**Rogowski Loop Designs for NSTX \***

B. McCormack, R. Kaita, H. Kugel, R. Hatcher

Princeton Plasma Physics Laboratory, Princeton University

P. O. Box 451, Princeton, NJ, 08543

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Abstract - The Rogowski Loop is one of the most basic diagnostics for
tokamak operations. On the National Spherical Torus Experiment (NSTX),
the plasma current Rogowski Loop had the constraints of the very limited
space available on the center stack, 5000 volt isolation, flexibility
requirements as it remained a part of the Center Stack assembly after
the first phase of operation, and a +120°C temperature requirement. For
the second phase of operation, four Halo Current Rogowski Loops under
the Center Stack tiles will be installed having +600°C and limited space
requirements. Also as part of the second operational phase, up to ten
Rogowski Loops will installed to measure eddy currents in the Passive
Plate support structures with +350°C, restricted space, and flexibility
requirements. This presentation will provide the details of the material
selection, fabrication techniques, testing, and installation results of
the Rogowski Loops that were fabricated for the high temperature
operational and bakeout requirements, high voltage isolation
requirements, and the space and flexibility requirements imposed upon
the Rogowski Loops. In the future operational phases of NSTX, additional
Rogowski Loops could be anticipated that will measure toriodal plasma
currents in the vacuum vessel and in the Passive Plate assemblies.

I. INTRODUCTION

The Rogowski Loop is one of the most important diagnostics from a plasma
current measurement and control aspect of tokamak operation. A loop is a
multi-turn uniformly wound solenoid having a uniform cross-sectional
area and completely wraps around the current to be measured. The winding
operation introduces a resultant one-turn advance which can be cancelled
out by an identical reverse winding overlaid or a single "return" loop.
The NSTX designs used the single "return" loop cancellation method.

The Rogowski Loop output voltage equation is:

Vo = 2 x 10-3[nA]{.underline} [dI (t)]{.underline}

R dt

where Vo = the loop output voltage in volts

n = number of winding turns

A = cross-sectional area of individual turn (cm2)

I (t) = current (megamps)

R = Rogowski Loop major radius (cm)

t = time (seconds)

Another way of expressing the Rogowski Loop output voltage equation is:

Vo = 0.03192 nA [dI (t)]{.underline}

dt

where Vo = the loop output voltage in volts

n = turns per inch

A = mandrel cross-sectional area in inches

I (t) = current (megamps)

t = time (seconds)

The constant takes into account metric/English conversions while the
equation form highlights the direct relationship of turns and
cross-sectional area to resultant output voltage.

II\. PLASMA CURRENT ROGOWSKI LOOP

A. Specifications

The plasma current Rogowski Loop had the requirements to fit in the
approximate 0.135" space between the inner diameter of the Center Stack
casing insulation and the OH Coil outside diameter. It would be
measuring Mamp plasma currents with a maximum dIp/dt of 5 Mamp per
second. The electrical isolation of 5000 volts between Center Stack and
vacuum vessel along with a 120°C-temperature compatibility were
additional requirements. Two loops were to be installed and the
installation required extreme flexibility.

B. Design and fabrication details

Due to the flexibility and temperature requirements, Teflon was chosen
as the mandrel material. In order to obtain a reasonable output voltage,
the mandrel shape was selected to be a rectangular with the dimensions
of 0.048" x 1.10" with rounded edges.

The winding pitch of 77 turns per inch was selected using an AWG No.30
gauge, Class 220(220°C rating), heavy polyimide-coated copper magnet
wire. The pitch was also chosen to maximize output voltage.

The length of the plasma current Rogowski Loop was conservatively
estimated to be approximately 34' as it traveled inside the Center Stack
casing, enclosing one poloidal field coil, and under the remaining
poloidal field coils. Also, the loops would be required to stand off
from the vacuum vessel due to the +350°C bakeout requirement.

The Teflon was procured as a 4' x 8' x 1/16" sheet and cut into 8' x
1/8" wide strips. The strips were pulled through progressive dies to
achieve a 1.1" wide by 1/16" thick mandrel shape.

A preliminary minimum length of 37' for the mandrel length was
established to allow for the winding operation. In order to achieve the
37' minimum length using the 8' long Teflon strips, each end was bias
cut at 45° and machined to half thickness for 2". The end machining was
configured to allow one piece to be end-fitted with the next strip.
After the machining, the Teflon strips were sodium ammonia solution
etched to prepare the Teflon for adhesive bonding. The etching process
colors the Teflon a dark brown, with negligible size or flexibility
change.

