Energy deposition in the LHC triplets vs. distance to the interaction point

Abstract

Approaching the triplet from the Interaction Point (IP) opens the possibility of an increased focusing and hence of a significantly larger luminosity. The energy deposition issue is consequently modified. The results of a parametric study about the energy deposition in the triplet as a function of the distance from the IP are presented. The insertion, consisting of the Collimator (TAS) and the triplet (Q1,Q2a,Q2b,Q3), is rigidly shifted from the actual design position (distance from IP to Q1=l*= 23 m) closer to the IP (up to a distance l*=13 m), the tracking of the secondary particles is performed and the energy deposition is computed by FLUKA code. INTRODUCTION Energy or power deposition in the insertion region (IR) magnets is a very important parameter to take into account because such energy/power can increase the temperature of the magnet above the quench threshold. It is better to deal with power instead of energy because the operation of a superconducting magnet depends on its cooling efficiency and cooling power. In addition this parametric study relates the behaviour of the insertion quadrupoles (i.e. the power deposed in the quads) as a function of the IP distance in order to evaluate the effect of the increased luminosity. For this reason power (that takes into account the luminosity value) instead of energy is the parameter to be used. Previous works [1] performed with a layout of the insertion region very close to the actual one (LHC version 6.5) showed an average power deposed in the quadrupole Q2a and in Q3 of about 30-35 W respectively and a peak power deposition of about 7-10 mW/cm, at the nominal luminosity of 10 cms.The positive (i.e. a reduction in the energy deposed) effect of wider aperture magnet was evidenced. This fact, together with the higher magnetic performances of Nb3Sn, makes this technology a good candidate for the LHC upgrade. As a matter of fact the higher beam focussing with the consequent increase in luminosity and power deposition, can be compensated by the wider magnet aperture. This study evaluates the power deposition in the low beta insertion as a function of l* in a 100 mm quadrupole aperture. CALCULATION METHOD AND HYPOTHESES 1300 p-p events at 7 TeV were considered from DTUJET event generator [2]. Before being passed to FLUKA [3][4] energy deposition MonteCarlo code, the charged secondary particles were tracked along the beam magnetic structure of the IR, while the neutral particles were directly passed to FLUKA. An accurate description of the insertion was used, taking into account the solenoid detector field with its fringing. The detector length was assumed 5.3 m centred at the IP with 2.2 m diameter and 2 T peak field. A hard edge approximation was used for the quad field. The beam pipe thickness is 1.75 mm, the TAS and beam screens are taken into account with their actual geometry [5] [6] For the FLUKA calculation the cut-off energy for hadrons was 1 MeV, 1.5 MeV for electrons and positrons, 0.2 MeV for photons, and 0.4 eV for neutrons. These cutoff values and some biasing option was set in order to have reasonable CPU time (∼60 hours for each configuration case), for this reason it is possible that the results are not as accurate as possible, nevertheless the relative variation of the power deposed with l* (that is the purpose of this study) is evidenced. The dimensions of the bins where the energy was deposed were about 0.5x0.5x50 mm (medium binning), 0.25x0.25x50 mm (fine geometry) and 0.25x.25x10 mm (very fine geometry). Power density is an important parameter, but the binning volume in which the power is deposed must always be specified, as a matter of fact a small bin volume well represent the peak distribution of the energy, while an energy deposed in a large bin volume will give an average energy distribution. Anyway care must be taken in defining how small the bin volume may be, because a too small volume has a poor statistic reliability. The regions of the quadrupoles where the energy is deposed was described as more detailed as possible (Fig.1), i.e. the two Nb3Sn 15 mm thick, 60° angular aperture coil layer, the G10 insulation between and around the coil layers (0.2, mm thick the innermost one 0.7 mm between the coil layers and 0.5 mm the outermost one), the stainless steel pole wedges and collar, (20 mm thick) and the 18 cm iron yoke. G10 Insulation (0.2, 0.7, 0.5 mm) Nb3Sn Current shells (15 mm, 60°)