Protecting Superconducting Magnets from Radiation at Hadron Colliders Accelerator Magnet Technology CARE Workshop on Beam Generated Heat Deposition and Quench Levels in LHC Magnets CERN March 3-4, 2005 Fermilab CARE AMT-2005 Nikolai Mokhov, Fermilab

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov2 OUTLINE Introduction Radiation Constraints at Hadron Colliders Tevatron Quench Limits and Experience with SC Magnets and Detectors Protecting LHC IR at Normal Operation Protecting IP1, IP5 and IP6 at Unsynchronized Beam Abort Upgrading LHC IR

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov3 INTRODUCTION Operational and accidental beam losses in superconducting hadron colliders can have a serious impact on machine and detector performance, resulting in effects ranging from minor to catastrophic. Only with a very efficient multi-component beam collimation system can one reduce uncontrolled beam losses to an allowable level. Such a system was implemented in Tevatron from the very beginning, and principles behind it, its evolution over two decades, operational experience with quench levels and transient beam losses are described below. Principal challenges arising from beam-induced energy deposition in SC magnets and a system designed to protect LHC IR magnets and detectors are discussed.

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov4 BEAM LOSS INDUCED RADIATION CONSTRAINTS AT COLLIDERS Sustain favorable background conditions in experiments. Maintain operational reliability in stores: quench stability and dynamic heat loads on cryogenics. Prevent quenching SC magnets and damage of machine and detector components at unsynchronized beam aborts. Minimize radiation damage to components, maximize their lifetime. Minimize impact of radiation on personnel and environment: prompt and residual radiation (hands-on maintenance).

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov5 SLOW AND FAST QUENCH LEVELS IN TEVATRON SC MAGNETS

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov6 QUENCH LIMITS IN TEVATRON MAGNETS Based on measurements and analyses by H. Edwards et al ( ), the following energy deposition design limits for the Tevatron SC dipole magnets (4.4 T, I/I c =0.9, 4.6 K) have been chosen in Tevatron design report (1979): 1. Slow loss (DC) 8 mW/g ( ~2 mW/g w/cryo) 2. Fast loss (1 ms) 1 mJ/g 3. Fast loss (20  s) 0.5 mJ/g

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov7 TEVATRON COLLIMATION SYSTEM EVOLUTION Design report, commissioning, initial operation: a few single 0.9 to 1.8-m long SS collimators in front of SC magnets (Edwards, Pruss, Van Ginneken, ). A set of two-unit collimators at optimal locations based on STRUCT/MARS modeling: 5-fold increase of 800-GeV proton beam intensity at fast resonant extraction (Drozhdin, Harrison, Mokhov, 1985). First two-stage system, two 2.5-mm thick L-shape tungsten targets with 0.3-mm offset relative to A0 scrapers: 5-fold reduction of beam loss rates upstream D0 and CDF detectors (Drozhdin, Mokhov et al., 1995). Genuine two-stage system proposed for Run-II with primary and secondary collimators at optimal locations optimized in STRUCT/MARS runs (Church, Drozhdin, Mokhov, 1999). Current system with tertiary collimators (Drozhdin, Mokhov,Still). Crystal primary collimator to be commissioned in March 2005.

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov8 TEVATRON RUN-II COLLIMATION Currently for proton beam, we have additionally D17(3) H/V, A11 (V) and A48 (V). Last two are for abort kicker prefire.

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov9 TEVATRON EXPERIENCE SINCE stage collimation system (primary, secondary and tertiary collimators, H/V, both beams) is mandatory. Quench levels at I/I c =0.9 are ~1mJ/g (fast) and a few mW/g (slow). With the collimators well tuned, the limiting factor is backgrounds in the collider detectors. Beam-gas scattering contributes substantially to the loss rate, particularly elastic between collimators and IR: improve vacuum as much as possible, no local pressure bumps. Beam-beam effects and longitudinal loss are another big items. Abort kicker prefires (AKP) take place a few times a year and is the main source of accidental beam loss. Tertiary collimators is a must-be last line of defense that protects the detectors and IR SC magnets against elastic beam- gas and AKP. Consequences of accidental beam loss are severe: abort philosophy.

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov10 TEVATRON BEAM LOSS PHILOSOPHY Initial Days of the Tevatron (only Fixed Target) –Protect against any possible quench –Unnecessary abort wastes a single beam pulse Early days of Collider (6X6, 900GeV, ~2E12) –Tevatron can survive a quench –An abort turns off collider for ~ 1 day –A quench is no worse than an abort Run II Intensities (36X36, 980GeV, ~1E13) –There is enough beam to damage Tevatron again –Improve protection of Tevatron components –Do not cause unnecessary down time (masking BLM abort inputs during a store saved falsely aborted stores)

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov11 ABORT LOOP PHILOSOPHY Abort Inputs –QPM (magnet Quench Protection Monitor) –Beam Loss monitors (masked during stores) –Power supplies (etc.) Abort Loop –Hardware fail-safe loop –Can abort beam within a couple revolutions –Aborts synchronized to single beam abort gap Courtesy D. Still

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov12 ’WAKE UP CALL’ TO CHANGE BEAM LOSS PHILOSOPHY December 5, 2003: Tokyo Pots move into beam Beam loss damages to 2 collimators and 3 correctors Two thirds of the ring quenches Two-week shutdown C18 spoolE03 1.5m collimator D49 target Courtesy D. Still

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov13 SEQUENCE OF DEC’05 EVENTS Losses generated quickly and quench A48U CDF Roman (Tokyo) pot moves into beam quickly Field in 5 dipoles start decaying (500 A/sec) Orbit moves everywhere Beam moves through D49 target, E11 spool piece, and E03 Collimator Protons are extinguished in E03 collimator in about several turns QPM detects quench in A48 upper in 16 msec Abort kickers fire (nothing to extract!) Courtesy D. Still

