Minutes of the 2nd FFIR meeting on 1/12/2000

The meeting was held in a room of 3-425 at KEK for 13:30-15:30. We discussed on (1) 3 Tesla detector solenoid (iron structure), (2) support tube, (3) super conducting final focus magnet (QC1) and (4) extraction line.

(1) H.Yamaoka presented the first design of 3 tesla solenoid magnet for "reduced" detector ,(Transparencies, pdf, 1.38GB or tif, 9 pages,1.44GB ), i.e. the dimension of CDC is scaled down to 2/3 of the original and the outermost boundaries of calorimeter are R<3.5m and |Z|<3.8m. The size of the new magnet is 8 diameter x 8.6m^2 while the 2 Tesla magnet was 9 diameter x 10m^2. As the result, the cold mass reduced from 290 to 191 tons. The field uniformity is <1.4% for 0.2< r < 2m and |z|<2m. Since the CDC region is 0.45 < r < 1.55m and |z|<1.55m, it must be better. Anyway, the 1.4% is better than the previous one of 1.5% which has been presented at the Seoul workshop(Nov.1999). Movement of iron-endcaps was also estimated to be 4.8mm while the previous one was 6.8mm which has been wrongly reported as 68mm at the Seoul workshop. Finally, he showed an improvement of return yoke to decrease a field saturation at a junction between endcap and barrel by increasing thickness of the outermost barrel iron slab. This improvement does not have much improvement on the field uniformity, but it can significantly reduce a leakage field.

Discussion: The total length of iron structure can be reduced from 14.4 to 13.5m since the longitudinal space for calorimeter is enough to be 7.7m ( 7.6 + 2x0.05 ). The good field uniformity is required only in the CDC region, i.e. r<1.55m and |z|<1.55m. Therefore, the total length of coil may be shortened for better cost-performance.

(2) We discussed on how to support the support tube. Since the endcaps would move longitudinally by 4.8mm with the solenoid ON, it is difficult support the tube there. It must be supported outside the iron structure. The total length of the tube should be 16~17m while it has been assumed to be 12m in the previous study. On resonance points, ground motions can be enhanced by a factor of 25 with 2% damping ratio. Although relative displacements between two QC1's are still negligibly small, we must take care of these resonant movements ? If so, we must look for a support system with large damping ratio or the support system must be vibration-proof. The latter solution effectively moves resonant frequency into lower value where a feedback system works well.

(3) K. Tsuchiya presented the first study towards a conceptual design of superconducting quadrupole magnet for QC1. (Transparencies, pdf, 1.3GB or tif, 8 pages,1.3GB ) Major assumptions are as follows; (a) in-coming beam passes through its center and out-going beam passes through in the aperture, which cross with 8 mrad horizontal crossing angle at IP, (b) there is no iron yoke, (c) SC-cable is assumed to be the same as that of LHC-IRQ (~10T at 1.9K) and (d) QC1 of L=2.2 m long is located at l*=2 m from IP. The field gradient of QC1 must be 225 T/m at Ebeam=250GeV by final focus optics.

Taking account of deflection of out-going beam, the aperture of QC1 is required to be a=48mm. With a thermal insulation, a radius of QC1-coil should be IR=60mm. So the field gradient is 160T/m. In order to fulfill the optics requirement (225T/m), QC1 has smaller aperture and has to be longer. At this moment, the value of 160T/m will be input to FF-optics people (K.Oide etc. ) for the optimization.

For smaller aperture, the thermal insulation can move inside if the aperture-volume is filled with non-magnetic material with two small holes for in-coming and out-going beams. Then, IR(QC1) becomes 50mm increasing the field gradient 183T/m. In this option, there is a benefit since additional superconducting coils can be placed around the hole of in-coming beam where the external field is 4.5Tesla at most. The coils can correct mis-alignment (2.25kG/1mm) and also dynamically correct oscillation at < 10Hz. The outermost radius was estimated to be 261~281mm without supports, which shall be inside of the 80cmdiameter support tube. He also showed the magnetic field calculations inside and outside of QC1. The leakage field is about 500 Gauss at r=50cm, where there is endcap calorimeter. It has no problem for detectors, anyway. For the optics optimization, super-QC1 prefers smaller horizontal crossing angle, which K.Yokoya mentioned that its minimum angle was 6mrad, and larger distance from IP, i.e. longer l*, say, l*=2.5m, for good solenoid-field uniformity in CDC-region etc. .

(4) We briefly discussed on a beam-extraction line, K.Kubo questioned on the dynamic range and accuracy of energy spectrum measurement there. Since a major interest is the spectrum in "delta" function at the nominal beam energy, 1-0.1%, it is difficult to measure the spectrum at 10% below. It is OK since our endcap calorimeter can measure such large energy tails. Change of crossing angle and l* (L) may affect a design of extraction line. The other comment was made on separate dumps for beam and beamstrahlung photons, where the former dump is a dominant neutron-background source and it can be placed far from the original beam line for shields. He promised that he will present the first design at the next meeting.

Next FFIR meeting will be held on 1/27 (Thus.) at 13:30-, 3-425, KEK, Japan. We will discuss on optimization on 3 tesla solenoid magnet (shorter length.. coil), support tube of 16~17m long, super-QC1 and extraction line and others.