detail in a
companion paper \[1\]. The vacuum vessel outer sections are formed using
a cylindrical section with upper and lower domes taken from the retired
S-1 machine at PPPL. Similarly the outer PF coils are taken from S-1.
NSTX will be installed in the Hot Cell of the D-site facility at PPPL
which supported TFTR until its recent retirement. Extensive use of the
D-Site infrastructure including magnet power supplies, and RF sources,
cooling water systems, etc., is made to minimize the overall cost of the
experiment.

Fig. 1. Isometric View of NSTX

Fig. 2. Cross Section of NSTX

*B. Baseline Machine Parameters and Ratings*

Main parameters and ratings are given in Table I.

**Table I**

**Baseline Ratings of NSTX Machine**

  -------------------------------------- ---------------- ----------------
  Major Radius (R0)                      85.4             cm

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  Aspect Ratio (R/a)                     1.26             

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  Current                                1.0              MA

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  Ramp Time                              0.2 - 0.4        sec

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  Flat Top (Inductive)                   0.5              sec

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  Repetition Period (Inductive)          600.0            sec

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  Flat Top (non-Inductive)               5.01             sec

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  Repetition Period (Partial &           300.0            sec
  Non-Inductive)                                          

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  Toroidal Field @ R0                    3.0/6.0          kG

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  TF Current/turn                        35.6/71.2        kA

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  TF Equivalent Square Wave              5.3/1.325        sec

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  OH Flux (double swing)                 0.6              volt-sec

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  OH Initiation Loop Voltage             5.0              volt/turn

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  OH Current/turn                        24.0             kA

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  OH Equivalent Square Wave              0.525            sec

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  PF1a Current/Turn                      15.0             kA

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  PF1b, 2, 3, 4 Current/Turn             20.0             kA

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  PF Equivalent Square Wave              5.0              sec

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  HHFW Power to Plasma                   6.0              MW

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  HHFW Duration                          5.0              sec

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  ECH2 Power to Plasma                   20               kW

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  ECH Duration                           5.0              mS

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  CHI Current Drive                      500.0            kA

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  CHI Current Rise Time                  5.0              mS

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  PFC Bakeout                            350              deg C

  -------------------------------------- ---------------- ----------------

  -------------------------------------- ---------------- ----------------
  Vacuum Vessel Bakeout                  ≥ 150            deg C

  -------------------------------------- ---------------- ----------------

1 - Toroidal Field flat top time 2 - Preionization only

*C. Upgrades Foreseen in the Baseline Design*

The baseline design anticipates the following upgrades:

• up to 2 Neutral Beam Injectors (2 x 5MW to plasma)

• long pulse (≈ 60 second) non-inductive operations

**II. FEATURES OF NSTX**

*A. Toroidal Field (TF) Coil System*

The 36 turn TF coil system (fig. 3) consists of discrete inner and outer
leg conductor sections. The inner legs are located along the central
axis of the machine and consist of an inner layer of 12 turns and an
outer layer of 24 turns. All the turns are water cooled but rated based
on adiabatic conditions for the 5 second NSTX pulse. The inner and outer
layer turns have different shapes but are of equal cross sectional area,
such that the utilization of the overall area taken by the inner leg
bundle is efficient and the thermal growth of all of the inner legs is
equal. The inner legs are connected to the outer legs via radial flags
and connectors at two elevations. Toroidal progression of turn to turn
current at the two elevations is such that the net

Fig. 3. Toroidal Field Coil System

amp-turns is zero. The joints between inner legs and flags are critical.
Contact area is ≈ 5 in2, contact pressure ≈ 2000 psi, and with silver
plated contact surfaces the expected resistance is ≤ 2µΩ-in2. The outer
legs are arranged in 12 groups of three and are much larger in cross
section and lower in current density than the inner legs. Parameters of
the TF coil system are given in Table II.

**Table II**

**TF Coil System Parameters**

  ------------------------------ -------------------- --------------------
  #Turns                         36                   turns

  ------------------------------ -------------------- --------------------

  ------------------------------ -------------------- --------------------
  Current/turn                   35.6/71.2            kA

  ------------------------------ -------------------- --------------------

  ------------------------------ -------------------- --------------------
  Open Circuit Voltage           1.0                  kV

  ------------------------------ -------------------- --------------------

  ------------------------------ -------------------- --------------------
  Resistance (10C)               6.3                  mΩ

  ------------------------------ -------------------- --------------------

  ------------------------------ -------------------- --------------------
  Self Inductance                4.7                  mH

