RF Strategy & the M uCool T est Area Meeting the RF in Magnetic Field Challenge Alan Bross M AP REVIEW 24-26 August, 2010 1
• Outline – The “ RF Challenge” – Science of RF Breakdowns – Current Program (Where we are) • Derun Li’s talk (next) will discuss where we are going • As will Katsuya Y onehara – The M uCool Test Area – Summary Alan Bross M AP REVIEW 24-26 August, 2010 2
Goals of This Talk • We have a well defined and measured experimental program • There are extensive scientific underpinnings for the program • The experimental effort is supported by detailed simulation work which is predictive Alan Bross M AP REVIEW 24-26 August, 2010 3
Just to Review Remember, from Long Ago – (Yesterday) Alan Bross M AP REVIEW 24-26 August, 2010 4
Normal Conducting RF R&D Issues and Present Status • M uon bunching, phase rotation and cooling requires Normal Conducting RF (NCRF) that can operate at high gradient within a magnetic field strength of up to approximately 6T – Required gradients (15-18M V/ m) easily obtainable in absence of magnetic field But Alan Bross M AP REVIEW 24-26 August, 2010 5
The RF Challenge • Significant degradation in maximum stable operating gradient with applied B field • 805 M Hz RF Pillbox data – Curved Be windows – E parallel B – Electron current/arcs focused by B • Degradation also observed with 201 M Hz cavity – Qualitatively, quite different Alan Bross M AP REVIEW 24-26 August, 2010 6
805 Pillbox Post-M ortem • Significant damage observed – Iris – RF coupler – Button holder • However – No damage to Be window Alan Bross M AP REVIEW 24-26 August, 2010 7
805 M Hz Imaging Hot Spot Arc forms Cavity Energy W=1/2 CV 2 ≈ 1-5 joule B All goes into melting Cu Surface Field Enhancement Initiates the event & B focuses the e - current which causes damage 8 Alan Bross M AP REVIEW 24-26 August, 2010 8
201 M Hz Cavity Test Treating NCRF cavities with SCRF processes • The 201 M Hz Cavity – Achieved 21 M V/ m – Design gradient – 16M V/ m – At 0.75T reached 10-12 M V/ m However, No observed damage! Alan Bross M AP REVIEW 24-26 August, 2010 9
201 M Hz Prototype Note: Stored energy available to sparks ≈ 100J Alan Bross M AP REVIEW 24-26 August, 2010 10
RF Breakdowns • Are not all equal – NCRF conditioning (B=0), process allows for higher gradient operation (“conditioning” ) – NCRF (B ≠ 0), process can cause damage and require re-conditioning at lower gradient in order to reach the same gradient attainable before breakdown Alan Bross M AP REVIEW 24-26 August, 2010 11
The Science of RF Breakdown Vacuum • In recent years, we have learned a great deal about the Science of RF Breakdown – Vacuum Arcs • An explanation of the formation of high β asperities – Surface Field Enhancements • Predictions of very high surface fields in arcs, consistent with measurements. • An explanation of the microstructure in arc pits • Preliminary results of arcs in static B fields. • Comparison with studies of unipolar arcs. • Calculations of sputtering and erosion rates. – Effects due to magnetically focused Field Emission • Including studies with SCRF Alan Bross M AP REVIEW 24-26 August, 2010 12
The Science of RF Breakdown II Vacuum Workshop on Uni-polar Arcs, ANL, Jan. 2010 Norem et al., 2001-2010 Breakdown Physics Workshop, CERN May 2010 Alan Bross M AP REVIEW 24-26 August, 2010 13
The Science of RF Breakdown III Vacuum • Advanced Simulation Code – OOPIC & VORPAL: Kevin Paul, Tech-X New particles Particles Particles added (lost accelerated moved based on This is removed) by the fields new velocity where all the { x α , v α } { v' α } { x' α } interesting physics for One Time Step RF breakdown New fields Currents Collisions and computed from “deposited” takes interactions old fields on the grid place!!! computed { E' i , B' i } { J i } Alan Bross M AP REVIEW 24-26 August, 2010 14
The Science of RF Breakdown IV Vacuum Alan Bross M AP REVIEW 24-26 August, 2010 15
The Science of RF Breakdown V Vacuum 0 0.1 T 0.25 T 0.5 T Dazhang Huang (IIT), Particle Studio Simulation Alan Bross M AP REVIEW 24-26 August, 2010 16
