# Presentation: 186918

## E-169: Wakefield Acceleration in Dielectric StructuresThe planned experiments at FACET

- J.B. Rosenzweig
*UCLA Dept. of Physics and Astronomy*
- AAC 2008 — Santa Cruz — July 30, ‘08

## E169 Collaboration

- H. Badakov, M. Berry, I. Blumenfeld, A. Cook, F.-J. Decker, M. Hogan, R. Ischebeck, R. Iverson, A. Kanareykin, N. Kirby, P. Muggli, J.B. Rosenzweig, R. Siemann, M.C. Thompson,        R. Tikhoplav, G. Travish, R. Yoder, D. Walz
***Department of Physics and Astronomy, University of California, Los Angeles*
***Stanford Linear Accelerator Center*
***University of Southern California*
***Lawrence Livermore National Laboratory*
- *Manhattanville College*
***Euclid TechLabs, LLC*
- Collaboration spokespersons

- UCLA

## E-169 Motivation

- Take advantage of unique experimental opportunity at SLAC
- FACET: ultra-short intense beams
- Advanced accelerators for high energy frontier
- Very promising path: dielectric wakefields
- Extend successful T-481 investigations
- Dielectric wakes >12 GV/m
- Complete studies of transformational technique

## Future colliders: ultra-high fields in accelerator

- High fields in violent accelerating systems
- High field implies high 
- Relativistic oscillations...
- Limit peak power
- Stored energy
- Challenges
- Scaled beams!
- Breakdown, dark current
- Pulsed heating
- What sources < 1 cm?

## Scaling the accelerator in size

- Lasers produce copious power (~J, >TW)
- Scale in size by 4 orders of magnitude
** < 1 m gives *challenges* in beam dynamics
- Reinvent the resonant structure using *dielectric *(E163, UCLA; see G. Travish talk)
- To jump to GV/m, only *need* mm-THz
- Must have new source...

**Resonant dielectric **
**Structure (HFSS sim.)**

## Possible new paradigm for high field accelerators: wakefields

- Coherent radiation from bunched, *v~c* e- beam
*Any impedance* environment
- Also powers more exotic schemes: plasma, dielectrics
- Non-resonant, *short pulse* operation possible
- Intense beams needed by other fields
- X-ray FEL
- X-rays from Compton scattering
- THz sources

## High gradients, high frequency, EM power from wakefields: CLIC @ CERN

- CLIC drive beam extraction structure

**Power**

## The dielectric wakefield accelerator

- Higher accelerating gradients: GV/m level
- Dielectric based, low loss, short pulse
- Higher gradient than optical? Different breakdown mechanism
- No charged particles in beam path...
- Use wakefield collider schemes
- CLIC style modular system
- Afterburner possibility for existing accelerators
- Spin-offs
- V. high power THz radiation source

## Dielectric Wakefield AcceleratorOverview

-  Electron bunch ( ≈ 1) drives ***Cerenkov wake*** in cylindrical dielectric structure
- Variations on structure features
- Multimode excitation
-   Wakefields accelerate trailing bunch

-   Design Parameters

- Ez on-axis, OOPIC

- *

## T-481: Test-beam exploration of breakdown threshold

- Leverage off E167
- Existing optics
- Beam diagnostics
- Running protocols
- Goal: breakdown studies
- Al-clad fused silica fibers
- ID 100/200 m, OD 325 m, *L*=1 cm
- Avalanche v. tunneling ionization
- Beam parameters indicate ≤12 GV/m longitudinal wakes!
- 30 GeV, 3 nC, z ≥ 20 m
- 48 hr FFTB run, Aug. 2005

- T-481 “octopus” chamber

## T481: Beam Observations

- Multiple tube assemblies
- Alignment to beam path
- Scanning of bunch lengths for wake amplitude variation
- Excellent flexibility: 0.5-12 GV/m
- Vaporization of Al cladding... dielectric more robust
- Observed breakdown threshold (field from simulations)
- 13.8 GV/m surface field
- 5.5 GV/m deceleration field
- Multi-mode effect?
- Correlations to post-mortem inspection
- Details in G. Travish talk, PRL

## OOPIC Simulation Studies

- Parametric scans
- Heuristic model benchmarking
- Determine field levels in experiment
- Show pulse duration...

