Design, fabrication, installation and testing of in-vessel control
coils\
for DIII--D

P.M. Anderson,\* C.B. Baxi, A.G. Kellmam, E.E. Reis, and J.I. Robinson

General Atomics,P.O. Box 85608, San Diego, California 92186-5608\
\*Phone: (858) 455-4748; Fax (858) 455-4190; email:
anderson@fusion.gat.com

Abstract

Since 1995, DIII--D has performed correction of magnetic field
imperfections using a set of six external picture frame coils located on
the vessel mid-plane. Recently, these coils have also demonstrated
significant benefits when used for feedback of the resistive wall mode,
an instability that limits the plasma performance at high beta. Modeling
has shown that substantial performance improvements can be achieved by
installing new coils inside the vessel and expanding the poloidal
coverage above and below the mid-plane. Two prototype internal coils
were installed in 2001 and have been tested successfully. Installation
of a set of twelve internal coils and magnetic sensors in the DIII--D
tokamak is to be completed in December 2002.

The design requirement for the new coil system was to maximize the
magnetic field at the plasma edge, operate with a frequency range of dc
to 1000 Hz, and fit behind the existing graphite wall tiles. The coil
design adopted and installed is a water-cooled hollow copper conductor
insulated with polyamide and housed inside a stainless steel tube that
forms a vacuum boundary. The coil is rigidly mounted to the inside of
the vacuum vessel.

The primary challenge in the design of these coils was in joining of
both the copper conductor and the stainless tube without overheating the
polyamide insulator.

**1.  Background**

DIII--D has performed correction of magnetic field imperfections since
1995 using a set of six external picture frame coils located on the
vessel mid-plane. Recently, these coils have also demonstrated
significant benefits when used for feedback of the resistive wall mode
(RWM), an instability that limits the plasma performance at high
beta \[1\]. A study was conducted to evaluate the performance of an
expanded array of 18 coils(6 on the midplane, 6 each above and below the
midplane). The study considered concepts replicating existing external
coil technology or installing new technology coils inside the vacuum
vessel in addition to the six existing external coils. Table 1 compares
coil design features.

Modeling using the VALEN 3D code has shown that substantial performance
improvements can be achieved by installing new coils inside the vessel
and expanding the poloidal coverage both above and below the mid-plane.
The advantage of the in-vessel design over the ex-vessel design is due
to the closer proximity to the plasma, better field penetration, lower
inductance and more uniform geometry. The expanded coverage provides a
better match to the mode structure of the RWM, improving our ability to
stabilize it. Comparison of a full set of 18 internal coils with a
reduced set of 12 (6 midplane coils removed) showed no significant
benefit to the 18 coil set. Twelve new internal coils with Bp sensors
should allow for operation near the ideal wall β limit, a key element in
the advanced tokamak mission of DIII--D.

# 2.  Design goals

The key design requirements for the new coil system are to maximize the
magnetic field at the plasma edge, operate with a frequency range of dc
to 1000 Hz, and fit in the space behind the existing graphite wall tiles
to avoid any plasma contact. The specified voltage standoff is 1000 Vdc
from conductor to vessel ground. This provides a factor of 3 margin
above the expected peak voltage of \~400 V; the coil power supply is
capable of a peak voltage of 300 V while disruptions can add up to 60 V
across the coil terminals. Disruption induced currents can add 4 kA
(17 kA for a faulted coil) to the normal 5 kA coil current, so the
mechanical attachment of the coil to the vessel must be adequate to
prevent damage to in-vessel components in the event of a faulted coil
condition. The in-vessel coil environment requires a robust design due
to high cyclic electromagnetic forces in vacuum and temperatures to
350°C during vessel baking.

Figure 1 shows the two prototype coils that were installed in DIII--D in
2001. The full in-vessel coil set will consist of two sets of six coils
each located on the inside of the outer wall on conical surfaces above
and below the midplane. The coils are single turn with a conductor
length of 5 m and an area of 1 m sq. In order to minimize error fields
and electromagnetic forces, the radially oriented power and cooling
water feeds are coaxial. The coils are shielded from the plasma by the
plasma facing graphite tiles that are shown partially installed.

