Subj:	BRAHMS forward spectrometer layout

Dear Collaborator

      We have been looking at the layout issues for the Forward Spectrometer
Arm in BRAHMS recently, since the magnet design for D1 and D2 will start next
week. In addition there was a meeting with the RHIC designers for the DX
magnets with the outcome that it is unlikely that the envelop will be less than
48" diameter. To accomodate sufficient clearence between magnets,
detectors and the RHIC DX and beam pipe a set of changes is proposed to the
layout of BRAHMS. The most important one is a change the minimum angle from
2.0 to 2.3 deg. 
      I will like, if you have comments to this proposed change, to get them as
quickly as possible to me. The proposal included below also refers to acceptance
figures. The overall aim is of cause to keep the minimum angle as small as
possible but it does seem difficult to maintain the 2 deg limit.
The can be faxed on request, but is also available as a postscript 
file "accept.ps" in brahms$group:[brahms] on hi2.hirg.bnl.gov or
/usr/brahms_group/brahms on sgs0.hirg.bnl.gov.

best regards
      Flemming


Proposed Change to the BRAHMS Forward Spectrometer Layout
D. Beavis
Feb. 26, 1995

There are several related issues which must be resolved before the
design of magnets D1 and D2 are started.  These magnets are scheduled
to begin engineering and design in a about one week requiring a
quick decision to these issues.  I will briefly state the current issues 
and propose a specific modification to the forward spectrometer layout.

Reference Ray

Most issues discussed in this proposal are related to the definition
of the reference ray and space in the horizonal plane.  For the present 
layout the following numbers are relevant to the reference ray:

	D1 bend			1.8 degrees
	D2 bend			1.8 degrees
	vertex to D1 center	3.5 meters
	D1 to D2 centers	3.0 meters
	D2 to D3 centers	6.0 meters
	D1 length (core)	2.0 meters
	D2 length (core)	1.6 meters
	D3 length (core)	2.0 meters

Issues

The following issues do not include longitudinal spacing concerns for the 
detectors:

1)Transverse space at the front of T1

The present layout has the reference ray entering T1 19cm from the 
collider beam. 8 cm are required for the beam pipe and clearance
to the beam pipe.  This provides 11 cm of space for fiducial volume
in the TPC, the field cage, and shielding.  This is
considered marginal.

2)Reference Ray at front of DX.

The best estimate for the radius of the collider magnet DX
is 24 inches (61cm).  The front of DX is about 930-950 cm from the vertex
depending on the design of the front of the cryogenics vessel.  The 
reference ray to DX space has between 2cm of interference to 0 cm of 
clearance depending on the exact shape of the cryogenic vessel of DX.

3)Clearance at the front of T3

The front of T3 is at 1050cm from the vertex. The reference ray is 10.5 cm
form DX at this location.  This space must accommodate the chamber frame
and fiducial space inside the active gas volume. This is presently considered
tight.

4)Clearance D3 to DX.

The front of the D3 magnet shield is 1120 cm from the interaction vertex.
At this location the reference ray is 78cm from the collider beam.
With the present conceptual design of D3 there exists 7cm of interference
between D3 and DX. This does not take into account any required spacial
clearance and potential magnetic coupling which may require additional
space between the magnets.

5)D1 current density and voltage

The present conceptual design of D1 has a maximum field of 12 kG with a 
voltage drop of 160V and a current of 3200A.  The current density in the
copper is 44,000A/in**2. In order to obtain a better match to AGS
supplies (should they become available) it would be desirable to have
the maximum voltage below 140V. For reliability it would be worthwhile
to reduce the current density as much as possible.

Possible approaches

1)Increase the minimum angle which is accepted by the forward spectrometer.

This is the "only" proposed solution which increases the T1 clearance.
The potential concern with this solution is it changes  the y-pt coverage of
the experiment. Plots of the y-pt coverage for a 2.3 degree minimum angle
for pi,k,p are attached along with a line indicating the present
coverage for a 2.0 degree minimum angle.

2)Decrease the maximum possible momentum of the reference ray.

This can always be done at a latter time . Again the y-pt plots can
be used to consider the compromises in the coverage.

3)Increase the Bdl of D1 and D2 and subsequently their bend angles.
If the minimum angle is not changed than an increase in the D1 bend
angle can only be achieved by a substantial change in the D1 design 
requiring both a poled design and then shimming the gap.


Proposed Change

The following changes are proposed:

1)Increase the minimum accepted angle to 2.3 degrees.  This will allow for
the design of D1 to be substantially unchanged except there will be
additional space for approximately 44% more copper in the coil.

2)Increase the field in D1 to 13 kG which can be done with the increased
copper.  Both the voltage and current density will be lower than the
existing design. In addition a few extra millimeters of clearance between
D1 and the beam pipe will be obtained.

The net result is:

 concern		old 			new
 T1 space		11 cm			13.5cm
 ref. ray to DX		-2cm to 0cm		5 to 7.5cm
 T3 to DX		10.5cm			18cm
 D3 to DX		-7cm			2cm
 D1 voltage		160V			130V
 D1 current dens.	44,000 A/in**2		31000-36000 A/in**2

new parameters:

	D1 bend is now 1.95 degrees (uses effective length of magnet)
	D2 bend is now 1.90 degrees (uses effective length of magnet)
	minimum production angle is 2.3 degrees.

Comments:

1) The change is the coverage is not judged serious.

2) It may be possible to increase the field in D2 but this has not been done
at present since the fringe field into the TPC volume needs to be carefully
considered.

3) The clearance for D3 to DX is still small at the front of D3.  However,
the D3 magnet design does not start for 3-4 months allowing time for the
DX design to become more firm. It is noted that the present conceptual design
of D3 has the current density at 5500 A/in**2. This could be increased allowing
for the coil to be smaller by 5 cm or more. In addition, moving D3 back
50 cm increases the clearance by 5.3 cm with this proposed design. Moving
D3 slightly farther from the vertex may be required to allow sufficient 
space for the detectors.