Malyshev O B

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Malyshev OB

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EUROPEAN LABORATORY FOR PARTICLE PHYSICSCERN - LHC DIVISIONLHC-VAC/OM Vacuum Technical Note 01-06July 2001THE CAPTURE FACTOR AND PUMPING SPEEDFOR SLOTS IN THE LHC BEAM SCREEN WITH BAFFLESOleg Malyshev2The capture factor and pumping speedfor slots in the LHC beam screen with bafflesO.B. Malyshev1 AbstractThis paper presents the continuation of the study of the pumping speed of the slots for thebeam screens [1]. The new analytical estimation of the pumping speed of the slots is made for apossible modification where the slots have a baffle to protect the cold bore against multipactingelectrons.2 IntroductionThe intensive studies of the beam induced multipacting in the LHC vacuum chamber shows[2] that during a commissioning period the power up to about 1 W/m from the electron clouds canbe adsorbed on a beam screen. (This limitation comes from cryogenic installed capacity of1.17 W/m per beam). Some electrons from the electron cloud may reach the cold bore via thepumping slots therefore their power will be adsorbed on the cold bore. This fraction reaching thecold bore can be estimated from 4.4% (the area of pumping slots in a beam screen) up to 50% (theslot coverage along a row in dipole magnetic field if the multipacting electrons are concentrated invery narrow strips [2]), in other words this could represent an unacceptable heat load to the 1.9Kcircuit power. Thus it may be necessary to protect the cold bore against the multipacting electrons.One proposal has been to intercept them with some kind of baffles. The aim of this study is toestimate how much the pumping speed will be reduced in the presence of such baffles.3 Results of simulationIn the LHC arc beam screen the pumping slots lie on 8 rows along the vacuum chamber (seeFigure 1). The slot length is randomly varied between 6, 7, 8, 9 or 10 mm, the average being8 mm. The average distance between slots is 8 mm. In these simulations the pumping slots are3replaced by continuos slots, having longitudinal symmetry, and this permitting a two-dimensionsimulation. This gives a lower value of capture factor (CF) than in the three-dimension simulation,but, given uncertainty of sticking probability of desorbed gases, these values give a reasonableestimate.3.1 A beam screen without baffles in a cold bore at T£3KThe first estimations were made for the arc beam screen without baffles for different slotpositions.  The simulation scheme is shown in Figure 1. The geometrical parameters are:- the inner cold bore diameter, ID;- the height of the beam screen (outer dimension), H;- the beam screen wall thickness, c;- the slot width, a.The baseline design parameters for the arcs are ID = 50 mm, H = 38.9 mm, a = 1.5 mm andc = 1.05 mm.The objective of these estimations, beyond those pervious for a beam screen publishedin [1], was to check how the capture factor (CF) of the slots changes with their location. Theresults of the estimations of the CF for H2 and CO for the different slot separation for the innerand outer rows are presented in Table 1. The baseline row separations are 10 mm and 18 mm, thealternative proposal was 15 mm and 23 mm [3]. The extreme separation of 27 mm is when the slotis placed at the edge of the flat part of a beam screen. The results are presented for three cases: (1)without SR, ions and/or electrons, (2) in presence of SR, (3) in presence of ions and/or electrons.The estimation with a wide slot (a=2mm) was performed to estimate the loss in pumping speed inthe case if two slots rows with a=1.5mm compare with by one with a=2mm.acHID4Figure 1. The actively cooled beam screen in the arcsTable 1. The Capture Factor for the pumping slots in the arc beam screen inside a cold bore with and withoutsynchrotron radiation (SR),  ions and electrons for a sticking probability a(H2)=0.1 and a(CO)=1.Capture Factor for H2 Capture Factor for COSlotsposition(mm)Slotssepara-tion(mm)Slotswidth(mm)no SR orionswith SR with ions orelectronsno SR orionswith SR with ions orelectronsThe beam screen in the arcsBaseline 10, 18 a=1.5 0.61 0.53 0.61 0.76 0.66 0.7415 0.53 0.61 0.66 0.7423a=1.5 0.610.51 0.580.760.63 0.73Wide23 a=2.0 0.6 0.47 0.56 0.80 0.61 0.7627 a=1.5 0.52 0.45 0.52 0.76 0.56 0.73Extreme27 a=2.0 0.50 0.42 0.49 0.80 0.53 0.75For all cases the CF and the pumping speed for H2 is always less than for CO due to thelower sticking probability. The slots pumping speed is proportional to the capture factor and theslot width: S µ CF ×a. For all studied slots the minimum pumping speed is in presence of reflectedphotons  (column 'with SR'), the reduction of pumping speed is up to 20% comparing to thecolumn 'no SR or ions'. The reduction of pumping speed by electrons or ions is much less (thecolumns and 'with ions or electrons').One can see there are no significant difference in a pumping speed between the