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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

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SLIDE 1

RF Strategy & the M uCool T est Area

Alan Bross M AP REVIEW 24-26 August, 2010 1

Meeting the RF in Magnetic Field Challenge

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SLIDE 2
  • 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
  • nehara

– The M uCool Test Area – Summary

Alan Bross M AP REVIEW 24-26 August, 2010 2

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SLIDE 3

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

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SLIDE 4

Just to Review

Remember, from Long Ago – (Yesterday)

Alan Bross M AP REVIEW 24-26 August, 2010 4

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SLIDE 5

Normal Conducting RF

R&D Issues and Present Status

  • M uon bunching, phase rotation and cooling

requires Normal Conducting RF (NCRF) that can

  • perate 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

Alan Bross M AP REVIEW 24-26 August, 2010 5

But

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SLIDE 6

The RF Challenge

  • Significant degradation in maximum stable
  • perating gradient with applied B field

Alan Bross M AP REVIEW 24-26 August, 2010 6

  • 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

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SLIDE 7

805 Pillbox

Post-M ortem

Alan Bross M AP REVIEW 24-26 August, 2010 7

  • Significant damage
  • bserved

–

Iris

–

RF coupler

–

Button holder

  • However

–

No damage to Be window

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SLIDE 8

805 M Hz Imaging

Alan Bross M AP REVIEW 24-26 August, 2010 8 8

B

Hot Spot Arc forms

Cavity Energy W=1/2 CV2 ≈ 1-5 joule All goes into melting Cu

Surface Field Enhancement Initiates the event & B focuses the e- current which causes damage

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SLIDE 9

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 9 M AP REVIEW 24-26 August, 2010

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SLIDE 10

201 M Hz Prototype

Alan Bross M AP REVIEW 24-26 August, 2010 10

Note: Stored energy available to sparks ≈ 100J

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SLIDE 11

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

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SLIDE 12

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

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SLIDE 13

The Science of RF Breakdown II

Vacuum

Alan Bross M AP REVIEW 24-26 August, 2010 13

Norem et al., 2001-2010

Workshop on Uni-polar Arcs, ANL, Jan. 2010 Breakdown Physics Workshop, CERN May 2010

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SLIDE 14

The Science of RF Breakdown III

Vacuum

  • Advanced Simulation Code

– OOPIC & VORPAL: Kevin Paul, Tech-X

Alan Bross M AP REVIEW 24-26 August, 2010 14

New particles added (lost removed) {xα, vα} Particles accelerated by the fields {v'α} Particles moved based on new velocity {x'α} Currents “deposited”

  • n the grid

{Ji}

One Time Step

Collisions and interactions computed New fields computed from

  • ld fields

{E'i, B'i}

This is where all the interesting physics for RF breakdown takes place!!!

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SLIDE 15

The Science of RF Breakdown IV

Vacuum

Alan Bross M AP REVIEW 24-26 August, 2010 15

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SLIDE 16

The Science of RF Breakdown V

Vacuum

Alan Bross M AP REVIEW 24-26 August, 2010 16

Dazhang Huang (IIT), Particle Studio Simulation

0 0.1 T 0.25 T 0.5 T

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SLIDE 17

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

E field contour Trajectories without external B field Trajectories with external B = 2-T field

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SLIDE 18

The Science of RF Breakdown

Gas

  • RF cavities filled with High-Pressure H2

– Paschen’s Law

Alan Bross M AP REVIEW 24-26 August, 2010 18

b pd pd a Vbd + = ) ln( ) (

Rolland Johnson

Shelter Island 2002

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SLIDE 19

The Science of RF Breakdown II

Gas

Alan Bross M AP REVIEW 24-26 August, 2010 19

Gas Filled Cavities

No focusing of electron avalanche

+

Electron Avalanche

H

2

H

2

H

2

H

2

H

2

H

2

H

2

H

2

H

2

H

2

H

2

H

2

H

2

Vrf = Vo Sin[ωt]

Cavity Energy W=1/ 2 C V2

≈ 1 joule → Heats gas

Collision frequency >> cyclotron frequency B has no effect !

Done?

Note: 40MV/m & 100 Atm E/p ≈ 5

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SLIDE 20

The Science of RF Breakdown III

Gas

  • However we want to operate with up to 1013

muons/ pulse – Beam-impact ionization + Ionization by secondary e-

Alan Bross M AP REVIEW 24-26 August, 2010 20

3 2

1000/cm ~ ) ( 1 ) eV 35 ( ) / ( muon 1 s r W s dx dE n

b i e

∆ × ≈ ∆ ≈ ∆ π ρ

µ + H2 µ + H2

+ + e-

e- + H2 H2

+ + 2e-

Most electrons (>90%) are quickly thermalized inside the cavity by elastic and inelastic collisions, and drift with RF until annihilated by recombination or attachment

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SLIDE 21

The Science of RF Breakdown IV

Gas

Alan Bross M AP REVIEW 24-26 August, 2010 21

  • 10
  • 5

5 10 15 20 25 30 35 40 45 50 0.0 0.2 0.4 0.6 0.8 1.0 1.2 RF off RF on Different beam intensity 10