The adhesive used was a 3M, Scotch-Weld Epoxy Adhesive, #2216 B/A Gray.
Alignment/clamping fixtures were used and the vendor cure instructions
were followed.

As mentioned above, the NSTX designs used the single "return" loop
cancellation method. To incorporate the single "return" loop, a groove
was machined along the long axis of the mandrel to enable the single
"return" winding to be recessed inside the mandrel (Fig.1). At the same
time, both sides of the mandrel were machined equally to reduce the
mandrel thickness to 0.049"+0.000/-0.002.

![](media/image1.pct){width="3.6944444444444446in"
height="2.9166666666666665in"}The winding of three Plasma Current
Rogowski Loops was achieved by first installing the single AWG No.30
magnet wire "return" winding and covering the wire with 0.002" thick
Teflon tape. The next step was to "fly wind" the "main" winding at a 77
turns per inch pitch. Splices were permitted and were performed per a
splicing procedure; any splices were placed along the 0.048" edge and
locally taped in place. One assembly had three splices and the other two
did not have any splices. Winding turn "crossovers" were permitted; one
unit had one and the other two did not have any "crossovers". The two
magnet wire self leads at each end would be terminated in the field
installation.

The first design specified a one-half lap wrap of 0.002" thick Teflon
tape for the loop section of approximately 8ft which was inside the
Center Stack Casing and the remaining length at both ends received a
one-third lap wrap of 0.002" thick Teflon tape. First article testing
revealed a 5000 volt breakdown in the one-half wrap section.

The insulation design was changed as follows:

a\) remove the one-half lap wrap of 0.002" thick Teflon tape from the
Center Stack Casing section for thickness build-up reasons; b) install a
0.002" thick kapton tape over the entire loop, one-third-lap wrap on the
end sections and one-half lap wrap in the Center Stack Casing section.
All modified loops passed the 5000 volt insulation testing.

Fig.1. Cross-sectional view of plasma current Rogowski mandrel.

C. Test Results

A Rogowski Loop test setup (Fig.2) was configured to perform scale
factor measurements.

The average tested scale factor was:

Vo = 0.198 [dIp (t)]{.underline}

dt

where Vo = the loop output voltage in volts

Ip(t) = plasma current( megamps )

t = time(seconds)

The tested scale factors were approximately 8% higher than the
calculated scale factor due to the inaccuracy of establishing the
effective mandrel thickness. This is a combination of the Teflon tape
thickness and the magnet wire conformance to the mandrel. The average
resistance of the main winding was an 805 ohms with an average
inductance of 6.4 µhenrys; the average resistance of the return winding
was a 4.5 ohms with an average inductance of 85 µhenrys.

Fig.2. Rogowski Loop Test Setup.

D. Installation

![](media/image2.pct){width="3.4444444444444446in"
height="1.8055555555555556in"}The first phase of the field installation
was positioning and attaching the two loops along the vertical axis of
the OH Coil. The angular orientation was established to be clear of port
covers and the linear orientation was established to place the
termination at a convenient lower dome location. The loops were Kapton
taped to the OH Coil. Voltage isolation tests at 5000 volts were
successful.

The second phase of the field installation was the 2lowering of the
Center Stack Casing over the thermally insulated OH Coil Assembly, which
included the two loop assemblies. The loops were pulled up through the
casing and had to have the flexibility to pass two 90° bends around the
Poloidal Field Coil, PF1A.

The third phase of the field installation was the lowering of the2Center
Stack Assembly through the vacuum vessel assembly for the attachment to
complete the vacuum vessel. The loops had to be tightly coiled up and
"tucked" away to clear the vacuum vessel diameters.

The final phase of the field installation was the uncoiling of the loops
and the welding of steel brackets to support the loops in insulated
clamps. This spaced the loops away from the vacuum vessel approximately
four to six inches in most areas and provided positional stability. In
close areas, additional Kapton sheet and tape was installed for
protection purposes.

The loop assemblies had been purposely wound longer than required. At a
bracket-supported junction, the length was reduced to that required at
the final installation and the appropriate terminations to the field
cabling were made. The resultant average loop length was 34'. The
average resistance of the complete loop was a 729 ohms with an average
inductance of 4.3 µhenries.