# 3.  Design solution

Because the size of the ports on DIII--D would not permit a fully
assembled coil to be passed into the vessel, the coil either had to be
formed in the vessel from a straight section introduced through a port
or assembled together inside the vessel from smaller subassemblies. The
space available for the coil beneath the graphite wall tiles is
approximately a 29 mm by 29 mm sq cross section. Due to the space
limitation imposed by the PF tiles, there is no room to allow for coil
turns to cross each other and the coil is limited to single turn
construction.

The in-vessel coil design adopted and installed is shown in Fig. 2. It
is water-cooled hollow copper conductor (14.4 mm o.d.) insulated with
polyamide and housed inside a stainless steel tube (19 mm o.d.) that
forms a vacuum boundary. The polyamide insulating layer is comprised of
0.5 mm Kapton tape (50% overlap), a 1.7 mm thick Duport vespel SP-1
spacer, and an additional layer of Kapton tape. The coil conductor
system is engineered to operate up to 7 kA dc steady-state with cooling
water flow of 5.5 m/s. Shop fabrication uses in-air induction brazing of
the oxygen free copper conductor joints at the corners of each coil. The
copper and polyamide insulation is encased in a welded 304 stainless
steel vacuum barrier that is the only material exposed to vessel vacuum.
Each coil is fully formed on a mockup in the shop. This process allows
the coil to be passed into the vessel in three pieces; a lower conductor
assembly, an upper conductor assembly and a 35 mm diameter concentric
lead assembly. This design required in-vessel joining of copper with
three induction brazes and six orbital welds of the stainless vacuum
jacket.

The design provides a double barrier of copper and stainless against
water leakage into the tokamak. The polyamide selected, DuPont Vespel
SP-1, is hydroscopic and contains about 1% by mass of water at room
conditions. This moisture is evolved as the material is heated to 350°C
during vessel baking. The insulator is located between the copper and
the stainless and could be exposed to atmosphere at both ends of the
conductor outside of the vessel. In order to detect water leaks in the
copper or leaks through the stainless into the vessel, the insulation
space is sealed in two places outside the vessel using machined
polyamide and "O" rings. During the initial bake of the vessel, this
space is vacuum pumped to remove moisture and gases that evolve. After
cool down, dry nitrogen gas is back filled to about 0.7 bar, and sealed
off. The pressure in this trapped volume is monitored to detect either
water leaks through the copper (higher pressure) or stainless sheath
leaks to the vacuum (lower pressure). During vessel baking, the cooling
water in the coils is replaced with dry nitrogen in order to limit
oxidization of the copper.

# 4.  Development issues

The primary challenge in the design of these coils was in joining of
both the copper conductor and the stainless tube without overheating the
polyamide insulator. Early testing indicated that the hollow copper
conductor could be brazed in air without overheating the polyamide
insulation located 50 mm from the braze joint. This process depends on
rapid localized heating of the copper. An axial thermal gradient of over
815°C at the braze to less than 400°C at the polyamide is required. This
was achieved with localized induction heating with about 10 kW for about
13 s. Similar polyamide overheating problems were evident when TIG
welding the stainless steel tubing which contacted the polyamide. A
solution was developed using automated orbital welding of the stainless
steel tube to specially designed joints that distribute the welding heat
throughout a large mass which is thermally remote from the polyamide. A
second issue with the welding process is trapping of gas pressure within
the stainless sheath due to outgassing of the polyamide during the
welding process. This caused weld blowout near the closure of the
orbital weld. This problem was corrected on the prototype coils with
manual weld repair. Gas vents were developed for the production coils to
prevent blowout. These 2.5 mm holes are then plugged with a set-screw
and seal welded manually. The brazing and welding processes are
performed both outside and inside of the vacuum vessel.

# 

# 5.  Thermal and stress analysis

Thermal analysis of the resistive heating and water cooling of the
copper as well as transient temperatures during vessel baking is
reasonably straightforward. The copper and water cross sections for the
14.4 mm o.d. conductor were optimized to maximize dc steady-state
current without exceeding a water outlet temperature of 90°C for water
flow of 5.5 m/s. The 5 m long conductor coil can handle 7 kA
steady-state, limited by 53°C water temperature increase. Steady-state
is reached after 8 s of constant power input.

