Dear EG1 collaborators,
at our meeting last week we decided that it is about time to freeze
some of the still unknown design parameters for the polarized NH3/ND3
target, so work can begin on the design of the "missing parts".
Specifically, we need to speficy:
1) The number and material of targets to be used (e.g., one NH3, one
ND3 and one dummy with either C, CO2 or N2)
2) The shape and size of the target cells and their placement on the stick.
3) The innards of the cryostat chamber which will contain the target cells.
Specifically, we need to discuss the shape and position of the microwave
trumpet, the depth and diameter of the chamber, the material and arrangement
of entrance and exit windows, etc. This will have a strong impact on the
accessible phase space for outgoing particles.
4) The beam entry channel (with windows)
5) The heat shield, especially where it can be seen by outgoing particles
and the beam.
6) The (large!) outer windows in the vaccuum chamber. Especially, we need
to discuss whether there will be different material/thickness for the beam
and the scattered particles, what the minimum and maximum theta angles
can and should be (and the overall angular coverage, also in sideways
direction), and whether we want to employ "spokes" on the window in the
coil shadows (? - note that the longitudinal field may wash out the definition
of what these are).
7) Related to the rest, where we want the target to be positioned relative
to the nominal unpolarized target point. There are good reasons to move it
back by maybe 1/2 meter to increase the coverage at low Q2.
I have arranged a meeting at JLab next Tuesday, 9/16, all afternoon. I will give you a room number as soon as I have one. If you cannot attend but would like to contribute to the discussion, please use our list to send email to everyone (just send your mail to eg1_run@cebaf.gov). However, please try to be very specific - we cannot any longer say things like "it would be nice if the windows were as thin as possible". Try to make estimates of what kind of window thickness would still be tolerable, and back up your claims with (at least "back-of-the-envelope") calculations. (Ideally a fullblown GSIM simulation should tell us what the impact of our design decisions might be, but that's asking a lot, I know). I hope to get at least a reference design out of this meeting, so we know what to scrutinize for possible Physics show stoppers.
-- Hope to see you next week,
- Sebastian
Subject: EG1 Polarized Target Meeting at JLab, 9/16
Date: Thu, 18 Sep 1997 09:39:44 -0400
From: Sebastian Kuhn
Dear EG1 collaborators,
here are the minutes of Tuesday's meeting. If you find that my personal
bias led me to forget or misrepresent anything, please send an email
to the distribution list to put me straight.
I think we had a rather productive meeting (if that isn't an oxymoron).
While we haven't exactly pinned down all the design details, we have
made some progress toward what I would call a "reference design". This
will serve us as a starting point; any changes should come from intense
scrutiny of this reference and hopefully can be incremental.
Starting from the front to the back, I describe the general outline
of the target as the beam (and scattered particles) see it:
*****
The first item we talked about is the beam line entering the target.
We agreed it should have a gate valve at the upstream end, and attach
directly to the target vacuum chamber downstream. The vacuum chamber
entry window will separate the beam line vacuum from the target vacuum;
meanwhile, this piece of beamline can be pumped out and then connected
to the rest of the accelerator vacuum by opening the gate valve.
ACTION NEEDED: We need to design the entrance window. It will be
straightforward, maybe 2 mil of Al or 1 mil Titanium, 4 cm diameter.
After entering the target vacuum chamber, the beam will go through a
tube that protrudes into the LHe target cryostat. The downstream end
of this tube will have to have a superfluid-tight window of about 4 cm
diameter. One possibility would be 1 mil of Ti, welded to the rim
or machined directly out of a rim piece of the tube. Up to this point,
the beam has then seen about 0.1% of a R.L.
ACTION NEEDED: Design of the end piece of this tube with window.
The beam then goes through the LHe of the cryostat and the target cell
itself. Our reference design for a target cell is the following:
a 1 cm long, 1.5 cm diameter cylinder made of Kevlar, Torlon
or similar material, with (thin) Al entrance and exit windows,
with an NMR coil embedded in the target material inside, but in a
"Helmholtz"-type arrangement which minimizes the coil material within
the central 1 cm diameter of the cylinder (the coils will be shaped
in a "drooping" fashion around the central 1cm dia. cylinder). We can
then choose to either raster over the whole volume (gives more accurate
polarization measurement) or over the central 1 cm only (better dilution
factor).