E. Subsequent Installations.

During the first brief operational run period, the Rogowski Loops were
successfully used to measure plasma currents up to 300kA. After this
time, the Center Stack Assembly was withdrawn from the vacuum vessel to
facilitate interior assembly work. Three strips of copper foil shielding
were applied to both exposed ends ( outside the Center Stack region )
along the length, leaving one edge exposed. The copper foil was
terminated to field cable shielding, which is connected to the
instrumentation rack ground. The Center Stack Assembly has been
re-installed and the Rogowski Loops re-terminated.

III\. HALO CURRENT ROGOWSKI LOOP

A. Specifications

The halo current Rogowski Loops are designed to detect currents flowing
through the Center Stack as a function of plasma location They had the
requirements to fit under the Center Stack carbon tiles in a 0.400"
radial by 1.000" axial space. The temperature requirement was 600°C and
four loops were to be installed around the 20" casing diameter. The
assembly under the tiles and around the casing required flexibility.

B. Design and fabrication details

![](media/image3.pct){width="3.1944444444444446in"
height="1.5416666666666667in"}Due to the temperature requirement, a
ceramic webbing was chosen as the mandrel material. This form of ceramic
also provided the flexibility required to fit behind the Center Stack
carbon tiles. The webbing material chosen had a nominal 0.125" by 0.750"
cross-sectional area. Based upon the space allocated and the desire for
as high an output signal as possible, three sections were bonded
together using a high temperature adhesive, Fortafix Fiborclad. In order
to maintain the required flexibility, the adhesive was applied in
approximate 1/4" vertical strips across the width at one inch
increments; this procedure maintain sufficient flexibility. The average
resultant mandrel cross-sectional area was 0.259" by 0.747"( See Fig.3
).

The same winding pitch of 77 turns per inch as the plasma current loops
was used. Due to the high temperature requirement, a ceramic insulated
AWG No.30 magnet wire, fabricated by Cermawire, was used. The ceramic
insulation has a degree of porosity and a significant coarseness that
must be considered in the Rogowski Loop fabrication.

A winding length of \~66" and a mandrel length of \~ 69" was chosen to
allow for trimming back to fit the 20" Center Stack Casing diameter. The
"return" winding was installed along one of the edges and the "main"
winding was "fly wound" over the "return" winding; the result was a
continuous winding loop.

Early tests revealed a design problem; the "return" winding was shorting
out to the "main" winding. The rework decision was to remove the
"return" winding from under the "main" winding.

The next step was to establish the loop length with an insulation wrap
that would result in a close fit around the diameter with the ends
nearly butting. The actual magnet wire winding was set back from the
mandrel ends by 1/8" to 1/4". Once established, the winding ends were
secured with a coating of the Fortafix adhesive. One of the field leads,
Awg No.24 Quartz insulated leads, was welded to one of the ceramic
insulated magnet wire self leads and routed along the mandrel edge to
serve as the "return" winding. The other field lead was welded to the
other self lead.

Due to the temperature requirement, welding was chosen to make the
connection between the Rogowski Loop and the field cable. There is no
requirement to "strip" the ceramic insulation or prepare the nickel
coated copper field cable. The termination connections were positioned
along the edge of the winding/mandrel assembly. The welded joint and the
field cable were installed inside a 1200°F fiberglass sleeving which was
continued up to the vacuum feedthru connector.

To protect and insulate the ceramic winding from the carbon tiles, a
Nextel one-inch ceramic tape was one-half lap wrapped around the
winding/mandrel assembly. The wrap was applied over the welded joints,
the "return" winding ( field cable/sleeving ), and the "main" winding
connection; the wrapping served as a strain relief for the field to
magnet wire connections. Both tape ends were ends were secured with a
coating of the Fortafix adhesive.

The average mandrel cross-sectional area after the wrapping process was
0.360" by 0.940" with individual locations approaching the 0.400" by
1.000" carbon tile cutout.

Fig.3. Cross-section of halo/eddy current Rogowski mandrels.

C. Test Results

The Rogowski Loop test setup ( Fig.2 ) was used to perform the scale
factor measurements.

The average tested scale factor was:

Vo = 0.476 [dIp(t)]{.underline}

dt

where Vo = the loop output voltage in volts

I(t) = halo current ( megamps )

The tested scale factors were identical to the calculated scale factor.
The average resistance of the loop with approximately 7' of field cable
was 113.3 ohms with an average inductance of 2.5 millihenrys.

D. Installation

The Center Stack Assembly has seven rows of tile on the larger diameter
upper and lower portions. Halo Current Rogowski Loops were installed
between the second and third row, and also between the fourth and fifth
rows of tiles; this was done on the top and bottom portions.