- Multi-mode excitation – short, separated pulse

- Single mode excitation

- Example scan, comparison to heuristic model

- Fundamental 

## E169 at FACET: overview

- Research GV/m acceleration scheme in DWA
- Push technique for next generation accelerators
- Goals
**Explore breakdown issues in detail**
**Determine *****usable***** field envelope**
**Coherent Cerenkov radiation measurements**
**Explore alternate materials**
**Explore alternate designs and cladding**
**Varying tube dimensions**
- Impedance change
- Breakdown dependence on wake pulse length
- Approved experiment (EPAC, Jan. 2007)
- Awaits FACET construction

## E-169: High-gradient AccelerationGoals in 3 Phases 

- Phase 1:  Complete breakdown study (when does E169->E168!)
- Coherent Cerenkov (CCR) measurement

-  explore (a, b, **z) parameter space
- Alternate cladding
- Alternate materials (e.g. CVD diamond)
- Explore group velocity effect

|   z | ≥ 20 m |
| --- | --- |
|   r | < 10 m |
| U | 25 GeV |
|   Q | 3 - 5 nC |

-  Total energy gives field measure
-  Harmonics are sensitive ***z* diagnostic

- FACET beam parameters for E169: high gradient case

## E-169 at FACET: Phase 2 & 3

- E-169 at FACET: Phase 2 & 3

- Phase 2:  Observe acceleration

-  10 cm tube length
-   longer bunch, ***z** *~ 150 m
-   moderate gradient, 1 GV/m
-  single mode operation

|   z | 150  m |
| --- | --- |
|   r | < 10 m |
|  U | 25 GeV |
|   Q | 3 - 5 nC |

- Phase 3:  Scale to 1 m length

- Ez on-axis

- Before & after momentum distributions (OOPIC)

- *

-  Alignment
-  Group velocity & EM exposure

- FACET beam parameters for E169: acceleration case

## Experimental Issues: THz Detection

- Conical launching horns
- Signal-to-noise ratio
- Detectors
- Test at Neptune now...

-   Impedance matching to free space
-   Direct radiation forward

-   Background of CTR from tube end
-   SNR ~ 3 - 5 for 1 cm tube

-   Pyroelectric
-   Golay cell
-   Helium-cooled bolometer
-   Michelson interferometer for autocorrelation

## Neptune THz CCR experiment

- Optics

- Waveguide

- See presentation by A. Cook

- Beam Energy  11 MeV

- Charge  300 pC

- Bunch length  200 um

- RMS radius  50 - 100 um

## Experimental Issues: Alternate DWA design, cladding, materials

- Aluminum cladding used in T-481
- Dielectric cladding
- Bragg fiber?
- Low HOM
- Alternate dielectric: CVD diamond
- Ultra-high breakdown threshold
- Doping gives low SEC

-   Vaporized at even moderate wake amplitudes
-   Low vaporization threshold due to low pressure
-      and thermal conductivity of environment

-   Lower refractive index provides internal reflection
-  Low power loss, damage resistant

- Bragg fiber

- A. Kanareykin

- CVD deposited diamond

## Alternate geometry: slab

- Slab geometry suppresses transverse wakes*
- Also connects to optical case
- Price: reduced wake amplitude
- Interesting tests at FACET
- Diamond example, 600 MV/m

- *A. Tremaine, J. Rosenzweig, P. Schoessow, Phys. Rev. E 56,  7204 (1997)

## E-169: Implementation/Diagnostics

- New precision alignment vessel?
- Upstream/downstream OTR screens for alignment
- X-ray stripe
- CTR/CCR for bunch length
- Imaging magnetic spectrometer
- Beam position monitors and beam current monitors
- Controls...

- Much shared with general SLAC goals, E168

## Conclusions/directions

_*Unique *_opportunity to explore GV/m dielectric wakes at FACET
- Flexible, ultra-intense beams
- Only possible at SLAC FACET
- Low gradient experiments at UCLA Neptune
- Extremely promising first run
- Collaboration/approach validated
- Much physics, parameter space to explore