We decided to have 4 target cells: one ^15NH_3, one ^14ND_3, one con-
taining the "background" material (see below) and one empty one (which
can be used to "park" the beam, or for additional background studies.
Possibly one could even hook it up to NMR as well and put a second
NH3 target in there). We plan to foresee the possibility to anneal
the targets during the run (may be needed once or several times per
week).
ACTION NEEDED: Both UVa and Genova will try to build prototype cells
following these general outlines. We need to study whether we can
really get the coil (mostly) out of the way, whether the NMR signal
(TE) will be large enough, what packing fraction we can get with these
rather small cells, etc. It will be hard to accomodate longer target
cells, since that would give us too big a beam spot downstream. The
present design is already cutting it close.
The 4 target cells will be spaced at 2.5 cm distance along the target
stick, so that an overall travel of 7.5 cm (+ additional room for
alignment) is needed - well within the maximum of 10 cm. We MIGHT
consider adding a "ring" of 1.5 cm right below the lowest cell to
have another spot for the beam to "park", which would increase the
travel distance to 9 cm + alignment.
The trumpet for the µwave will come in from behind, snake around and
blast the cell from the side (about 90 degrees). Marco and Don made
a visit to a company that claims they can fabricate the corresponding
wave guide; it is probably impossible to move the trumpet into the
shadow region at 60 or 120 degrees. However, because the target volume
is rather small, the trumpet can be small, too, and won't obstruct
too much of the sideways openings.
The target material for the "background" target is still open to dis-
cussion. Ideally, it would be N2. However, there may be difficulties
associated with this (it obviously freezes below LN2 temperature - LNe
would work), and we might not know how much we got into the cell.
Alternatives range from conservative (Carbon, CO2) through
exciting (N2O) to exotic (N2H2 = hydrozine?, N3H = hydrolysic acid ??).
Mike Seely (and perhaps others?) will look into the latter materials,
to determine freezing point, density, radiation properties (if known),
and possible hazards (explosion, poisoning, uncontrollable laughter).
It would be better if we wouldn't have to introduce NEW nuclear species
(like O, C) into our background target.
In any case, I estimate that the beam will be going through an extra
0.5 - 1cm of LHe in front and behind the target cell, since the
windows (especially the outgoing one) will be bulging, and we need
enough clearance for the vertical motion. All in all, I expect the
target plus LHe plus entry windows to add up to about 1.8% of a
radiation length. We need to try and stay below that for the rest
of the material that the beam and the outgoing particles have to
go through. Specifically, we have 3 large windows (exit window of
cryostat, heat shield, and vacuum exit window) in forward direction
(which have to accomodate up to +/- 50 degrees in theta without
shadowing anything beyond 7 degrees) and the same in the 6 sideways
ports.
Coming to the latter first: The 6 sideway ports on the cryostat
chamber will simply be machined-down sections of the (stainless steel)
walls. Hopefully they will be thin enough (<5 mils = 1% R.L.)
ACTION NEEDED: Marco (or Don or Mike?) will contact Oxford to determine
what the present dimensions of these sideway ports are, and how thin
we can have them shave them.
The front window will have to allow for the beam AND the scattered
particles up to 50 degrees to go through unshadowed. This precludes
the use of any "spokes" or "rings" on this window. However, we can
design a lid (2 possibilities were shown at the meeting) which brings
this window rather close to the target cell exit (maybe 0.5 cm), which
is desirable anyway. In that case, a 5-6cm diameter would be sufficient,
which could be accomodated by another 1-2 mil Ti foil.
ACTION NEEDED: We need to design and draw the lid and decide how
the foil will be attached without any bolts/rim getting in the way
of vertical target motion.
The first really big window will be in the heat shield. Oxford may
have a design already, but most likely it won't fit our bill. Again,
there are side windows for the 6 side ports, plus a huge (about 67 cm
diameter) forward window. The problem is that this window needs to
have good enough heat conductance to cool it to near LN2 temperatures
right in the middle, where the beam goes through. Obviously, we could
make some composite design, with a large, somewhat thicker (1 mm?)
plate having a small (4 cm dia) hole in the center, which in turn is
covered by a thinner foil for the beam to go through. Fortunately this
is not a vacuum/pressure window, so few mechanical constraints exist.
