AP Implications of a Proposed Fix-em for Recent LHC Q1/Q3 Woes

J.A. Johnstone

Boy Eats Own Head After Radioactive Exploding Magnet Disaster!

Q1 Failure During Pressure Test“On Tuesday, March 27, structural supports to a Fermilab-built quadrupole magnet, one of an "inner triplet" of three focusing magnets, failed a high-pressure test in the tunnel of the LHC accelerator under construction at CERN. The force generated in the pressure test broke the supports in magnet Q1 that hold the magnet's cold mass in place inside the cryostat, the magnet's outer metal jacket. The support structure broke because it was not designed to withstand the amount of longitudinal force applied during the pressure test.”

Fermilab Today, April, 2007

CERN & Fermilab studied several potential solutions.

One solution could produce longitudinal motion of the Q1 & Q3 cold masses by as much as ±3 mm after each quench. These movements would be random, uncorellated, irreproducible, and unmeasurable.

What is the impact on the LHC optics?

Optics Modeling (Analytic)

Quad(RS of IP)

βx( )m

∆β/βx (max)(%)

∆νx βy( )m

∆νy ∆β/βy (max)(%)

1Q us 1059 0.286 0.42 -E 3 1058 2.48 -E 3 1.673

ds 1266 2269

3Q us 3588 1.562 2.31 -E 3 2841 -1.63 -E 3 1.103

ds 4718 2042

Tot al for L S + R S Quads 4.62 6.00 -E 3 -6.00 -E 3 4.62

T.Sen, Conf-99-304 (c.f. J.A.Johnstone, TM- 2157, 2001)

These results are for one IP & ∆s = ±3 mm. For 2 IP’s the ß wave could be as bad as ~9.2% and |ν| ~0.012.

Optics Modeling (Simulation)• 16 combinations of ∆s = ±3 mm assigned to Q1 & Q3 at IR5 and identical configuration at IR1 (256 combinations if IR1 configured independently; 65,536 possibilities for 4 IR’s!)

In the ‘worst’ case ∆ßx/ßmax = 13.8% (8.9% rms); ∆ßy/ßmax = 8.2% (3.8% rms). In the ‘average’ case ∆ßx/ßmax = 5.2% (3.4% rms); ∆ßy/ßmax = 3.6% (1.7% rms).

Simulation II• The LHC ß-wave budget is 20%, which could conceivably be consumed entirely by the Q1/Q3 movements at 4 IR’s.• The IR’s could be re-tuned after every quench to re-match optically to the arcs, but ………

Residual ß-wave from 1 IP after re-matching in the ‘worst’ case scenario. Gradient changes in the main magnets are ~ a few parts in 10^3 at most.

The FIX

To strengthen the support structure of the magnets, cartridges will be attached to one end of the Q1 magnets and to one end of the Q3 magnets.

The Q1 cold mass with cartridges attached.

• A review held April 24th - 28th examined proposed repair methods to hold the magnet's cold mass in place inside its cryostat to keep the magnet from moving when it experiences an asymmetric longitudinal force. • The solution accepted by the review panel uses four long metal cartridges installed parallel to the magnets long axis. The cartridges will be welded to the magnet's cold mass and to a bracket attached to the cryostat.

Summary

• Through modeling of the adverse impact of random Q1 & Q3 longitudinal shifts on LHC operations it was possible to help eliminate the cheapie, quick-fix engineering solution.