9 protons/bunch

10

8 protons/bunch

10

7 protons/bunch

10

6 protons/bunch

Amplitude of pickup signal (Arb.) Time (µs)

Beam on Beam off

  • 1. Rapid decay of pickup

signal according to the ionization rate

  • 2. Saturation level and

recovery rate determined by the recombination rate Gray line: normal signal without beam

Solution:? Electron “getter” Electro-neg. Gas see K. Yonehara’s talk

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SLIDE 22

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

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SLIDE 23

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 H2O rinse – 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 (H2) cavities

Alan Bross M AP REVIEW 24-26 August, 2010 23

Vacuum

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SLIDE 24

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)

24 M AP REVIEW 24-26 August, 2010 Alan Bross

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SLIDE 25

201 M Hz Cavity Running Summary I (B=0)

Design Gradient Limited by RF Power 25 M AP REVIEW 24-26 August, 2010 Alan Bross

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SLIDE 26

201 M Hz Cavity Running Summary II (B>0)

26 M AP REVIEW 24-26 August, 2010 Alan Bross

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SLIDE 27

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

27 M AP REVIEW 24-26 August, 2010 Alan Bross

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SLIDE 28

Advanced Processing Concept

Atomic Layer Deposition (ALD)

  • Field enhancements in RF cavities root cause of surface breakdown

– Due to high local electric fields that exist at surface asperities (field

enhancements)

– Cover asperities with a conformal coatings applied with Atomic Layer

Deposition (ALD)

– Has been applied to SCFR

Alan Bross M AP REVIEW 24-26 August, 2010 28

108 109 1010 1011 Quench @ Eacc = 32.9 MV/m

Q0 Eacc [MV/m]

5 15 20 25 30 35 10 Atomic Layer Deposition (10 nm Al2O3 + 3 nm Nb2O5)

Previous Best Cavity Performance (Initial Electro-Polish and Bake) Cavity As Received For Coating

Single Cell Cavity Test (J Lab 6/27/08) Argonne Cavity Coating Procedure

1 example of ALD processing of 1.3GHz SCRF cell

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SLIDE 29

NCRF R&D Program

Potential paths towards a solution: Phase I: Technology

Assessment (continuation of existing program) M ulti- pronged approach: – Surface Processing

  • Reduce (eliminate?) surface field enhancements, dark current

– SCRF processing techniques » Electro-polishing (smooth by removing) + HP H2O rinse – 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 – High-Pressure Gas-filled (H2) cavities

Alan Bross M AP REVIEW 24-26 August, 2010 29

Vacuum

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SLIDE 30

M aterial Studies

  • “ Button” system in pillbox cavity

designed for easy replacement of test materials

– T

ested so far: TiN-coated Cu & M o, bare M o and W

– Results to date indicate that M o can

improve performance at a given B field by somewhat more than 50%

– 16.5M V/ m → 26M V/ m

Alan Bross M AP REVIEW 24-26 August, 2010 30

Molybdenum buttons

(1.7x field enhancement factor on button surface)

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SLIDE 31

Button RF Cell Data Summary

Alan Bross 31 M AP REVIEW 24-26 August, 2010

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SLIDE 32

NCRF R&D Program

Potential paths towards a solution: Phase I: Technology

Assessment (continuation of existing program) M ulti- pronged approach: – 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

– Surface Processing

  • Reduce (eliminate?) surface field enhancements, dark current

– SCRF processing techniques » Electro-polishing (smooth by removing) + HP H2O rinse – M ore advanced techniques (Atomic-Layer-Deposition (ALD)) » Smooth by adding to surface (conformal coating @ molecular level)

– M agnetic Insulation – High-Pressure Gas-filled (H2) cavities

Alan Bross M AP REVIEW 24-26 August, 2010 32

Vacuum

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SLIDE 33

M agnetic Insulation

  • Although lattices that employ magnetic

insulation have drawbacks with respect to the required RF power, we are studying the concept using a newly completed 805 M Hz box cavity

Conceptual Design

33 M AP REVIEW 24-26 August, 2010 Alan Bross

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SLIDE 34

Alan Bross M AP REVIEW 24-26 August, 2010 34

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SLIDE 35

Box Cavity in Solenoid

35 M AP REVIEW 24-26 August, 2010 Alan Bross

  • M ax angle w/ r to

horizontal ≈ 120 – E at 78o w/ r to B

  • M ax Gradient (B=0)

– 40M V/ m

slide-36
SLIDE 36

NCRF R&D Program

Potential paths towards a solution: Phase I: Technology

Assessment (continuation of existing program) M ulti- pronged approach: – 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

– Surface Processing

  • Reduce (eliminate?) surface field enhancements, dark current

– SCRF processing techniques » Electro-polishing (smooth by removing) + HP H2O rinse – M ore advanced techniques (Atomic-Layer-Deposition (ALD)) » Smooth by adding to surface (conformal coating @ molecular level)

– M agnetic Insulation – High-Pressure Gas-filled (H2) cavities

Alan Bross M AP REVIEW 24-26 August, 2010 36

Vacuum

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SLIDE 37

High Pressure H2 Filled Cavity Work with M uons Inc.

  • High Pressure T

est Cell

  • Study breakdown properties of materials in H2 gas
  • Operation in B field

– No degradation in M .S.O.G. up to ≈ 3.5T

  • Next Test – Repeat with beam

No Difference B=0 & B=3T

37 M AP REVIEW 24-26 August, 2010 Alan Bross

Well beyond gradient requirement for HCC

slide-38
SLIDE 38

High Pressure H2 Filled Cavity Results

In Surface Breakdown Regime

Alan Bross M AP REVIEW 24-26 August, 2010 38

Pit distribution fit to Emax (ANSYS) ≈ Fowler-Nordheim

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SLIDE 39

RF T est Facility

  • M uCool Test Area (M TA)

– RF power

  • 201 M Hz (5M W)
  • 805 M Hz (12 M W)

– Class 100 clean room – 4T SC solenoid

  • 250W LHe cryo-plant

– Instrumentation

  • Ion counters, scintillation

counters, optical signal, spectrophotometer

– 400 M eV p beam line

Alan Bross M AP REVIEW 24-26 August, 2010 39

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SLIDE 40

M TA Layout

Alan Bross M AP REVIEW 24-26 August, 2010 40

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SLIDE 41

M TA RF

Alan Bross M AP REVIEW 24-26 August, 2010 41

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SLIDE 42

M TA Cryo

Valve Box & Transfer Line

Alan Bross M AP REVIEW 24-26 August, 2010 42

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SLIDE 43

M TA Hall – Clean Room

43 M AP REVIEW 24-26 August, 2010 Alan Bross

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SLIDE 44

M TA Hall – Clean Room II

Alan Bross M AP REVIEW 24-26 August, 2010 44

  • Goal for Clean room : Class 100

– Achieved better than Class 10

  • Even with 3 people inside: Class 40
  • Goal for Hall: Class 1000

– Achieved Class 500

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SLIDE 45

M TA Instrumentation

Alan Bross M AP REVIEW 24-26 August, 2010 45

Counters

Optical Diagnostics

DAQ RF Pickup X-ray Optical

slide-46
SLIDE 46

Current M TA Beam Line

Alan Bross M AP REVIEW 24-26 August, 2010 46

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SLIDE 47

M TA Beam Line Status

  • Beam Line Installation

–

Complete

  • Beam Line commissioning

to first beam stop.

–

Complete

  • First pass @ Radiation

assessment review

–

Complete

  • First beam experiments

by the end of the year

The MTA is a World-Class Facility (& Unique): High-Power RF; High-Intensity Beam; H2 handling, SC Magnet

47 M AP REVIEW 24-26 August, 2010 Alan Bross

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SLIDE 48

Phase I RF Program (2 year)

  • D. Li, Next Talk
  • Complete tests on M agnetic Insulation

– Second box test series with E ⊥ B – Box with orientation E || B

  • Be materials tests

– Button cavity test – Be wall cavity

  • ALD coated button test

– And with purpose RF cavity

  • Beam tests of high pressure H2 filled cavity
  • 201 M Hz tests in higher B field

– Need new SC magnet - FY2012

Alan Bross M AP REVIEW 24-26 August, 2010 48

slide-49
SLIDE 49

RF Strategy

Down Selecting

  • Down selection of RF cavities will be based on

the outcome of the experimental studies. The successful cavity technology must work at an acceptable RF gradient (requirements are, of course, dependent on the position along the channel, i.e., phase rotation, bunching, initial cooling, final cooling, etc.) in a multi-tesla magnetic field. Engineering, fabrication, integration, and cost of the cavity and RF power must also be considered

Alan Bross M AP REVIEW 24-26 August, 2010 49

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SLIDE 50

Summary

  • We have a comprehensive program aimed at

developing a solution to the “ RF Challenge”

  • The experimental program is robust and

scope-appropriate, is coupled with mature simulation efforts and is backed by a good understanding of the physics of breakdown/arcs in RF structures

  • This multi-pronged approach gives us

confidence that we can meet this challenge and minimize the risk to a future M uon Collider

Alan Bross M AP REVIEW 24-26 August, 2010 50

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SLIDE 51

Acknowledgments

The ongoing effort on NCRF has a long list of contributors who have contributed to this presentation. I thank them all (in no particular

  • rder):

Bob Palmer Jim Norem Alvin Tollestrup M oses Chung Bob Rimmer Derun Li Al M oretti Pierrick Hanlet Zubao Qian Dazhang Huang Katsuya Y

  • nehara

Y agmur Torun M ilorad Popovic Diktys Stratakis Kevin Paul Steve Virostek Thomas Prolier

Alan Bross M AP REVIEW 24-26 August, 2010 51

slide-52
SLIDE 52

END

Alan Bross M AP REVIEW 24-26 August, 2010 52

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SLIDE 53

But it is not a Cliff

Alan Bross M AP REVIEW 24-26 August, 2010 53

µ/p after initial (4D cooling)