It was also suggested that one could add "fingers" to the window
that are much thicker and lie in the shadow of CLAS' acceptance;
these could aid with the heat transport away from the center. However,
we can NOT have a central ring, since it would either clip the beam
or shadow some of the 7 degree scattered particles. Another idea
was to reduce the need for a truly thick heat conductor by having about
10 layers of superinsulation (that adds about 1mil) between this
heat shield and the outer chamber surface.
ACTION NEEDED: Obviously, here we need major design/prototyping work.
We also need to tap the experience of seasoned cryogenic target builders
(Hall C/A ?) and Oxford to assess what combination of thin foil,
fingers and superinsulation works best and lets us get away with the
least amount of material in the way of outgoing particles. Maybe we
will need to do tests with different configurations, using the real
target and reducing the heat shield thickness until the boil-off rate
becomes prohibitive. Don and Mike can help with info (and contact
Oxford to see what they've designed so far and what they think might
work), but clearly we need more help on this one.
Finally, we need the large vacuum exit window on the downstream side
of the can. (Again, we also need 6 side windows; however, they will
be relatively small and shouldn't pose huge design problems. That doesn't
mean we DON'T need to start working on their design now!). Here, we
need to cover about 80 cm diameter to withstand 1 atm +. Again, the
best solution is probably a composite, involving "spokes" (and here we
could have a small "ring" right outside of 4 cm diameter without
shadowing 7 degrees), a thicker large window (made out of a low-Z,
lightweight material - Kevlar, Mylar, you name it) with a hole covered
by a smaller (2-3 mil) Al foil (this foil can and SHOULD cover the
WHOLE outside of the window for electric and light shielding.
ACTION NEEDED: Again, major design, test and fabrication work. Needs
as much input from experts as we can get.
*****
Miscellaneous other items:
1) You'll find some action items above that don't have a specific
name attached to them. Of course, that should encourage you to take
over the task in question. However, we seriously need some engineering/
drafting manpower to design and draw all the parts we need. Unfortunately,
it didn't become clear to me at our meeting who this could be, or where
the manpower will come from (note that Walter Tuzel is no longer
available). Certainly, it will HAVE to be someone stationed at JLab
working closely with Mike Seely.
2) Volker reported that a straightforward beam transport calculation
showed that the solenoidal field from the target can wreak a lot of
havoc on the electron beam optics, especially at the lowest energy
(1.6 GeV). We need more detailed studies and consider possible
remedies (i.e., altering the incoming beam optics and/or the rastering
scheme) to at least partially compensate for this.
3) Mike reminded us that alignment will be a recurring problem unless
something is done to the insertion cart rails that would allow us to
move the target in in one smooth move. Right now, the rails are crooked,
which requires several LATERAL adjustments to be made during insertion
(to avoid bumping into the R1 drift chambers). We should discuss with
the Hall engineers (O'Meara?) whether and how the rails could be
shimmed/readjusted to make this possible. Our only alternative is to
have an alignment crew come in EVERY time we move the target in or out.
(We MIGHT be able to avoid that if we put reference marks close to the
once-surveyed "in" position; at least, this might speed the process up).
This reminds me that we haven't really understood how Oxford is proposing
to reference the position of the magnet relative to the outside of
the can, so we don't have to insert the target "open", but rather can
use survey marks (tooling balls etc.) on the outside for final alignment.
4) Now that we have a "reference" design, we can implement it in our
favorite CLAS Monte Carlo and see what all that material does to
our resolution, acceptance and reconstruction. We also still need
to look into optimal target position in z (back from the nominal
target point), CLAS field strength and polarity, etc. We should have
another meeting "soon" - maybe in late October - and hopefully some
results/answers to these questions.
OK, this is probably at least MY personal record for a lengthy email
message. 'nuff already! You can always erase it - our distribution
list keeps an archive, so you can go back there to read it again later...
Greetings -
--
- Sebastian
Subject: Meeting 11/10
Date: Tue, 11 Nov 1997 18:24:26 -0500
From: Sebastian Kuhn
Dear members of the Polarized Target in CLAS group,
yesterday we had our meeting to discuss progress on the design and
fabrication of our target. We received drawings from Genova showing
the present target stick design (see separate message to Marco Anghinolfi).
