Transport System

Reason for a transport system?

Atoms need to be shuffled from the glass MOT chamber (at ~10-9 torr) to the much higher vacuum steel lattice chamber (at 10-11-10-12 torr) in order to increase the lifetime of the atoms during the "science" part of the experiment. The lifetime of the atoms is the average time between collisions with background molecules in the chamber environment - collisions that knock atoms out of the weak optical lattice trap.

2chambers

Theory

Basics

Atoms are magnetic. This means that they feel a potential in a magnetic field:

(1)
\begin{align} U=\mu_B\mathit{g}_Fm_FB \end{align}

For 40K, $g_F=2/9 > 0$, so atoms will be attracted to the magnetic field minimum. A magnetic field can be created by running current through a coil. If two coils are placed a diameter apart from each other like so:

qtcoils

the resulting magnitude of the magnetic field will be:

(2)
\begin{align} B=B'\left(x^2+y^2+4z^2\right)^{1/2} \end{align}

where $B'$ is the value of the magnetic field gradient and $z$ points in the axial direction. Note that the field is linear rather than harmonic. This is a quadrupole magnetic trap. If a bunch of these coil pairs are put side by side, the magnetic minimum - and therefore atoms - can be made to move smoothly along some trajectory (see Alan's wicked animation of this on the top left of this page).

Controlling Atom Transport

The position and shape of the magnetic field is controlled by three currents at any instant in time. In order to keep a cloud of cold atoms moving smoothly from one place to another with minimal heating, we solve for these three currents according to three conditions:

-> setting the gradient

(3)
\begin{align} \frac{dB}{dz}=100G/cm \end{align}

-> setting the aspect ratio

(4)
\begin{align} \frac{\frac{dB}{dx}}{\frac{dB}{dy}}=1.72 \end{align}

-> setting the field minimum to be zero wherever the atoms are

(5)
\begin{align} B\left(r_{atoms}\right)=0 \end{align}

Transport system

This project was started by Josefine Metzkes and Alan Stummer in January 2008. Josefine designed the first current scheme, ordered coils and designed the system mounts. Alan took care of all the electronics. Michael Yee then took over from Josefine, integrating the transport system to an experiment that was redesigned in summer 2008, designed and wrapped the "push" coil and mount, and put things together at the end with Dylan. Dylan helped Alan with wiring the electronics.

Horizontal and Vertical Transport

  • Michael Yee has written has written a couple summary documents:
  • All coils, including the MOT but not the push, were wrapped by Oswald. Oswald has poor customer service, is slow, expensive, but produces good quality coils. However, given our accumulated coil wrapping expertise, we would now choose to spend the time to wrap coils ourselves.
  • Calculations were done using the following mathematica scripts:
  • The transport system mounts were made at the physics dept. machine shop. They are made out of brass, with an insulating plastic layer to inhibit eddy currents. See Josefine Metzkes lab book.

Push Coil

  • Coils were wrapped using MWS wire industries .040"x.090" HPN-155 RED rectangular wire.
  • Wrapping the push coil required making a jig with a specified ID and parallel plates some specified distance apart. The jig was machined from a standard piece of PVC or other machinable plastic. The distance between the plates should be as close to the total height of the coil, with less than a mm of extra space. By specifying this distance exactly, you can wind a tighter more exact coil.
  • In our case, LePage 24 hour epoxy was used.
  • Acetate transparencies were used to cover the jig so that the coils could be removed from the jig after the coils had set. The acetate could then be easily ripped off from the coils. We found that the epoxy actually did not adhere the acetate to the coil and was therefore ideal.
  • Perhaps it's worthwhile using thermal epoxy rather than LePage epoxy. It was also shown that the thermal epoxy doesn't glue acetate to the coils and can therefore be used as well.
  • We waited a day for the epoxy to fully set.
  • Coils were wound in a "double-stack" fashion, starting with the jig at the middle of the wire length, with one person wrapping the top layer in a clockwise direction, and another person wrapping the bottom layer in a counter-clockwise direction. This is a very important point, and allows us to finish with the coil leads on the outer coil windings. If you start from one end and wrap the coil, you'll end up with one lead starting at the inner diameter and this can cause problems in tight places.
  • LePage 11 epoxy was applied on the inner side of the wire.
  • Separate double-stack coils were epoxied together using a thermal epoxy: Epoxies Etc resin model # 50-3150 RF with catalyst CAT.150CL13….In retrospect, this epoxy - or some other thermal epoxy - should have been used in wrapping the coils as well. However, because it is on for so short a time (<1s), the push coil is not expected to heat up too much so this shouldn't matter much.
  • See Michael Yee lab book #4 for details.
  • Jigs used to wrap the push coil subcoils. Notice that the inner diameter was lathed to the desired dimension and the distance between the top and bottom plate are set.
jigtogetherjigapart

Final construction (Feb 10-20, 2009)

  • A system mock-up was put in place to determine the "choreography" of water cooliing - which pipes should connect to which, the placement of swage connections, etc.
  • Copper pipe was bent to fit into place on the brass mounts
  • The Copper pie was soldered to the brass mounts
  • Coils were encapsulated with a thermal epoxy (Epoxies Etc resin model # 50-3150 RF with catalyst CAT.150CL13 - has a thermal conductivity of 2 W/mK)
L1010097width500.jpg

Electronics

Alan Stummer's page on the transport system

L1010102pix300.jpgback500pix.jpg

Operation

1) CATS power supply must be plugged in
2) Main coil supply must be set between 8V and 20V - the smaller the better.
3) Press Green button in the front of the CATS


Coil Resistances

Coil Measured Resistance ~ Calculated Resistance
Push 338mOhm 316mOhm
MOT 403mOhm 358mOhm
Horiz #1 86mOhm 70mOhm
Horiz #2 85mOhm 70mOhm
Horiz #3 87mOhm 70mOhm
Horiz #4 87mOhm 70mOhm
Horiz #5 87mOhm 70mOhm

Power Supply Operation

Set Supply Voltage (or Feed Forward)

  • This allows us to control the power supply voltage using the AdWin
  • 1V AdWin input corresponds to 4V supply voltage (for a maximum of 40V supply voltage)
  • Although one can always put 0-10V input voltage from the AdWin, the supply voltage will never exceed the manually set value on the Master supply (set using the voltage knob).
  • The AdWin channel that controls this is analog channel #18

MOT fast switch

  • to switch off the MOT quickly, send a digital signal to the MOT switch
  • "1" is no current, "0" allows current.
  • This is digital channel #16
  • To turn off the MOT quickly, send in a digital signal
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