The installation process required the field leads to be installed into a
cable channel machined in the carbon tiles and run to a feedthru tube
and connector. At the same time, the mandrel/winding assembly was
installed behind one of the rows of tiles. The loop was closed in by the
installation of the next row of tiles.

IV\. EDDY CURRENT ROGOWSKI LOOP

A.  Specifications

The eddy current Rogowski Loops are intended to measure the distribution
of currents induced in conducting structures surrounding the plasma.
They had the requirement to be able to wrap around the Primary and
Secondary Passive Plate assemblies. The temperature requirement was
350°C and four loops would be required for the top Primary Passive
Plates and one for the top Secondary Passive Plates; a mirror image
complement would be installed on the bottom Passive Plates.

B. Design and installation details

Even though the temperature requirement was lower, the same ceramic
webbing mandrel used on the Halo Current Rogowski Loop was chosen and
design fabrication was the same; the only difference was the
winding/mandrel lengths. This mandrel form also provided the flexibility
required to fit around the Passive Plate assemblies. The average
resultant mandrel cross-sectional area was 0.259" by 0.747"(See Fig.3).

The same winding pitch of 77 turns per inch as the halo current loops
was used. Due to the high temperature requirement, the same ceramic
insulated AWG No.30 magnet was used.

The average winding length was 40.5" for the Secondary Passive Plate
installation and the average winding length was 59" for the Primary
Passive Plate installation. The mandrel lengths were longer for the
winding process.

The first tests revealed the same design problem with windings shorting;
the rework process was identical.

For the eddy current loops, maintaining a critical length was not
necessary; therefore, the ceramic mandrel ends were trimmed to achieve a
space of 1/8" to 1/4" from the actual fabricated magnet wire winding
lengths.

To protect and insulate the ceramic winding, a Nextel one-inch ceramic
tape was one-third lapped wrapped around the winding/mandrel assembly;
thickness build-up was not critical. All the other fabrication details
were identical to the halo current loops.

C. Test Results

The Rogowski Loop test setup ( Fig. 2 ) was used to perform the scale
factor measurements.

The average tested scale factor was:

Vo = 0.457 [dIp(t)]{.underline}

dt

where Vo = the loop output voltage in volts

I(t) = eddy current ( megamps )

The tested scale factors were approximately 4% lower than the calculated
scale factor. The eddy current loops were the first units to be
reworked; there probably are some shorted turns in these assemblies. The
average resistance of the secondary passive plate loops with
approximately 7' of field cable was 69.7 ohms, with an average
inductance of 1.6 millihenrys. The average resistance of the primary
passive plate loops with approximately 7' of field cable was 96.5 ohms,
with an average inductance of 2.1 millihenrys.

D. Installation

In between the passive plate assemblies, there is a 2" vertical space
with a partial overlap of the carbon tiles which results in a 1" opening
to the plasma. In selected openings on both primary and secondary
plates, there is a stainless steel bracket with some Mirnov coils
attached to them. The field installation of the eddy current loops used
one side of these brackets, and required a second bracket welded on to
the opposite side of the plate assemblies for mounting. The loops were
held to the brackets by stainless steel straps that clipped on to the
brackets. After making the loop snug to the bracket, the other side of
the strap was welded to the bracket. The brackets provided the vertical
support for the loops; the loops were tied off to various cooling tubes
on the top and bottom using 1/16"OD, 1200°F fiberglass sleeving to
provide the remaining support. The signal leads were routed to the
nearby feedthru connectors.

Due to scheduling demands, only the eddy current Rogowski Loops on the
bottom half passive plates were installed at this opening for a total of
five loops.

V. FUTURE ROGOWSKI LOOP REQUIREMENTS

Currents in the Center Stack and the passive plate supports were
successfully measured with the Rogowski Loops. Any future requirements
for Rogowski Loops depend on operational results, budget, and time
restrictions but the following are possibilities: a) install the balance
of the eddy current loops, b) install loops around some portion of the
inner and outer divertors. The choices require more investigation and
operational data analysis.

Acknowledgements: Many people provided assistance and insight; I would
like to thank E. Bush, J. Carson, W. Derry, S. Edwards, G. Gibilisco, K.
Gilton, L. Guttadora, T. Holoman, P. Kivler, R. Knoll, G. Rossi, and F.
Simmonds.

\* Work supported by U. S. D. O. E. Contract DE-AC02-76-CH03073.