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PHENIX-MVD-96-7

1.0 Introduction

1. Electronics Cooling
2. Fluid-Induced Vibrations
3. Gravitational Displacement
4. Hydroscopic Expansion of Rohacell.

1. Summary: brief summary of the ESA report on that particular topic
2. Comments: Statements which clarify already complete R&D tests or mechanical plans, as well as additional tests which we agree to perform in the future
3. Future tests: Additional tests which we propose be conducted by ESA-DE. This section is divided into two lists. The "Class A" tests are those which we feel could be started right away, assuming that we agree on the ESA financial and time estimates. The "Class B" tests are those which we will likely want to be conducted in the near future. We would like to request from ESA, estimates which describe the costs and time needed to perform these tests. Please make separate estimates for the "Class A" and the "Class B" tests.

2.0 Electronics (MCM) Cooling

2.1 Summary

• Utilize a heat spreader on the base of the MCM to minimize the temperature non-uniformity across the MCM.
• Investigate possibility of lowering plenum inlet air temperature via a water chiller.
• Construct a full-scale prototype air cooling system in lab with resistive strip heaters and measure temperature of MCM and air.
• Study thermal stratification of flow in cooling plenum.

2.2 Comments

• We agree that we should utilize a heat spreader to minimize the temperature non-uniformity across the MCM.
• We always intended to use a water chiller if air cooling is the technology choice.
• We already have constructed full scale mechanical prototypes of air-cooling systems in which we simulate heat loads and measure air and MCM temperatures. Please refer to NIMA 345(1994)284. General agreement exists between the ESA cooling calculation results and the MVD results described in the article. We have also built a six layer cooling prototype and an MVD note is currently under construction. We recognize the importance of building a final full scale prototype of our cooling system once a technology choice is made and the mechanical design is complete.
• Mechanically, no further progress can be made concerning a cooling technology choice until progress is made on the MCM technology and electronic component characteristics. However, we did perform some simple cooling estimates based on the current knowledge of the MCM size, power dissipation and cooling calculations from the ESA report. These results can be found in "Air Cooling Status Report", PHENIX-MVD-96-4.. They indicate that air cooling is still a feasible option.

2.3 Future tests

2.3.1 Class A

• Study distortion of truss structure as a function of inside environment temperature.
• Study efficiency of Aluminum Nitride MCM substrate.

2.3.2 Class B

• Study feasibility of cooling the silicon detectors with electronics cooling system vs. independent cooling system (After cooling technology choice).
• Studies to maximize efficiency of cooling system design (After cooling technology choice). For example, adding fins to MCM in air cooling.
• Study thermal stratification of flow in cooling plenum. (IF air cooling is technology choice.)
• Finite element calculation of final cooling system with final electronics dimensions and characterizations (After cooling technology choice).

3.0. Fluid-Induced Vibrations

3.1 Summary

• High velocity cooling air may induce vibrations that create noise pickup in the electronics and deterioration of Rohacell plenum mounting grooves.
• Measure velocity and pressure in the final plenum as a function of time and frequency. ESA would then be able to perform a COSMOS finite element analysis to determine normal nodes of vibration and possible resonances. ESA also offered to aid in measurements of induced-vibrations with the use of transducers and anemometers.
• Exercise care when designing cooling loop to avoid high turbulence levels.
• Attention to design MCM configuration to produce uniform heat load transversely in the plenum.

3.2 Comments

• We are aware that air induced vibrations may lead to noise pickup and deterioration in the Rohacell plenum. With the use of an accelerometer, we have measured in three dimensions the displacement of the prototype plenum as a whole and a centrally located MCM in this plenum. Preliminary results indicate that all measured displacements are less than 100um. There is some evidence that the plenum and centrally located MCM enter resonant states in narrow and different velocity ranges. This may potentially be a concern, mechanically, but we do not feel it is efficient to study the problem further until a cooling technology choice has been made and the cooling system fully designed, as the degree of mechanical instability introduced will largely depend on the final mechanics of the plenum, assuming air cooling is the technology choice.
• We have studied the effect of adding mechanical robustness to Rohacell by coating it with a thin layer of parylene. R&D results on this subject can be found in "Parylene Conformal Coating for Rohacell", PHENIX-MVD-95-12. If air cooling is the technology choice and if it found that the induced vibrations do deteriorate the Rohacell at the MCM mounting groove, then we may consider coating the Rohacell plenum with parylene.
• Of more importance is the sensitivity of the electronics to induced vibrations. The effect of mechanical vibrations on the performance of the electronics will be studied in the lab and during a beam test in April 1996 at the AGS.

3.3 Future tests (IF Air Cooling is the technology choice):

3.3.1 Class B

• We agree that we should construct a final plenum and make velocity and pressure measurements once the cooling design is complete so that ESA can perform a COSMOS finite element analysis. We would be pleased to work with ESA in making these measurements.
• Simulations to optimize plenum performance.
• Evaluation to optimize design of plenum transitions (adapters) and cooling loop.
• Study mechanical deterioration of Rohacell due to induced vibrations and effect of Parylene coating.

4.0 Gravitational Displacement of MVD Frame.

4.1 Summary

• ESA calculations indicate that the MVD detector assembly does not experience any significant gravitational displacement and that the mechanical design is acceptable.

