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Magnet
These pages cover different aspects designing a VPPEM instrument
Page 1
Page 2
Instrument Design
The magnet was required to be compact, and magnetically well shielded
with low stray fields. Because the magnet is for use with a microscope
the working volume can be small. A small volume implies the stored
magnetic energy is low, and with low stray fields, it makes the magnet
safe while operating, and during quench events.
The spatial resolution of the VPPEM depends directly on the strength of
the magnet so that a high field superconducting magnet is desirable.

Naturally, the performance of the electrostatic elements are degraded in
the presence of stray magnetic fields. Therefore, it is important to have the
electrostatic elements well shielded, and ideally at a distance from any
magnetic fields.

However, the experimental arrangement includes dual photon beams
intersecting at the sample position which is at the center of a strong
magnet. The geometry of the incoming photon beams, and the CHA
means that the magnet, and the CHA input lens with its mu-metal shield
are in close proximity.

Having a well shielded magnet is an intrinsic part of the design for the
VPPEM because the sudden termination of the magnet field along the
optical axis using a ferromagnetic shield is the defining action producing
the image.

Preventing stray fields from escaping a strong magnet is challenging.
However, the VPPEM microscope is only imaging a few tens of microns
the working volume of the magnet can be made relatively small (1x1x1
mm). This means the magnet itself with its shielding can be made small.
Shielding a small magnetic volume is much easier.
We chose to use a superconducting magnet in an electromagnetic
configuration. That is to say the ferromagnetic shielding, and the
superconducting solenoid are closely coupled in a magnetic circuit. This
has the advantage that the ferromagnetic shielding adds to the magnetic
induction, while remaining compact.
High temperature superconducting (HTS) YBCO tape was used for this
prototype. This choice allows us to either use a liquid cryogen or a close
cycle Gifford-Mahone cryocooler. The microscope is in an open laboratory
at an endstation, and handling cryogens in that environment means extra
oversight in operations. The Gifford-Mahone cryocoolers are moderately
compact making them a suitable fit for use with a microscope.
One difficulty with winding a compact HTS tape coil is the requirement to
make connections between the pancakes. With larger coils these
connections are made with straps soldered across the center of the
pancakes. With a small coil having straps soldered arcoss the center of
the pancakes is virtually impossible.

We used a novel winding method. This novel method does not use any
soldered joints within the magnet. This new winding method is
straightforward to implement, and is extensible to multiple pancake coils.
An additional benefit is that the inner coil winding does not have any
heating from the joints under load, removing a source of failure.
The tape for an HTS magnet is wound as a set of ‘pancakes’, not layers.
The tape itself can be bent around but must be carefully layered, it cannot
be folded over.

If the magnet has more than one pancake coil, the current from the center
turn of the coil must be connected with the current at the outside turn of the
next coil if the coils are wound in the same direction. If the coils are wound
separately, and then reversed in pairs together then the inner turns must
be connected together by soldering a strap across. Soldering a strap
across is the normal method used for large pancake coils.
Cooling is based on a Sumitomo CH204S cold head, using a HC-CE1 helium compressor. The CH204S cold head has
two cooling stages. The first state a has a cooling power at 77K of 16.2 Watts. The second stage has a cooling power at
20K of 8.1 Watts.

We decided to use a cryogen free design using solid conduction to cool the magnet. To affect this the coil shown in the
previous page  was encapsulated in a thermally conducting shell using OFC copper, this is shown attached to the cold
head above.

A thermal paste of diamond grit in an Apeizon Type N cryogenic vacuum grease matrix made into a stiff putty was
molded onto the conductors holding them in thermal contact with the body of the cold head along their length.  The
encapsulation of the conductors can be seen as the grey putty on the sides of the cold head in the first picture.

Because the putty is electrically nonconducting no additional isolation such as a Kapton film was required. This makes
assembly easier and more reliable.
The ferromagnet shield was made from low carbon steel ASTM A
108. It was attached to only one side of the copper shell using
three #6 Ti-6Al-4V bolts, this titanium alloy having a very low
thermal conductivity at cryogenic temperatures.

The final temperature of the ferromagnetic shield was just under
80K. We can estimate the heat loss to the room temperature
vacuum system.

The ferromagnetic shield has an area of 2x10-2 m2, and with an
estimated emissivity of 0.2 we would expect the radiation loss to
be approximately 1.9 W. With a high conductivity from the 8.1 W
(20K) cold head final stage to the magnet, we can afford to
operate the magnet without a radiation shield.  Note that the
ferromagnetic shield is not acting as a radiation shield in this
arrangement because it is cooled directly from the magnet, not
from the cold head first stage.
The magnet was modeled using  Field Precision LLC, PerMag.
A plot of the simulation is shown in Figure 11. The results are
rotationally symmetric around the horizontal axis.
The magnet has two central pancake coils of width 4 mm,
minimum radius 8 mm, and maximum radius 25 mm.

The two coils are separated by 4 mm. For the simulation the coils
are split into four layers of 4, 4, 4, and 5 mm thickness.

Smaller layers than this make a minimal difference to the
simulation. The currents flowing in each layer are calculated as
the number of 56.5 micron turns in each layer times the supply
current.  
The supply current was adjusted to 68 Amps to give a central
on-axis field of 2.0 Tesla. The maximum field below the bottom
edge of the coils is then 2.3 Tesla.

The stored energy is calculated to be in the region of 25 Joules.
There is a very significant contribution to the field from the mild
steel yoke. Without the yoke, the on-axis field is only 1.45 Tesla.
The as-calculated yoke acts as an efficient magnetic shield. At 50 mm along
the axis the calculated on-axis field has decayed to 10 Gauss, at 85 mm the
field is less than 5 Gauss. It is less than 5 Gauss 60mm off-axis
The magnet reached a 2.0 T central field using a current of 66.5 Amps
compared with the calculated 68 Amps.
The electromagnetic properties of the magnet are ideal for our purposes. The
stray field is very low, less than 2 Gauss, and the current required is a modest
66.5 Amps. Most importantly the failure mode is very benign. The field decay
after a quench takes approximately 5 seconds.
The magnetic circuit is a core part of the instrument. For the new VPPEM
at NSLS II we designed and built a custom superconducting magnet.
Compact 2.0 Tesla fully shielded superconducting magnet.
A two-coil pancake wound using our new method