The Science of RF Breakdown VI Vacuum • Numerical studies at BNL and SLAC (in collaboration) using Omega-3P and Track-3P codes, – Cavity with flat windows: 5 M V/ m on axis; 2-T uniform external magnetic field; scan of a few points from one cavity side Trajectories without Trajectories with E field contour external B = 2-T field external B field
The Science of RF Breakdown Gas • RF cavities filled with High-Pressure H 2 – Paschen’s Law ( ) a pd = V bd + ln( ) pd b Rolland Johnson Shelter Island 2002 Alan Bross M AP REVIEW 24-26 August, 2010 18
The Science of RF Breakdown II Gas Gas Filled Cavities No focusing of electron avalanche V rf = V o Sin[ ω t] + H H H 2 2 H H H 2 H H 2 2 2 H 2 2 H Electron Avalanche 2 2 H H H 2 2 2 Note: 40MV/m & 100 Atm Cavity Energy E/p ≈ 5 W=1/ 2 C V 2 ≈ 1 joule → Heats gas Collision frequency >> cyclotron frequency B has no effect ! Done? Alan Bross M AP REVIEW 24-26 August, 2010 19
The Science of RF Breakdown III Gas • However we want to operate with up to 10 13 muons/ pulse – Beam-impact ionization + Ionization by secondary e - + + e - e - + H 2 � + + 2e - µ + H 2 � µ + H 2 H 2 ∆ ρ ∆ n ( dE / dx ) s 1 ≈ × 3 e ~ 1000/cm ≈ π ∆ 2 1 muon ( 35 eV ) ( ) W r s i b Most electrons (>90%) are quickly thermalized inside the cavity by elastic and inelastic collisions, and drift with RF until annihilated by recombination or attachment Alan Bross M AP REVIEW 24-26 August, 2010 20
The Science of RF Breakdown IV Gas see K. Yonehara’s talk 1.2 1. Rapid decay of pickup Amplitude of pickup signal (Arb.) Beam on Beam off signal according to the 1.0 ionization rate Different beam intensity 2. Saturation level and 9 protons/bunch 10 0.8 recovery rate determined 8 protons/bunch 10 by the recombination rate 7 protons/bunch 10 0.6 6 protons/bunch 10 0.4 Gray line: normal signal 0.2 without beam 0.0 Solution:? -10 -5 0 5 10 15 20 25 30 35 40 45 50 Electron “getter” Time ( µ s) RF off RF on Electro-neg. Gas Alan Bross M AP REVIEW 24-26 August, 2010 21
Science of RF Breakdown Summary • Although the study of breakdown in RF cavities is an active (& continuing) field of research, and academic study of RF breakdown is not M AP’s mission • We know: – Without surface field enhancements, there is no field emission – Without field emission, the events that lead to the damaging breakdowns (B ≠ 0 will not occur) • So: – Eliminate ( ameliorate ) the surface field enhancements – Or mitigate the damaging effects to the cavity from the resulting events Alan Bross M AP REVIEW 24-26 August, 2010 22
NCRF Program R&D Strategy � Technology Assessment (continuation of existing multi- pronged program) – Surface Processing • Reduce (eliminate?) surface field enhancements, field emission – SCRF processing techniques » Electro-polishing (smooth by removing) + HP H 2 O rinse Vacuum – M ore advanced techniques (Atomic-Layer-Deposition (ALD)) » Smooth by adding to surface (conformal coating @ molecular level) – M aterials studies: Use base materials that are more robust to the focusing effects of the magnetic field • Cavity bodies made from Be or possibly M o – M agnetic Insulation • Inhibit focusing due to applied B – High-Pressure Gas-filled (H 2 ) cavities Alan Bross M AP REVIEW 24-26 August, 2010 23
201 M Hz Cavity Test Treating NCRF cavities with SCRF processes • 21 M V/ m Gradient Achieved (Design – 16M V/ m) – Limited by available RF power (4.5 M W) Alan Bross M AP REVIEW 24-26 August, 2010 24
201 M Hz Cavity Running Summary I (B=0) Limited by RF Power Design Gradient Alan Bross M AP REVIEW 24-26 August, 2010 25
201 M Hz Cavity Running Summary II (B>0) Alan Bross M AP REVIEW 24-26 August, 2010 26
201 M Hz Cavity B Field Tests Summary • Sparking @ B=0 did condition the 201 cavity • Sparking @ B ≠ 0 causes damage (B relatively low) • Although we “ broke” the 201, it did re-condition @ B=0. • But upon inspection of the cavity – No observed damage • SCRF processing techniques help Alan Bross M AP REVIEW 24-26 August, 2010 27
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