4.2 Comments

• The calculations are consistent with experimental measurements which are detailed in the technical note "Multiplicity Vertex Detector Subsystem Cage Deflection Studies", PHENIX-MVD-96-3.

4.3 Future tests

• None

5.0 Hydroscopic Expansion of the Foam

5.1 Summary

• Expansion of Rohacell due to moisture absorption will cause distortions in support framework.
• Expansion of Rohacell due to moisture absorption will cause high stresses in the area of the foam-silicon glue surfaces. Small changes in humidity may result in values of stress that exceed tensile stress of foam.
• "Potentially serious problem exists, with potential for non-functioning of the assembly."
• We should bond Rohacell frame around single silicon strip and expose to humidity cycles to study cracking, separation, tearing from foam.
• ESA should make stress-strain curve for Rohacell to obtain Young's modulus and elastic limit for foam.
• We should make more careful determination of moisture coefficient of Rohacell. Allow expansion to reach equilibrium and study time-dependent behavior. Measure foam at ambient conditions of New Mexico then expose to higher humidities, as high as 100% relative humidity. Tests should be performed with and without coatings.
• We should consider testing parylene coating to measure effect on short term and long term expansion.
• Consider flexible bonds between silicon and foam to minimize effects from distortion and stresses.
• Consider alternative materials to foam.

5.2 Comments

• We feel that many valuable suggestions have been made regarding additional tests which we should perform and methods in which we can improve the environmental tests already conducted. However, we do not feel that the use of Rohacell in the MVD cage assemblies poses a "serious problem". We would like to emphasize that our preliminary results from environmental tests of Rohacell indicate that we do not approach any mechanical or physics tolerances ("Influence of Silicon Wafers on the Expansion and Contraction of Rohacell Cages when Exposed to a Changing Environment, PHENIX-MVD-95-11). We have not yet begun to study the stress at the silicon/foam joint and we acknowledge the concerns described by ESA. We are pleased to see that several options exists which we can explore to minimize this effect.
• Results from ESA calculations are based on cycling relative humidity between 30-80%. It is intended that the MVD operate in a stable and controlled environment with relative humidity ranging between 30-40% (the same conditions under which the silicon/cage sections were assembled). Of course, it is important that we understand how the Rohacell behaves at high humidity in case of enclosure failure. It is expected that the relative humidity of the experimental hall be less than 40%, therefore we are more concerned with accidental exposure prior to installation in the experimental hall (such as shipping).

• The data shown in figure 1 was obtained from the Rohacell Manufacturer. This plot shows that the water diffusion into Rohacell in the case studied by ESA (30-80 %cycling in humidity) is nonlinear . Assuming linear behavior and extrapolating ESA results to real operating conditions will then lead to pessimistic results.
• We do not currently intend to test single frames of Rohacell as suggested. It is not clear as to what additional information would be obtained in comparison to past tests (PHENIX-MVD-95-11) and future tests described below. We are willing to reconsider if the benefits of this test are made more clear.

• Figure 2 shows data from the Rohacell Manufacturer. The % Strain in the plot refers to % L/L. The experimentally measured number given to ESA (on the order of 1%) is a worse case situation and is still well within the elastic properties of the Rohacell (falls on the linear portion of plot in figure 2).
• Several detailed environmental tests on Rohacell cages have already been performed (See PHENIX-MVD-11, PHENIX-MVD-12).
  • Our approach is already to begin the tests at the ambient conditions of New Mexico then increase the humidity.
  • We have always studied the time dependence of the foam behavior.
  • We agree that we may not have allowed the expansion to reach equilibrium before changing humidity- this will be investigated.
  • We do not feel that it is necessary to carry the tests to 100% relative humidity which can be complicated. It is unrealistic to expect that the MVD will ever be exposed to 100% relative humidity unless Jan drops it into the bath tub.
  • We intend to test coated and non-coated cages.
  • We wish to repeat our environmental tests with a partially populated silicon cage so that the results can be directly compared to ESA predictions (Our previous results are on a fully populated cage).
  • We wish to add the change in length to our measurements; in the past, we concentrated on change in radial and width dimensions.
  • We have already tested the short-term effect of parylene on Rohacell ("Parylene Conformal Coating for Rohacell, PHENIX-MVD-95-12). We agree that we should test the long term effect of parylene on Rohacell.
  • We agree that we should investigate options which will lead to a flexible bond at the silicon/foam joint and that we should also investigate mechanical design changes which would minimize stress at this joint. We intend to evaluate glue candidates during the Summer of 1996.
  • We feel it is unnecessary to consider alternative materials to the foam as our results indicate that we do not approach mechanical or physics tolerances and options exist which can minimize stress at the silicon/foam joint. We did estimate the thickness of a cage made of aluminum which has the same radiation length as the Rohacell cage. The aluminum thickness would be 15um (0.0006"). Such a cage would likely be difficult to construct.

5.3 Future tests

5.3.1 Class A

• Stress-strain test of Rohacell. Please inform us of sample size requirements.

5.3.2 Class B

• Perform simulations with 0.5mil parylene coated cage (after we have evaluated long-term effect of parylene).
• Perform simulations on proposed design changes and glue candidates that minimize stress at silicon/foam joint.