Derek Branford reported from his visit to Oxford on the 4th. He saw
bits and pieces of the target, and witnessed a cold test of the coils
(they have already exceeded the 5 T design value, but will add some
more correction coils). He also brought with him a quite extensive set
of drawings, which should help our design work. Everything seems like it
is still on track for January delivery, although the schedule slipped some
due to Oxford's move to a new factory.
We discussed in some detail the design of heat shields. Oxford has
build a 5mm window for the forward direction of the "80K" heat shield, which
we will want to replace by the thinnest window we can get away with.
David Kashy, the new Hall B engineer (with extensive cryogenic experience)
will look into this. He will need to know what is the maximum acceptable
heat load on the target and on the magnet respectively,
and design everything else from there. Oxford has agreed that superinsulation
is certainly a viable option; we could also consider adding a second
heat shield at 4K that connects to the front face of the magnet to
reduce heat load on the 1K refrigerator. We probably want to try to
make the large heat shield window as simple as possible, so the idea
of incorporating "fingers" in the "shadows" is presently in disfavor.
One question that came up (but nobody knew the answer to) is the
thickness and design of the sideways ports on the "80K" heat shield -
can we leave those as built by Oxford? (Any input is strongly appreciated).
We looked then at possible designs for the large forward vacuum window.
We will probably go with 2 different possibilities (and decide which one
is best once we've done some prototyping): Volker and Yury will work on
a single window made of Kapton/Carbonfiber/Kapton composite, possibly
with a hole (covered with Al) in the middle for the beam. The ODU group
(Jeff Helderlein, Junho, and myself) will prototype a design with
"spokes" and a central ring that carries a Kapton/Kevlar/Kapton
window, again with hole and Al cover. We will have to optimize the
thickness vs. bulging of the window (NO touching of heatshield/superinsulation)
and safety.
Other topics we need to worry about:
- alignment of Helmholtz coils (target field) with beam line - even a
0.8 mrad misalignment can steer the beam considerably at low energies.
We need to set a date for our technical readiness review BEFORE FEBRUARY 1.
We proposed tentatively to shoot for Friday, January 30th (we will need
to check availability of the people involved). Also, the full EG1 collaboration
should meet before that date; Ralph has graciously offered us the morning
of the Sunday, January 11, E1 meeting for this purpose (right after the
CLAS collaboration meeting). Send your votes for/against these dates in NOW.
There are (at most) 7 more available meeting dates before that meeting -
I believe it is necessary for us to make use of these to keep on top
of developments. I know that Don usually cannot attend Monday or Tuesday
meetings, but Derek has requested that we have our next meeting on a
Monday again. As a compromise, I propose we meet next Monday, 11/17,
at the same time (and location) (3:00 p.m. Room L207), and then again
on Wednesday, 11/26 (time is open to mutual agreement - the earlier, the
better, 'cause it's the day before Thanksgiving). If I get more affirmative
votes for this plan than negative ones, I'll go ahead and schedule these
dates.
See you soon -
Sebastian
Subject: Minutes of yesterday's meeting
Date: Tue, 25 Nov 1997 11:07:51 -0500
From: Sebastian Kuhn
Minutes of the Polarized Target in CLAS meeting 11/24
1) We had some more discussion on background targets/material. The list
of proposed materials is still growing; each with their own problems:
a) HN3 (Hydroazoic acid) - ignitable, explosive, reactive (airbags!)
On the annealing question, Volker showed data from Bonn that indicated that
one can anneal at 75 K. However, this is still above the melting point of
Nitrogen, and uncomfortably close to the boiling point. Unless we can
accomodate a feed line to the outside, I believe pure N15 is pretty
hopeless. We need a DEFINITE answer on that feed line ("CAN do" or "NO CAN do")
from our Italian collaborators SOON.
We decided to look into the relative PHYSICS merits of these proposed materials
and make a decision which of them might be acceptable for the dilution
correction. Note: Unless a given material yields DEMONSTRABLY smaller
systematic errors on the extraction of A|| than C12, we should go with C12,
since it is by far the least complicated solution. We will have a special
meeting in December just between people who want to look into this, and come
up with a final recommendation. So far, Volker Burkert and Derek Branford
have expressed interest to attend that meeting (and do some homework for it).
