Engineering and Design of the Steady Inductive Helicity Injected Torus (HIT-SI)
P. E. Sieck, T. R. Jarboe, B. A. Nelson, J. A. Rogers, U. Shumlak
University of Washington
|
|
Engineering and Design of the Steady Inductive Helicity Injected Torus (HIT-SI)P. E. Sieck, T. R. Jarboe, B. A. Nelson, J. A. Rogers, U. Shumlak University of Washington |
|
Steady Inductive Helicity Injection (SIHI) is an inductive current drive method that injects helicity at a nearly constant rate, without open field lines, and without removing any helicity or magnetic energy from the plasma1. SIHI directly produces a rotating magnetic field structure, and the current profile is nearly time independent in the frame of the rotating field. The Steady Inductive Helicity Injected Torus (HIT-SI) is a spheromak designed to implement SIHI so that the current profile in the rotating frame is optimized. The geometry of HIT-SI is presented, including the manufacturing techniques and metallurgical processes planned for construction of the close-fitting flux conserver. The flux conserver is made of aged chromium copper with 80% the conductivity of pure copper. The detailed electrical insulation requirements in the helicity injector design lead to a complex o-ring seal and a plasma-sprayed alumina insulation coating. This has prompted the construction of an o-ring prototype test fixture having the main features of the o-ring design and the alumina coating. The design and evaluation of this fixture is presented with vacuum and voltage test results.
1 T.R. Jarboe, Fusion Technology, 36 (1), p. 85, 1999.
The Equilibrium Region of HIT-SI is a "bow tie" spheromak with a close-fitting flux conserving shell.
There are two helicity injectors on HIT-SI. The injectors are located on opposing sides of the equilibrium region, and are oriented at right angles to one another.
 |
 |
Side A |
Side B |
Each helicity injector uses an iron core transformer. The transformers are shown below, with the vacuum tank removed for clarity.
The injectors require another coil, shown below with the transformers removed.
 |
 |
A prediction of the HIT-SI equilibrium |
Shear profiles for a "tuna can" and a "bow tie" spheromak |
Pressure-driven modes in a spheromak can be stabilized by shear. HIT-SI is designed to take advantage of shear stabilization by shortening the symmetry axis (r=0) to form a bow tie shape, drastically lowering q at the symmetry axis. A greater change in q results in higher shear over the equilibrium region and a higher beta.
Helicity injection at the edge will drive a hollow current profile. This gives a lower lambda at the magnetic axis, also contributing to higher beta.
If we look at a steady inductive helicity injector by itself, we see that it operates like one half of a reversed-field pinch.
The injector flux is varied sinusoidally, with one half cycle shown below.
The transformer provides an electric field in the injector, and the injector flux coil provides complete flux tubes to link with the equilibrium plasma.
 |
|||
t0 |
t1 |
t2 |
t3 |
Now we look inside the black boxes. Notice that the field lines reconnect with the equilibrium as the injector flux changes direction.
 |
|||
t0 |
t1 |
t2 |
t3 |
Other methods oscillate the equilibrium toroidal or poloidal flux by a small amount in phase with an oscillating voltage. Helicity and power are ejected from the plasma when the voltage is negative, leading to plasma wall interaction.
In this method the voltage is only applied to the injector flux which changes sign with the voltage, allowing helicity and power to always be injected.
Helicity injection is constant giving a constant-optimum profile. The profile for helicity ejection is greatly different from that of injection.
The picture below shows the flux conserving shell of the HIT-SI device.
All the pieces shown must be electrically insulated from each other. This insulation scheme is necessary because there are many time-variant magnetic fluxes directed around the device. The following table names the magnetic flux responsible for each insulating break.
Chromium Copper (CDA designation C18200) is a copper alloy containing 0.6% to 1.2% chromium by weight.
C18200 is age hardenable. A typical heat treatment consists of solution treatment at 1000°C, a rapid quench, and aging 2 hours at 500°C.
During aging, the chromium forms a fine precipitate that strains the surrounding lattice structure. This impedes the motion of dislocations, thereby hardening the material.
The electrical conductivity of the surrounding material increases as the chemical composition approaches pure copper.
After the thermal cycle, the conductivity will be approximately 80% that of pure copper.
|
The inner cone (red piece, left) and the outer cone (light blue frustrum, shown at a smaller scale) will be spun from flat plates of chromium copper. |
|
|
The bulk of the injector duct cover (dark blue, right) will be rolled into shape from a plate. |
|
Electron beam welding (EBW) will be used to join parts when brazing is not feasible. In EBW, a high-energy electron beam is used to quickly melt the base metal in a small area surrounding the joint. EBW was chosen over other welding methods for its performance on high thermal conductivity metals. EBW is used as little as possible in this design because the welding process is highly sensitive to impurities in the C18200 alloy.
EB Welded joints in HIT-SI:
|
|
Joining a flange to the outer cone |
Joining the small cone to a flat plate |
There are two options under consideration for the construction of the injector ducts. The first option is to forge the duct into rough shape, then machine details. This would be less expensive, but there would be no guarantee that the forging would be free of voids.
 |
 |
Forged Injector (dimensions in inches) |
Machined Injector |
The second option is to machine the injector duct out of a single large billet of chromium copper. Despite the cost of this method, it may be the only way to produce a strong, leak-free injector.
Brazing or welding the duct together from separate parts would be difficult, as the fixtures necessary for such operations would become highly complicated.
Brazing is the most efficient means of joining Stainless Steel to copper. A gold braze (BAu-4 Au+Ni eutectic) was chosen because the recommended brazing temperature is near the solution treating temperature of Chromium Copper. Combining the brazing and heat treating operations results in:
Brazed joints in HIT-SI:
 |
 |
Conflat |
Diagnostic Gap Ring |
 |
|
One o-ring lies completely inside the other so that the o-rings don’t intersect. The double o-ring seal gives a better vacuum than a single o-ring. |
 |
The complicated sealing geometry of HIT-SI makes it necessary to bring o-ring sections together in a tee.
At right is the molded o-ring joint for HIT-SI. A solid molded tee is less likely to fail than a tee made from glued circular stock. Also, greater detail can be put into a mold. |
 |
 |
|
Alumina will be plasma sprayed onto chromium copper surfaces to provide added electrical insulation between sealing surfaces. Surfaces will be masked so that plasma-facing surfaces do not receive an alumina coating. The bottom of the o-ring groove will also be masked. The sealing surface opposite the o-ring groove will receive a 10-mil alumina coating, and the coating in this area will be lapped and polished into a suitable sealing surface. The coating should have a dielectric strength of 500 V/mil.
Four test strips were created to evaluate the performance of the alumina coating on the curved surfaces of HIT-SI o-ring grooves. Each groove had different edge radii.
 |
 |
Uncoated test strip |
Test set-up |
Despite the varying groove geometry, the breakdown potential was approximately 1.7 kV in each case.
A test fixture is being constructed to:
 |
O-ring Test Fixture |
The o-ring test fixture has provided experience with the EB welding, brazing, aging, and machining of C18200. The test fixture o-ring is functionally equivalent to the HIT-SI o-ring:
|
|
Test Fixture O-Ring |
HIT-SI is a high-beta bow tie spheromak designed to implement SIHI. The close fitting and flux conserving vacuum vessel is made of aged chromium copper. The implementation of SIHI necessitates a complex insulating o-ring seal. The o-ring groove is insulated with plasma sprayed aluminum oxide. A test fixture is being constructed to ensure that the o-ring design will hold vacuum and voltage.