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov14 OTHER FAST BEAM LOSS EVENTS IN TEVATRON AND NEW QPM AND BLM SYSTEMS Separator spark before fast QPM aborts –July 8 – B11 Vert sep spark (4 house quench) Separator spark after fast QPM abort –Dec 13 – B11 Vert sep spark (E1 house only quenched) Kicker Prefires – Nothing we can do with loop Kicker building loss of power Vacuum valve move into beam (upgraded vacuum abort + fast QPM abort) May 15, 2004 – Unknown cause – Damaged E03 collimator again Implemented new QPM code to abort on detection of quench within 1-2 msec, instead of 16 msec, but still masking BLM during stores. New BLM system in Courtesy D. Still

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov15 LHC IR1/IR5 PROTECTION SYSTEM At the LHC, after thorough optimization of the IR layout, an IR protection system was designed to protect SC magnets against debris generated in the pp-collisions and in the near beam elements. The optimization study was based on detailed energy deposition calculations with the MARS code at Fermilab. The system includes a set of absorbers in front of the inner triplet (TAS), inside the triplet aperture and between the low-beta quadrupoles, inside the cryostats, in front of the D2 separation dipole (TAN) and between the outer triplet quadrupoles. Their parameters were optimized over the years via MARS runs to provide better protection and to meet practical requirements at the same time.

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov16 IR1/IR5 INNER TRIPLET PROTECTION

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov17 LHC IR PROTECTION: DESIGN CONSTRAINTS 1. Use ultimate design luminosity of cm -2 s Geometrical aperture: keep it larger than “n1 = 7” for injection and collision optics, including closed orbit and mechanical tolerances. 3. Quench stability: keep peak power density  max, which can be as much as an order of magnitude larger than the azimuthal average, below the quench limit with a safety margin of a factor of Radiation damage: with the above levels, the estimated lifetime exceeds 7 years even in the hottest spots. 5. Quench limit: tests of porous cable insulation systems and recent calculations concerning the insulation system to be used in the Fermilab- built LHC IR quadrupoles (MQXB) have shown that up to about 1.6 mW/g can be removed while keeping the coil below the magnet quench temperature. 1.2 mW/g was used as a limit in ’90s in these studies. 6. Dynamic heat load: keep it below 10 W/m. 7. Hands-on maintenance: keep residual dose rates on the component outer surfaces below 0.1 mSv/hr. 8. Engineering constraints must always be obeyed.

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov18 MARS MODELING IN IP1/IP5

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov19 PEAK POWER DENSITY AND DYNAMIC HEAT LOAD

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov20 SUMMARY FOR IP1/IP5 As a result of optimization of the protection system, it became possible to meet design constraints for the LHC IR at luminosity of cm -2 s -1, with  max < 0.45 mW/g, Q < 10 W/m, and lifetime in the hottest spot of about 7 years. Note that the power density and dose in the SC coils always peaks in the horizontal or vertical planes at the coil inner-most radius. Dynamic heat loads at cryo temperatures are about 30 W in each quad (114 W total), 19 W in correctors and feedbox, 2 W in D2 dipole, and 0.5 to 2 W in outer triplet quads. At room temperature, the main players are TAS (184 W), D1 dipole (50 W), and TAN (189 W). Residual dose from several hundred mSv/hr in TAS and thick beam tube to below 0.1 mSv/hr on outer vessel.

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov21 IP5: USYNCHRONIZED BEAM ABORT (UBA) (1999)

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov22 TERTIARY COLLIMATOR IN IP5 FOR UBA (1999) A factor of reduction of radiation loads in inner triplet quads. A factor of 4000 reduction in peak dose rates at the CMS inner pixels.

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov23 IP6 PROTECTION UBA SYSTEM ( ) IP6 collimators close to the cause. CMS and IP5 are perfectly protected at UBA. A few IP6 SC magnets will quench without any damage. STRUCT/MARS modeling

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov24 LHC UPGRADE SCENARIOS 1.“Modest” upgrade to 2.5x10 34 cm -2 s -1 : traditional quadrupole-first design (large bore Nb 3 Sn); energy problems are manageable! 2.“Ultimate” upgrade to cm -2 s -1 : double- bore inner triplet with separation dipoles placed in front of quads (reduced # of long- range beam-beam collisions, beam on-axis in quads, local corrections for each beam), but severe radiation problems with about 3.5 kW dynamic heat load on the first dipole!

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov25 DIPOLE-FIRST IR LAYOUT

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov26 DIPOLE-FIRST: COS-THETA vs BLOCK COILS Cos-theta:  max =50 mW/g (probably can be reduced to 13 mW/g with low-Z spacers), i.e. well above the quench limit. Open mid-plane block coil design with tungsten rods at LN temper: peak  max < 1 mW/g, significant fraction of heat load is dissipated in tungsten roads. But substantial R&D is needed. MARS15 power density in open-midplane block coil dipole at maximum (non-IP end)

CARE AMT – CERN, March 3-4, 2005Protecting SC from Radiation - N.V. Mokhov27 SUMMARY There is a good understanding of quench levels in SC magnets, beam loss mechanisms and collimation at Tevatron. 3-stage collimation system is mandatory at SC hadron colliders. Quench levels in the LHC IR quads are well understood, more work is needed on other magnets. IP1 and IP5 SC magnets and CMS and ATLAS detectors are adequately protected at normal operation and accidental conditions with the local protection systems, main collimation system in IP3/IP7, and the IP6 collimators (TCDQ etc). LHC upgrade scenarios are quite challenging from energy deposition standpoint, first simulation results are encouraging, but much more work is needed.