  ------------------------------ -------------------- --------------------

  ------------------------------ -------------------- --------------------
  ESW                            5.3/1.325            sec

  ------------------------------ -------------------- --------------------

  ------------------------------ -------------------- --------------------
  Repetition Period              300.0                sec

  ------------------------------ -------------------- --------------------

*B. Ohmic Heating (OH) Solenoid*

The OH solenoid is a 4 layer, 962 turn coil, consisting of 8 separate
continuous lengths of conductor wound in two-in-hand pairs. With the
two-in-hand winding pattern the maximum conductor length (≈ 360 feet) is
such that it can be produced in a single extrusion, and the water flow
required for cool down between pulses can be realized at a practical
pressure, flow rate, and flow velocity. The coil is wound over a Teflon
layer which covers a tension cylinder. Spring washers working against
the flanges at the end of the tension cylinder are used to impart an
axial preload the OH coil winding. Parameters of the OH system are given
in Table III.

The ESW and repetition period in Table III correspond to a double swing
of the OH current, applicable to fully inductive pulses, with flat top
time ≤ 0.5 seconds. For longer pulses with partial inductive drive a
single swing of the OH coil current (to zero) is used, and a repetition
rate of 300 seconds is possible.

**Table III**

**OH System Parameters**

  ------------------------------ -------------------- --------------------
  #Turns                         962                  turns

  ------------------------------ -------------------- --------------------

  ------------------------------ -------------------- --------------------
  Current/turn                   24.0                 kA

  ------------------------------ -------------------- --------------------

  ------------------------------ -------------------- --------------------
  Open Circuit Voltage           6.0                  kV

  ------------------------------ -------------------- --------------------

  ------------------------------ -------------------- --------------------
  Resistance (10C)               93.4                 mΩ

  ------------------------------ -------------------- --------------------

  ------------------------------ -------------------- --------------------
  Self Inductance                13.0                 mH

  ------------------------------ -------------------- --------------------

  ------------------------------ -------------------- --------------------
  ESW                            0.525                sec

  ------------------------------ -------------------- --------------------

  ------------------------------ -------------------- --------------------
  Repetition Period              600.0                sec

  ------------------------------ -------------------- --------------------

*C. Poloidal Field (PF) Coils*

The inner PF coils, consisting of PF1a (upper and lower), and PF1b
(lower only) are provided for plasma shaping. The outer PF coils consist
of PF2a/2b, PF3a/3b, and PF4a/4b/4c, and are used for plasma shaping as
well as vertical and radial position control. These coils are taken from
the S-1 machine. They are mounted on the upper and lower domes of the
vacuum vessel, within the bore of the TF coils.

Configuration, current control modes, and ratings of the PF coil system
are given in Table IV.

**Table IV**

**PF Coil System**

+---------+---------+-------------+----------+-------+-------+-------+
| **Ckt** | **      | **Config.** | *        | **V** | **I** | **    |
|         | Coils** |             | *Curr.** |       |       | ESW** |
|         |         | **Upper/**  |          |       |       |       |
|         |         |             | **       |       |       |       |
|         |         | **Lower**   | Cntrl.** |       |       |       |
+---------+---------+-------------+----------+-------+-------+-------+

  ---------- ---------- -------------- ----------- -------- -------- --------
                                                   (kV)     (kA)     (sec)

  ---------- ---------- -------------- ----------- -------- -------- --------

  ---------- ---------- -------------- ----------- -------- -------- --------
  PF1a       PF1a       Separate U&L   Unipolar    2        20       5.0

  ---------- ---------- -------------- ----------- -------- -------- --------

  ---------- ---------- -------------- ----------- -------- -------- --------
  PF1b       PF1b       L Only         Unipolar    2        15       1.0

  ---------- ---------- -------------- ----------- -------- -------- --------

  ---------- ---------- -------------- ----------- -------- -------- --------
  PF2        PF2a, PF2b Separate U&L   Unipolar    2        20       5.0

  ---------- ---------- -------------- ----------- -------- -------- --------

  ---------- ----------- -------------- ----------- -------- -------- --------
  PF3        PF3a,PF3b   Separate U&L   Bipolar     1        20       5.0

  ---------- ----------- -------------- ----------- -------- -------- --------

  ---------- ---------------- -------------- ----------- -------- -------- --------
  PF4        PF4a,PF4b,PF4c   Combined U&L   Unipolar    2        20       5.0

  ---------- ---------------- -------------- ----------- -------- -------- --------

*D. Center Stack Casing and Ceramic Insulator*

The Coaxial Helicity Injection (CHI) current drive technique requires
that an electric potential ≤ 2kV be applied between the inboard and
outboard regions of single null x-point divertor plasmas. To permit this
mode ceramic insulator assemblies are installed at the upper and lower
ends of the center stack, and the vacuum vessel is thus divided into two
parts, namely the inboard center stack (CS) casing and the main outer
section. The ceramic insulators are adapted from spare TFTR assemblies
which were used for the NBI and vacuum pumping system ducts.

