The Outside Vacuum Chamber: The outside shell of the magnet is made from stainless
steel and is about 1/8" (3 mm) thick. The shell took about 8
hours to cut open with an air-powered cutoff grinding tool. The
picture right shows the outside of the liquid nitrogen vessel with a
portion of the outer can and aluminized Mylar super-insulation removed.
The Mylar insulation reflects the infra-red heat radiation from the
inside of the room temperature surface. There are about 165 layers of
the aluminized Mylar insulation. During normal operation of the magnet
this 'space' is evacuated under high vacuum. The high vacuum is
maintained by the cryo-pumping action of the 4K liquid helium can.
The Liquid Nitrogen Vessel: The liquid nitrogen vessel is made of
approximately 3/16" (4.8 mm) aluminum. The vessel required
about 30 minutes to cut open with a panel saw. The picture at the
right shows the interior of the liquid nitrogen vessel. During normal
operation this space is filled with liquid nitrogen (77K). The
liquid nitrogen level sensor and the passageway for the magnet charging
lead can be seen inside of the liquid nitrogen vessel. Note that
the liquid nitrogen reservoir space is mostly above the magnet. The
purpose of the liquid nitrogen is to act as a less expensive
refrigerant to block infra-red radiation from reaching the liquid
helium vessel.
The 20 Kelvin Radiation Shield: The 20 K (Kelvin) radiation shield is made of
aluminum and was wrapped with alternating layers of aluminum foil and
open weave gauze. The purpose of the 20 K shield is to block
infra-red radiation coming from the 77 K liquid nitrogen vessel. The
elimination of infra-red radiation lowers the liquid helium boil-off
rate. The 20 K radiation shield is thermally isolated from the liquid
helium (4.2 K) and liquid nitrogen (77 K) and reaches an intermediate
temperature near 20 K. The picture at the right shows the 20K
radiation shield after the inside of the liquid nitrogen vessel is
removed. During normal operation the 20 K shield is surrounded by high
vacuum.
The Liquid Helium Vessel: The liquid Helium vessel is made of stainless steel and is wrapped with a single layer of aluminum foil which acts a radiation shield to help lower radiant heating. The liquid helium can is about 1/16" ( 1.6 mm) thick and took about 1 hour to cut with the air-powered cutoff grinder. In the picture at the right, the 20 Kelvin shield has been removed, showing the outside of the 4.2 K liquid helium vessel. Note that the copper bore tubes are clearly visible in the picture.
The Liquid Helium Baffle: Inside the liquid helium vessel around the
magnet is an aluminum baffle. This baffle acts both as an infra-red
radiation shield and protects the superconducting magnet from any
fluctuations in the liquid helium reservoir, particularly during a
liquid helium refill. This is a critical feature because
superconducting magnets at low fields, such as a 54 mm bore 270 MHz,
are not fully submerged in liquid helium. Higher field superconducting
magnets, such as 500 MHz, must maintain the superconducting solenoid
fully immersed in liquid helium. The helium vapor above the liquid is
actually sufficient to maintain superconductivity of the 270 MHz magnet.
When the magnet is near empty the top of the magnet is at about 6 K,
the magnet will stay superconducting to about 10K before it quenches.
However, this also means that any disturbance of the vapor temperature
could quench the magnet. The only path for the liquid helium to reach
the magnet surface is a small opening (1/4", 6 mm) at the very
base. During a liquid helium refill, the magnet is protected from an
accidental discharge of helium gas that might otherwise cause a quench.
The picture at the right shows the liquid helium baffle with the
outside stainless steel liquid helium vessel removed. The black
section is cloth tape wound around the baffle to secure the liquid
helium syphon tubing.
The Superconducting Magnet: The picture at the right shows the internal
liquid helium baffle cut away to expose the superconducting solenoid
wrapped in black tape. A section on the black tape was cut away to
expose a clear tape through which the superconducting shim coils are
visible. The superconducting shim coils are wound on the outside of the
solenoid and are used to adjust the magnetic field gradients at
the probe in much the same way as room temperature shim coils. The
superconducting magnet wire is made of a copper-clad niobium-titanium
alloy. This magnet contains approximately 12 miles (19 km) of
superconducting wire.
Quench Resistor & Charging Plug: The picture at the right shows a close-up of one of the quench resistors and the plug for magnet charging partly inserted into the magnet. The quench resistors protect the magnet during a quench by dissipating the heat generated from the 85.5 kJ of energy that is stored in the energized magnet. A quench is what happens when a superconducting magnet stops superconducting and has a finite resistance.
Superconducting Shims: The superconducting shims can be seen in
the picture at the right. The superconducting shims are wound on
the outside of the magnet. This allows the amount of wire
required for the shim to be adjusted to meet the shim strength needs
for the magnet. The vertical wires are from one of the radial
shims, X, Y, ZX, ZY, XY, or X2-Y2.