Stress analysis was done for I×B forces and thermal stresses during
vessel bake-out. With alternating current at frequencies as high as
1 kHz, there are concerns for high cycle fatigue of the copper and
stainless. The coil conductor is clamped rigidly to the Inconel vessel
wall at intervals less than 180 mm. Although finite element analysis
predicted a first resonance at 980 Hz, none were detected during
frequency sweeping to 1000 Hz in a strong magnetic field. The cyclic
bending stress in the copper due to a 5 kA coil current reacting with
the 2.2 T toroidal field is 35 Mpa and compares with a room temperature
yield strength of 69 Mpa for annealed copper. This provides an expected
fatigue life of greater that 1010 cycles, sufficient for 106 full
current, 10 s pulses at 1 kHz. Of greater concern is the low cycle
fatigue experienced during baking cycles that occur about 10 times a
year. The temperature rise from 20 to 350°C takes 5 hrs and the
temperatures of the Inconel vessel, and the coil components of
stainless, polyamide and copper are reasonably uniform due to conduction
between materials. Thermal stresses develop during baking due to
differences in the rate of thermal expansion between the Inconel 625
wall, stainless 304 sheath and the oxygen free copper conductor. These
thermal expansion rates are 0.0000176/°C for stainless and copper and
0.0000155/°C for Inconel. The main concern is for compressive stresses
that develop in the copper as the thermal expansion of the copper is
restrained by the stainless and Inconel. The copper is fully annealed
during each bake cycle to 350°C. With this type of thermal cycling, the
thermal stresses shake down to acceptable levels with the highest
stresses occurring during the first bake since most of the copper starts
in the hard, high strength condition. Based on a finite element elastic
analysis of the coil/vessel structure, the total strain at the worst
case OFHC elbow joint during cooldown from bakeout is 0.14%. This total
strain applied to the stress curves for annealed OFHC translates to an
alternating plastic strain of 0.04%. The allowable number of cycles at
this plastic strain range is 10,000 cycles based on the lower bound
fatigue curve of Ref. \[2\]. The two prototype coils have experienced
about twelve bake cycles to 350°C without problems.

# 6.  Prototype testing

The plan for initial operation of the twelve internal coils in 2003 is
to power them with supplies capable of 5000 A. The two prototype coils
installed in Summer 2001 have been tested to the full capability of
those supplies. This included the following:

> • Coil cooling tests: 4.5 kA dc for 7 s which generated 22.7 kW of
> heat in the coil as compared to the steady-state design rating of 59
> kW. No anomolous heating was observed. A slight increase in coil
> resistance was measured (\~200% @ 1 kHz) by water flow calorimetry.
> This increase is expected due to the decreasing skin depth in the
> copper conductor at 1 kHz.
>
> • Power testing: 4.5 kA delivered from dc -- 300 Hz as limited by the
> power supply with delivered current decreasing to 1.5 kA at 1000 Hz as
> limited by the inductance of the supply system cabling.
>
> • Mechanical Testing: frequency sweeping from 0 to 1000 Hz with
> maximum tokamak poloidal and toroidal fields to monitor for coil
> mechanical resonances. None were detected by strain gages on the
> stainless coil sheath. Operation at 4.5 kA, 250 Hz for 12,500 cycles
> at maximum poloidal and toroidal fields to perform fatigue tests. No
> failures were observed.
>
> • Hi pot testing: the installed coils were tested to 4.8 kV for early
> mechanical assemblies before brazing or welding. The installed coils
> were tested to 1.2 kV after installation. Prototype developmental
> component parts were tested to failure that occurred between 6 and
> 9 kV. AC hi pot testing was performed with a ± 400 V, 10 kHz square
> wave for \~36 hrs. with no insulation failure.

Based on successful manufacturing, installation, and operation of the
prototype coils, we are confident that the full 12 coil set should be
ready for operation in 2003.

**Acknowledgment**

> Work supported by U.S. Department of Energy under Contract
> DE-AC03-99ER54463.
>
> **References**

\[1\] A.G. Kellman, "Recent Progress from the DIII--D Program," to be
published in Fusion Engineering and Design.

\[2\] "Properties of Copper and Copper Alloys at Gryogenic
Temperatures," NIST Monograph 177, National Institute of Standards and
Technology, (1992) 4-22.

Fig. 1.  Installation of 2 prototype 1 coils, October 2001.

Fig. 2.  Construction of internal control coils for a set of 12 single
turn, actively water-cooled, copper loops. The coils will be located
above and below the midplane and behind the protective armor tiles.