If you want to be involved too, send me an email!
2) We talked about the schedule and the upcoming readiness review. Don told
us about schedule slippage at Oxford. Right now, the plan is to have a complete
system test early in January, and then ship the target to CEBAF by the end
of January. A full EG1 collaboration meeting will occur in conjunction with
the E1 meeting after the CLAS meeting, on Sunday January 11. The readiness
review should be scheduled for January 29th or 30th. Clearly, the focus will
be on the target.
We have identified several things that still need to be addressed in a serious
way to get ready (both for the review and for the run):
a) The beam line. We need to make drawings for the beam line before and
after the polarized target (keeping in mind that it will be pulled back
0.6 m from the nominal CLAS target position). Dave Kashy will be burdened
with much of the work, but we need to get him started. Our next scheduled
meeting, on Monday 12/1 3:00 pm A110, will be exclusively dedicated to that
topic (please come if you can contribute). We will pore over the existing
drawings and sketch out the overall design. But there are still several
outstanding questions which will have to be solved:
b) Alignment: The target must be aligned to about 1.6 mrad (1/10 of a degree)
and about 1 mm in all directions. How are we going to do that? Also, it would
make life a lot easier if the rails for the insertion cart were parallel to
the beam line. Mike Seely will find out what we know about these rails, and
possible schedules for surveys/corrective action.
c) We still need to design entry and exit windows for the "Banjo". Also, we
need to know the schedule for the target insert ("the stick"), when it will
be here, so we can integrate it into the target.
d) We need to plan for NH3/ND3 irradiation (that includes ACQUIRING the
material!), pre-installation of supply lines for the target, commissioning
of rastering and Moeller, cold in-situ irradiation, and x-raying.
Some of these things require the accelerator or accelerator personnel -
in either case, the competition will be tough. The Moeller commissioning
tests are presently scheduled for March 26/27, but beyond this there seem
to be only 6 days in early June which haven't been assigned yet ("development
time"). In any case, we will have to come up with a realistic work schedule
that accomodates the time constraints for the readiness review (hopefully
Mike will be able to do much of this work).
As I said above, next Monday will NOT be a general Pol. Target meeting,
but rather a meeting on the beam line alone. The meeting on the Monday
thereafter (12/8) will have to be rescheduled to a different time - maybe
we can have a meeting of the "Background material study subgroup" instead.
We should shoot for another "general target meeting" on December 15
(probably the last one for this year).
- Sebastian
- Alignement of the insertion rails - David Kashy will look into this
together with Mike Seely.
- New beam line design: Everything downstream from the target will have
to be built new, including optimized shielding (lead) to get rid of
as many Moellers as possible. In this context, we will have to "freeze"
the target position rather soon; my proposal is to have the target
permanently set back from the nominal CLAS target position by 60 cm.
Junho is presently running GSIM to study the acceptance effects of this;
of course shielding is a major concern. Also, we need to make sure that no
"loose" iron pipes are in the vicinity of the target (e.g., inside the
cryostat bore).
b) H4N2 (Hydrazine) - extremely hazardous, bad H/N ratio.
c) H5N5 (Hyrazine acid) - similar to HN3(?), but worse H/N ratio.
d) N15 pure - melting point too low (conflict with annealing, see below).
e) N14 - same as d), but not even the right nucleus.
f) C14 - a very attractive choice (N15 "=" C14 + p), but we need 10 Curie!
g) C12 - no hazards, but a clearly different nucleus.
h) CO2 - average A = 14.7 (close to N15), but TWO new nuclear species.
i) CF2 (Teflon) - totally safe, but I don't see the advantage of that.
- What kind of lead shielding can we get in? Will it be sufficient to
shield against the Moellers? What about Moellers generated in the target
entrance and exit windows (1-2 mils of Al foil, outside the central B field
region)? We need more GSIM studies on this, and Volker will request help
directly from our Italian collaborators.
- Magnetic parts (iron pipes) and the like - how far away are they, and how
securely fastened?
- Can we install a small correcting steering magnet downstream of CLAS, to
counteract possible missteering by the target B-field? We need up to 50
Gauss-meters in this case (maybe 2000 Amp-Windings).