* *

*E. Plasma Facing Components (PFCs)*

The PFCs consist of the passive stabilizers, inner wall tiles, and
inboard and outboard divertors. Maximum heat loads are given in Table V.
All PFCs must withstand the 5 second pulse with Tmax \< 1200C.

**Table V**

**Max PFC Heat Loads**

  --------------------------------------------- -------------------------
  **Component**                                 **Max Heat Load
                                                (MW/m\^2)**

  --------------------------------------------- -------------------------

  --------------------------------------------- -------------------------
  Passive Stabilizers                           0.1

  --------------------------------------------- -------------------------

  --------------------------------------------- -------------------------
  Inner Wall                                    1.9

  --------------------------------------------- -------------------------

  --------------------------------------------- -------------------------
  Inboard Divertor                              7

  --------------------------------------------- -------------------------

  --------------------------------------------- -------------------------
  Outboard Divertor                             17.1

  --------------------------------------------- -------------------------

Existing PPPL inventory of POCO AXF-5Q and FMI 4-D materials will be
utilized for the PFCs.

To minimize the radial build the inner wall tile thickness is limited to
0.5 inches, and active cooling is not provided. A special tile
configuration and rail mounting scheme has been developed for the inner
wall tiles. A heat shield is provided to limit the influx of heat into
the center stack to ≤ 0.5W/cm2, and cooling is effected by radiation
outward, mainly to the passive plates.

Divertor and passive plate PFCs, 1 inch thickness, are actively cooled
using a high temperature heat transfer fluid (Dowtherm \[2\]).

The passive plates consist of 12 segmented 1/2 inch thick copper plates,
with flexible connecting straps at all except one gap, at which location
a saddle connection joins the plates in the upper and lower half planes.
The plates are mounted to the vacuum vessel via 1/2 in thick stainless
steel gussets. At the toroidal gap where the saddle connection is made
these gussets present an effective resistance of ≈ 37.5 µΩ in each half
plane.

* F. Integrated Center Stack (CS) Assembly*

At the midplane, proceeding from the centerline (machine axis) outward,
the integrated CS assembly (fig. 4) consists of:

• the TF inner legs and ground insulation

• a clearance gap

• the tension cylinder

• a Teflon layer

• the OH coil, with ground insulation and ground plane

• magnetic diagnostics (flux loops and Rogowski coils, overwrapped with
Kapton

• a layer of Microtherm \[3\] thermal/electrical insulation

• a clearance gap

• the Inconel CS casing

• various diagnostics (thermocouples, Langmuir probes, Mirnov coils

• the inner wall PFCs

The total radial build at the midplane is 18.4 cm (7.25 inches).

![](media/image1.pct){width="3.2222222222222223in"
height="3.0277777777777777in"}

Fig. 4. Cross Section of Center Stack at Midplane

*G. Main Outer Vacuum Vessel*

The outer vacuum vessel consists of three pieces, namely the upper and
lower domes (taken from S-1) and a new central cylindrical section. The
upper and lower domes each include 11 ports, and the cylindrical section
12 ports, 10 of which are 24 inch diameter circular style, and 2 are
designed to accommodate the TFTR style NBI ducts.

The outer vacuum vessel supports the outer PF coils, whose holders are
lined with a low friction material which permits radial growth of the
vessel during bakeout without imparting stress on the coils. The outer
vacuum vessel also supports the outer legs of the TF coils via a strut
system which is in tension during periods of TF energization, but which
is relaxed during bakeout as the vacuum vessel expands radially.

The outer vacuum vessel is supported by four legs attached to the lower
dome and anchored to the test cell floor. A sliding interface joint is
provided to permit the radial growth of the vessel during bakeout.

*H. Ancillary Systems*

1\. Power Systems

One of the two existing PPPL D-site Motor-Generator (MG) sets
(475MVA/2.25GJ) is more than adequate to support the pulsed load
presented by the NSTX TF, OH, and PF power supply systems (250MVA/180MJ
max.), plus the NBI systems in case of the upgrade. For non-inductive
operations (without the OH system) the required power can be taken
directly from the grid under most circumstances.

