Notes on ATF research program (how it may evolve with transition to ATF-II)

1. Transition period at a glance

(This is an outlook on the transition period to ATF II and beyond from the perspective of developing a strong flagship experimental subprogram with notable ATF leadership aimed to heighten the facility’s scientific impact. We do not address here other aspects of ATF-II as SC NUF, such as productivity, etc.)

Based on evolution of facility capabilities, the following stages within the transition period can be identified:

FY16-17 (ATF continues operations, ATF-II Phase I)

Short term goals for the ATF→ATF-II transition: Maintain momentum with serving user experiments (including new ones) and obtaining new scientific results based on new (short term developed) capabilities.

Approach: In order to reduce impact of ATF dark time and gradual restore of capabilities at ATF II, we shall use most efficiently remaining resources at ATF.

Plan:

a. UED on line - Important reportable item, however, scientific impact is moderate. Therefore, we need to think at the same time about next accomplishments listed below under b-e:

b. Initiate a strong program around x-band RF deflection cavity with several active experiments providing enhanced scientific output (AE52, 62, 65, 68, 73).

c. Increase number of users and experiments that use CO2 only. In addition to present ion acceleration (AE66, NRL – Imp. Coll.), 3 more experiments will become productive: LWFA (AE71, SUNY-led collaboration), laser plasma machining (AF69, Tsinghua Un.), CO2 channeling in air (AE74, UCLA).

d. Consider installing strong-field laser at ATF EH will 3-4 experiments served (AF69, AE70, AE71, new Trojan Horse). Together with other active experiments, this plan (b-d) fills FY17 and guarantees that ATF program looks strong all the way until ATF terminates.

e. Initiate 1st ATF-II user experiment early. This could be electron noise experiment after linac (AE48, UCLA-TelAviv Un.).My additional comment on this period: Consider postponing dark time until FY18. We shall afford completing Phase I with (e) without stopping ATF operations. More time in preparation to ATF shut down may allow to reduce dark time.

FY18 (ATF tapering and terminating operations, CO2 moved to ATF –II operates at 10 TW delivered to LEH, ATF-II EH1 and EH2 beamlines being constructed, x-band moved to EH2)

Mid-term goals for the ATF→ATF-II transition: Restore e-beam capabilities, create 10 TW CO2.

Approach: Restore selected categories of active experiments at ATF-II at earliest opportunity.

Plan:

f. Restart full program of laser only experiments at higher power.g. Restart full program of electron only experimentsh. Initiate some flagship experiments

FY19-20 (ATF-II status to beginning of FY19: Phase I of Stage I is completed; Phase 2 in progress; