MuCool Test Area Experimental Program Summaryvietnam.in2p3.fr/2016/nufact/transparencies/... · Linacs, RLA or FFAG, RCS Cooling ... • MTA has a 5-T superconducting solenoid that

Alexey Kochemirovskiy

The University of Chicago/Fermilab

MuCool Test Area Experimental

Program Summary

Alexey Kochemirovskiy | NuFact'16 (Quy Nhon, August 21-27, 2016)

Outline

• Introduction

– Motivation

– MTA facility

• 201 MHz MICE cavity program

• Gas filled cavity program

• Study of vacuum RF breakdown in 805MHz

cavities

• Conclusion

August 23, 2016 Alexey Kochemirovskiy | NuFact'16 (Quy Nhon, August 21-27, 2016) 2

Why are we interested in Muons?


Muon accelerator R&D is focused on developing a

facility that can address critical questions concerning

two frontiers:

• The Intensity Frontier with Neutrino Factory

generating well-characterized neutrino beams for

precise high sensitivity study

• The Energy Frontier with a Muon Collider capable

of probing multi-TeV energies

Muon Ionization Cooling in called for


Buncher

PhaseRotator

InialCooling

CaptureSol.

ProtonDriver FrontEnd

MW-ClassTarget

Accelera on

DecayChannel

µ StorageRing

ν

281m

Accelerators:Single-PassLinacs

0.2–1GeV

1–5GeV

5GeV

ProtonDriver Accelera on ColliderRing

Accelerators:Linacs,RLAorFFAG,RCS

Cooling

µ+

6DCooling

6DCooling

FinalCooling

Bunch

Merge

µ−

µ+ µ−

Share same complex

n Factory Goal: 1021 m+ & m- per year within the accelerator

acceptance

NeutrinoFactory(NuMAX)

MuonCollider

m-Collider Goals: 126 GeV

~14,000 Higgs/yr

Multi-TeV Lumi > 1034cm-2s-1

ECoM:

HiggsFactoryto

~10TeV

Cool-ing

InialCooling

ChargeSep

arator

ν µ+

µ−

Buncher

PhaseRotator

CaptureSol.

MW-ClassTarget

DecayChannel

FrontEnd

SCLinac

SCLinac

Accumulator

Buncher

Accumulator

Buncher

Combiner

Muon cooling concept

August 23, 2016 5

Short lifetime of muon(2.2µs) require accelerating gradients of tens MV/m

Gradient is restricted by RF breakdown, rate of which should be kept small

It was experimentally shown that strong magnetic fields aggravates problem

Study of RF breakdown in strong magnetic field is needed


Fermilab’s Mucool Test Area (MTA)

Facility built specifically for muon cooling hardware R&D

• Capacity to test 201 and 805MHz

cavities in strong magnetic field

• H- beamline passes through the

center of magnet bore

• Infrastructure for clean room

assembly and inspection

• Extensive instrumentation for BD

characterization

• Run control system to detect

breakdown events and

record relevant data streams

August 23, 2016 6

MagnetCavity Beamline


Data acquisition and run control system

August 23, 2016

• Trigger system for breakdown detection and run control

• Fast oscilloscopes to record time sensitive signals <- Labview

– Cavity pickups

– Light signal from optical fibers

– Scintillators ( X ray detection)

– Forward and reflected power

– Radiation detectors

• Vacuum pressure data

• Temperature sensors

• Acoustic spark localization

• Etc.

7

Run control station at the linac gallery


MTA experimental programs

• 201MHz MICE cavity


• 805MHz vacuum program







International Muon Ionization Cooling

Experiment (MICE)


• 201 MHz cavity module very similar to that of the full ionization cooling

demonstration in MICE tested in MTA

• Design gradient of 10.3 MV/m, with cavities operated in fringe fields of multi-

Tesla solenoid magnets

• MTA has a 5-T superconducting solenoid that provides operational conditions

similar to MICE channel

201MHz MICE cavity


• Pillbox geometry (~1m in

diameter, 40cm RF gap)

• External vacuum vessel

• Mechanical tuners

• Extensive instrumentation

MICE cavity program: operation results


• The experimental program complete

• Cavity has been tested under operating conditions similar to MICE

• Stable performance is demonstrated in both B=0T and B=3T external

field configurations with copper and beryllium windows

• Zero breakdown rates at MICE design peak gradient (~10.3MV/m)

• Measured radiation rates are within limits for tracker backgrounds

• Gained a lot of experience in surface preparation, design and operation






Helical Cooling Channel


• The main idea is to combine absorber

and RF cavity together

• Gas filled RF cavities operating inside

helix of solenoids

• Emittance exchange from dispersion

in magnetic field – 6D cooling

• Pressured gas inside the cavities acts

both as an absorber for beam cooling

an as RF breakdown mitigator

• Hydrogen is an ideal absorber due to

large radiation length and stopping

power

Gas breakdown vs metal breakdown


Maximum stable gradient in hydrogen filled cavity for different electrode materials

• Maximum gradient increases with gas pressure (Paschen curve)

• Gradient behavior gets dominated by metallic RF breakdown at high pressures

• Baseline 20MV/m gradient is demonstrated for gas pressure as low as 20 atm

• Gradient is not affected by external magnetic field (green vs magenta)

Pressured gas cavity – beam considerations


• Beam passing through the cavity must not trigger breakdown –

experiments at MTA showed it does not

• Resultant plasma loads the cavity which leads to degradation of

accelerating gradient - needs to be minimized

Plasma loading effect in gas filled cavity

Factors effecting plasma loading:

• Gas pressure

• Dopant concentration

• Beam intensity

Dielectric Loaded Pressurized Cavity


HCC design challenge: 325 & 650 MHz pillbox cavities do not fit in bores of

present magnet technology

Possible solution – load cavity with dielectric to decrease resonant frequency

Alumina - ideal dielectric candidate due to low losses

Specially designed alumina donut insert to study effect on RF breakdown

Dielectric Loaded High Pressure Cavity with alumina donut insert

Dielectric Loaded High Pressure cavity -

performance


• Gas breakdown observed up to ~10atm pressure

• Comparison with baseline “no-insert” performance shows gradient

limited by alumina

• No clear dependence of gradient on alumina purity

High pressure gas cavities: summary


• Demonstrated that high pressure gas filled cavities can be

operated in external multi-Tesla magnetic fields

• Baseline 20MV/m gradient in multi-Tesla field is reachable with

hydrogen pressure as low as 20atm

• Gas breakdown and metallic breakdown curves investigated

for different gas species/dopants and metal material

• Established breakdown limits for alumina of 11-17MV/m

depending on alumina purity and gas composition






Why do we see the deterioration of cavity

performance in strong B fields?

August 23, 2016

• Dark current: electron field emission from surface

imperfections

• B field focuses dark current into “beamlets”

• Beamlets cause pulsed heating of the surface

• Pulsed heating leads to surface degradation

• Breakdown is triggered

Potential mitigations:

• Surface treatment

• Use higher radiation length materials ( for ex. Be)

• Decrease impact energy density of electrons

D.Stratakis , J.Gallardo, R.Palmer, Nucl. Inst. Meth. A 620 (2010), 147-154

21Alexey Kochemirovskiy | NuFact'16 (Quy Nhon, August 21-27, 2016)

Mitigation technique: surface treatment

August 23, 2016

201 MHz MICE cavity - electro

polished, clean room environment

Conditioned to design gradient of

10.3MV/m with no breakdowns

All Seasons Cavity – stainless

steel with copper coating

Modular Cavity - chemically

polished, clean room environment.

45MV/m in 0T field reached

Alexey Kochemirovskiy | NuFact'16 (Quy Nhon, August 21-27, 2016) 22

Mitigation technique: geometry

• Box cavity: magnetic insulation

August 23, 2016

E

B

Magnetic field deflects

dark current electrons

to low E field regions

Works well only for angles very close to 90o

Small shunt impedance compared to pillbox cavities


Mitigation technique: geometry

August 23, 2016

All Seasons Cavity: longer RF gap length

15 cm

Electron energy as a function of

cavity length

Θ = 0o

Θ = 30o

Θ = 60o


Mitigation Technique: geometry

August 23, 2016

Grid windows

• Allow the beam to pass though

• Increase shunt impedance of the cavity

• Allow dark current beamlets to exit cavity volume


Performance of various 805MHz cavities

August 23, 2016

Factors that affect fit quality:

• Condition history

• Local field enhancement around coupler regions

• Surface treatment


No surface prep

for this cavity

Pillbox 805MHz “modular” cavity

August 23, 2016 27

• End walls can be un-mounted easily

• Allows for end wall material swap

• Low E fields in the coupler region


• Allows for careful control over experimental conditions

• Evaluate different materials

• Perform frequent inspections to track surface state

Goal: to build a coherent picture of processes inside the cavity during breakdown

Unique design features:

Surface inspection after B=3T run

August 23, 2016 28

All clearly visible damage was inflicted during B=3T run (!)

Endplate after B=0T run with peak gradient of 50MV/m

Endplate after B=3T run with peak gradient of 12MV/m

First time inspections were carried out separately after run at zero

magnetic field and run at high magnetic field


Inspection after B=3T run: damage

microstructure

August 23, 2016 29

Typical

breakdown

damage

• Characteristic diameter of a BD damage ~1.5mm

with melted core up to ~70um deep

• Traces of splashing

• Damage is much more “violent” after B=3T run,

although stored energy is 16 times lower than in

B=0T run

Splashing traces around BD damage

5mm

2mm


0.5mm

0.2mm Crater depth ~ 30µm

Inspection after B=3T runs: damage pattern

August 23, 2016

30

Endplate Center

Coupler

Map of “volcanos”

• Perfect 1-to-1 correspondence between

354 “volcanos” on each endplate (new

result) - supports the model of BD being

induced by focused dark current

• Damage distribution is denser in high E field

region

• Mystery: detected 136 sparks, but observed

354 damage sites


Next set of measurements will be with

Beryllium endplates

August 23, 2016 31

• Radiation length of Beryllium (~35cm) for electrons

is higher than of copper (~1.4cm)

• Mitigation of breakdown triggers on the surface

• Better gradient performance

There are several measurements enabled by Beryllium:

– Direct measurement of dark current (Faraday

Cup)

– Measurement of transverse emittance of dark

current beamlets (film/glass)

Inner surface of Beryllium endplate

2.6 mm-thick window

What we expect to observe:

*Beryllium – copper configuration is also being discussed


Conclusion

• We have gained a lot of hands-on experience operating cavities in external magnetic fields at MTA facility

• Applying advanced surface preparation techniques resulted in meeting design gradients for 201MHz MICE cavity

• We have demonstrated > 20 MV/m operation of 805 MHz vacuum cavities at B = 5 Tesla. That is an encouraging result towards demonstrating the feasibility of vacuum cavity-based ionization cooling channel

• Using RF cavities with high-pressure gas, we have demonstrated a general solution to the cooling problem

• A lot of interesting results coming from studying the problem of RF breakdown in strong magnetic field

• The work is still ongoing, including data analysis and high-power tests


“Thank you” slide


Thank you!

Documents

MuCool Test Area Experimental Program Summaryvietnam.in2p3.fr/2016/nufact/transparencies/... · Linacs, RLA or FFAG, RCS Cooling ... • MTA has a 5-T superconducting solenoid that