The existing PPPL D-site rectifier inventory (74 6-pulse units @
1kV/24kA) is more than adequate to support the requirements of the NSTX
TF, OH, and PF coil systems (30 units required). Two of the D-site
15kV/1MJ capacitor banks will be used for CHI.

A new cable run will be installed from the TFTR Test Cell basement to
the NSTX Test Cell, ≈ 280 feet, ≈ 60 1000 MCM cables

2\. RF Systems

A new 12 strap antenna will be built for the NSTX High Harmonic Fast
Wave (HHFW) system. Six of the existing TFTR ICRF generators, each at
30MHz, 1MW, will be utilized. Extension of the existing transmission
lines from the D-site Mock-Up building to the NSTX Test Cell is
required.

An existing 28GHz system @ 20kW/5mS will be provided by ORNL.

3\. Diagnostics Systems

The baseline NSTX diagnostic set is listed in Table VI.

**Table VI**

**Baseline NSTX Diagnostics Set**

  -----------------------------------------------------------------------
  Plasma current Rogowski coils

  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  Eddy current Rogowski coils

  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  Flux loops

  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  Bθ, Bρ coils

  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  Mirnov coils

  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  Visible TV camera

  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  IR camera

  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  "Slow" diamagnetic loop (using toroidal field coil)

  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  Multichannel bolometer

  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  1 mm microwave interferometer

  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  Survey spectrometer (SPRED)

  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  Soft X-ray imaging system

  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  Hα detectors

  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  CHERS

  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  High-throughput CHERS background array

  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  X-ray pulse height analysis

  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  Neutral particle analyzer

  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  Visible spectrometer

  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  Langmuir probes/thermocouples

  -----------------------------------------------------------------------

Except for the magnetic diagnostics, nearly all of the diagnostics will
utilize existing equipment at PPPL. The implementation of the magnetics
diagnostics is noted to be especially challenging. Because of the
premium on space within the center stack, the maximum allowed radial
build for Ip Rogowski and flux loops is 2mm, and the Rogowski must coil
up to permit insertion the CS during machine assembly. The Mirnov Br/Bz
coils located behind the inner wall PFCs must be limited in dimension to
2.5mm x 25mm x 25 mm, and must be rated to withstand 600C.

4\. Vacuum and Gas Injection Systems

Existing PPPL equipment (mechanical, booster, and turbomolecular pumps,
etc.) will be used to a large degree,

along with new piping and duct systems, and a new PLC based local
controller.

5\. Bakeout System

Bakeout heating will be accomplished by passing a DC current through the
center stack casing (≈ 5kW), and by passing a high temperature heat
exchange fluid (Dowtherm \[2\]) through the PFC heating/cooling circuits
(≈ 50kW). In addition, to ensure that appendages to the vacuum vessel
(windows, valves, etc.) do not exceed their temperature limits
(typically ≈ 150C), additional cooling circuits will be utilized to
maintain 150C at those locations. During operations the same Dowtherm
systems will be used for cooling the PFCs (≈100kW on average basis).

6\. Cooling Water Systems

Existing PPPL equipment (pumps, chillers, etc.) will be used to a large
degree. A total of ≈ 60 coil cooling circuits must be serviced, with a
total flow of ≈ 200GPM, maximum 150MJ per pulse, 500kW average.

7\. Central Instrumentation & Control (CIC) System

The CIC consists of three main parts, namely:

• routing process control system

• data acquisition system

• real time control system

NSTX is considering joining the Experimental Physics Instrumentation &
Control System (EPICS) collaboration for process control, and the MDS
Plus (Modular Data System Plus) collaboration for data acquisition. A
highly desirable feature inherent in both of these systems is the
ability to interface with CAMAC devices, of which there are a large
number in the D-site complex.

For the real time control system, the Digital Plasma Control System
(DPCS) developed for PBX and TFTR, will be adopted for NSTX \[4\].

**III. KEY DESIGN SOLUTIONS**

The main engineering challenges on NSTX arise from the desire to
minimize the aspect ratio, the 350C bakeout requirement, and the need to
accommodate the CHI mode.

To minimize the aspect ratio, space within the center stack is set at a
premium. Components within the center stack must be engineered to the
limits of mechanical and thermal stresses. Engineering features must,
simultaneously, provide for:

• large thermal ranges, and displacements associated therewith;

• support of many nested components in a small area;

• large thermal gradients.

The key design solutions described herein were driven mainly by these
requirements.

*A. TF Coil Inner Leg Design*

The TF inner leg design makes efficient use of area via two conductor
shapes of equal CSA. The method of connecting the inner to outer legs at
two elevations, and the design of the joints at the interface, are key
features.