baselineslots separation (10 and 18 mm), the wider separation one (15 and 23 mm). The pumping speed ofone 2-mm wide slot with a slot separation of 23 mm will have 59% of baseline design pumpingspeed and this figure is 53% for a slot separation of 27 mm.3.2 A beam screen with baffles with a cold bore at T£3K3.2.1 V-type bafflesOne proposal is to have a simple screen to protect the cold bore against the electrons. Such abaffle could be made from a thin metal strip with V-shaped profile and welded between two rowsof pumping slots. Since in a dipole magnetic field the electrons can move only along magneticfield lines the baffles should overlap the pumping slot. An overlap of 0.5 mm from each side waschosen for the estimations.5The simulation scheme is shown in Figure 2. The results of the estimations of the CF for H2and CO for the different angles of V's from horizontal position and slot separations are presentedin Table 2.Figure 2. The actively cooled beam screen with V-type baffle3.2.2 ¤ -type bafflesAnother proposal was also studied: to replace two rows of the pumping slots with one row withwider pumping slots, for example with a=2 mm, this simplifying the baffle shape.The simulation scheme is shown in Figure 3. The results of the estimations of the capture factorfor H2 and CO for the different angles of V's and slots separation are presented in Table 2.The same as without baffles there are no significant difference in a pumping speed betweenthe baseline slots separation (10 and 18 mm) and the wider separation one (15 and 23 mm). Thepumping speed of one 2-mm wide slot with a slot separation of 23 mm and /-type baffle will have68% of pumping speed for the baseline design slot separations with V-type baffle.By comparing results in Table 1 and Table 2 it can be concluded that, in presence of baffles,there is a reduction of pumping speed about 40%.0 2 4 6 8 10 12 14 16 18161820222426X (mm)Y (mm).6Figure 3. The actively cooled beam screen with a  /-type baffleTable 2. The Capture Factor for the pumping slots with baffles in the arc beam screen inside a cold bore  withsynchrotron radiation ,  ions and electrons for a sticking probability a(H2)=0.1 and a(CO)=1.Baffle Angles ofbafflesSlotsseparation(mm)Slotswidtha(mm)CaptureFactor for H2Capture Factorfor CO45° and 45° 10 and 18 1.5 0.31 0.4458° and 45° 10 and 18 1.5 0.30 0.4745° and 30° 15 and 23 1.5 0.28 0.43V-type58° and 30° 15 and 23 1.5 0.28 0.4750° 18 2.0 0.29 0.4550° 18 2.5 0.26 0.5140° 23 2.0 0.32 0.50/-type40° 23 2.5 0.26 0.490 2 4 6 8 10 12 14 161820222426X (mm)Y (mm).73.3 A beam screen with V-type baffles in a cold bore at T>3K.In the Long Straight Sections there are cryogenic elements with a cold bore at T>3 K. The coldbore of these elements do not pump H2 and a cryosorber is required. It looks very natural to place thecryosorber inside the 'V' of the baffle where the cryosorber is shielded against photons, electrons andions. Another advantage of this is that the gases like CO, CO2 and CH4 will be mainly adsorbed on theouter walls of the beam screen or on the cold bore and so the cryosorber will have a higher capacity forpumping of H2. Since the sticking probability of H2 depends on cryosorbing material the estimationsare made for two different sticking probability 0.1 and 1. The results of the estimations of the capturefactor for H2 and CO for the different angles of V's and slots separation are presented in Table 3. Thebest results for H2 are for baseline design slot separation with 45° angles of baffles. Meanwhile it is only43–55% of pumping speed without baffles.Table 3. The CF for the pumping slots with V-type baffles and a cryosorber in a beam screen inside a cold bore withsynchrotron radiation, ions and electrons for a sticking probability a(H2)=0.1, a(H2)=1 and a(CO)=1.Capture Factor for H2Angles ofbafflesSlotsseparation(mm) a=0.1 a=1CaptureFactor forCO30° and 30° 10 and 18 0.22 0.28 0.4445° and 45° 10 and 18 0.23 0.29 0.5558° and 45° 10 and 18 0.22 0.28 0.5845° and 30° 15 and 23 0.19 0.26 0.4958° and 30° 15 and 23 0.17 0.22 0.524 Conclusions1. The estimation of capture factor for different slot row separations is presented.2. The estimation of capture factor for slots with baffles shows the reduction of about 40% in the slotpumping speed.3. In a vacuum chamber with a cold bore at T>3K the baffles loaded with a cryosorber will provide43–55% of pumping speed estimated for a vacuum chamber with a cold bore at T£3K without baffles.8References                                                                1. O.B. Malyshev. The capture factor and pumping speed for slots in the LHC beam screen.Vacuum Technical Note 00-17, August 2000, CERN. - 20 p.2. F. Zimmerman. Electron cloud simulations for SPS and LHC. Proc Chamonix 10,CERN-SL-2000-007-DI.3 N. Kos. Private communication.

The capture factor and pumping speed for slots in the LHC beam screen with buffles.

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