Convection Cell
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http://www.jlab.org/~zwzhao/polhe3/ccell
from Al Tobias
As a first step toward A1N target development, it was agreed to produce two (2) prototype all glass cells of convection style. As a first step toward a prototype design, I am sending you some preliminary drawings. The first drawing contains nominal dimensions of our current convection cell. The second and third drawings show a cell with larger pumping chamber and larger target chamber, respectively.
1) Can a convection cell of dimensions shown in the first drawing fit into the JLab target oven? 2) What is the largest pumping chamber the current JLab target oven can handle? 3) We may want to extend the length of the transfer tubes, so that the distance between the pumping chamber and target chamber is greater than the amount shown in all of the attached drawings. What limit would be imposed given the current JLab target?
http://www.jlab.org/~zwzhao/polhe3/ccell/New%20Convection%20Cells.pdf
from Alan Gavalya
Here are a few more views to help. The largest unobstructed opening (window) is 3.13 inches. The target was angled at about 41 degrees to allow vertical pumping (EPR_COIL_IN_OVEN_SECTION_CUT attached) so moving the target in line with the oven requires moving the support over with respect to the design position. We may not have space to do this. There are probably more considerations to be found in the future.
http://www.jlab.org/~zwzhao/polhe3/ccell/EPR_COIL_IN_OVEN_SECTION_CUT.pdf http://www.jlab.org/~zwzhao/polhe3/ccell/img-330113807-0001.pdf http://www.jlab.org/~zwzhao/polhe3/ccell/img-330114936-0001.pdf
from Yi Qiang
The designs are pretty good to me. I'm thinking about another variation with narrower transfer tube for the wall effect. The inside dimension of the current oven which holds the pumping chamber is 6.4 inch (W) x 6.4 in (L) x 7.0 in (H). The side windows which allow laser passing through are 3.8 inch in diameter, they are horizontally centered on each side, and vertically, their centers are 2.625 inch above the bottom of the volume. So in principle it should be large enough to hold all your designs with maximum 2 inch pull-offs, but the room for the laser spot is really tight. Also, a new bottom piece needs to be designed to mount the new cells. More drawings of the current oven and the Transversity/d2n cells can be found here: http://www.jlab.org/~esp/Transversity/Target_Ladder_with_Oven_Rev-/CS85_Oven_Assy_RevC/
from Al Tobias
Attached is the final version of the 1st Convection cell (with string) we'd like to have constructed. It has smaller diameter transfer tubes and its "height" is slightly larger.
The 1-inch bulb was suggested by Gordon as a well defined and larger volume of gas on the transfer tube (TT) to perform pulse NMR. The bulb would allow us to test polarization measurement techniques on the TT in case any future designs of a target cell limit us the ability to do such measurements on the target chamber.
Scott used 1-inch spherical cells in his research.
As for the bulb, some of the motivations: a) to provide for more volume than the transfer tube alone: keep in mind that a TT cylinder with 6mm I.D. is much less volume then a sphere with 22mm I.D. b) allow for doing "Spin Echo" measurements: for various reasons, a spin echo may allow one to more consistently measure the NMR amplitude for polarization. For example, in cases where the FID signal is too short (decays too quickly due to field inhomogeneities ie. short T2*) it becomes difficult to fit the FID back to time zero to extract an amplitude. This is especially true when there is much RF "ring down" from the initial pulse that contaminates the beginning of the FID. The solution is to do a spin echo in which case one just measures the peak of the echo. To do a spin echo successfully, you want most of your gas in a region of good RF (H1) field homogeneity since the pulses used in a spin echo are important to flip the 3He spins properly. Gordon's idea is to use a solenoid twice the length of the bulb so most of the gas lies well inside a homogeneous RF field during pnmr measurements, with minimal gas within the TT and other edges of the solenoid where the field is less homogeneous.
In a typical spin echo, one first applies a pulse that tips the spins as much as 90-deg, just enough to get a large signal. During this time, the spins dephase. After a time t, one applies a 180-deg (pi) pulse causing the spins to rephase an additional time t later to give the spin echo. We simply measure the amplitude of the echo.
As for tube tolerance, I can specify an outer diameter on the drawing if you prefer. I spoke to Mike and asked him what he thinks the OD of the tube would be if I ask him to make an ID of 5 to 6mm. He said the OD would be about 8 to 9mm (ie. 1.5mm wall thickness).
http://www.jlab.org/~zwzhao/polhe3/ccell/Next_Convection_Cell_16may2011.pdf http://www.jlab.org/~zwzhao/polhe3/ccell/uva_GE180_string_22apr2011.pdf
from Mike Souza
This new design represents some major changes compared to the Transversity Experiment done several years ago.. Though this particular cell does not require precision target windows. The Pumping chamber is larger and more complex and each of the transfer tubes are significantly longer. All of this has to be made from 16 mm od. tubing.
Based on those parameters I am quoting as follows:
1 ea GE 180 Convection cells w/ standard wall windows $1,800
1 ea 3 -piece GE 180 String manifold graded to pyrex $220
For GE 180 Convection Cell as shown using precision blown windows There will be an additional $600 per cell depending upon window thickness and tolerances identical to the Transversity Cells