*B. OH Coil Design*

The use of the \"two-in-hand\" winding scheme to reduce cooling path
lengths and avoid joints in the conductor is a key feature selected to
facilitate the tall, thin coil.

*C. Center Stack Load Paths & Restraints*

Required functions are as follows:

• react torsion on TF inner leg due to radial PF and OH fields;

• react torsional and axial loads on TF lead flags due to axial

PF fields and TF field;

• react axial loads on OH due to interaction with PF1b;

• react axial loads on PF1a and PF1b;

• allow relative motion between parts to permit thermal growth.

Key solutions developed are as follows:

• hub assembly and torque collar are provided on TF inner leg bundle to
react center stack torsion and reduce TF insulation shear (fig. 5).

Fig. 5. Hub Assembly and Torque Collar

• top umbrella structure (fig. 6) attaches to hub assembly and outer
legs to transmit torsion from center stack to vacuum vessel without
stress on electrical joints. Spline joint feature

(see fig. 1) permits axial growth of CS assembly. At bottom of machine
torsional loads from center stack are transmitted via hub assembly and
umbrella structure to center stack pedestal.

Fig. 6. Upper Umbrella Assembly

• OH coil is wound on tension cylinder fixed to machine base,

with slip plane between cylinder and TF inner leg bundle. OH coil
winding is preloaded in compression via spring washers acting against
flanges at end of tension cylinder. Axial loads on OH coil are reacted
by tension cylinder.

• PF1a and PF1b coils are restrained against axial loads by straps
supported off of OH tension cylinder.

• aforementioned slip planes, spline joints, and gaps between the TF
inner leg bundle, tension cylinder, OH coil, and PF1a and PF1b coils
allow for relative displacement of these parts due to thermal growth.

*D. Center Stack Thermal Management*

Required functions are as follows:

• accommodate on the center stack 2.1MW out of the total 6MW-5s/300 sec
heat load during plasma operations;

• provide 350C bakeout of the PFCs while limiting temperature of vacuum
vessel appendages to 150C;

• accommodate a potential of ≤ 2kV during CHI operations on the CS
casing;

• limit OH ground insulation temperature ≤ 120C;

• avoid significant non-ohmic heat load on OH conductor;

• react forces due to halo currents and disruptions;

• accommodate magnetic diagnostics.

Key solutions developed are as follows:

• gap space between OH coil and CS casing containing\...

\- OH ground insulation covered by conducting ground plane

\- diagnostics applied over ground plane w/Kapton over wrap

\- Microtherm insulation (≈ 0.25\") applied over Kapton layer,

\~0.025 W/m-°K, \~ 50 volts/mil

\- clearance gap for installation (≈ 0.125\")

• halo currents found to be benign because aspect ratio of CS casing
results in current distribution with very little toroidal asymmetry;
disruption induced forces are small. As a result the use of the
Microtherm (which has little compressive strength) is feasible since the
pressure will be ≤ 1 psi.

• during operations, with passive plates and VV at 50C, CS casing
ratcheted temperature is found to be \< 600C and inflow to OH coil ≈
3kW.

• OH coil insulation will be ≤ 120C even with loss of CS coil water
cooling during operations.

• OH coil insulation will be ≤ 120C for 5 hours after loss of CS coil
water cooling during bakeout.

• ceramic insulator assembly adapted from TFTR provides the required
isolation, and serves to accommodate the relative displacement of the CS
casing and outer VV section due to thermal differences during operations
and bakeout.

• I2R heating of CS VV casing, circulation of ≈350C Dowtherm fluid
through PFC cooling circuits, and circulation of ≈150C DowTherm fluid in
tubing around VV appendages accomplishes the required bakeout
conditions.

**SUMMARY**

NSTX will be a flexible experiment which will explore the physics of the
Spherical Torus, and enable the development of non-inductive start-up,
current drive, and profile control. Significant engineering challenges
associated with the compact Center Stack have been addressed. PPPL
infrastructure and resources are highly leveraged.

**REFERENCES**

\[1\] J. Chrzanowski, et. al., \"NSTX Center Stack Design and
Assembly\", *17th SOFE, October 1997*

\[2\] Dowtherm A Heat Transfer Fluid, Dow Chemical Company, Midland
Michigan 48674

\[3\] Microtherm Inc., Maryville, TN 37804

\[4\] P. Sichta, et. al., \"TFTR Realtime Control Applications using the
Digital Plasma Control System\",*17th SOFE, October 1997*

[^1]: \* Work Supported by USDOE Contract #DE-AC02-76-CHO3073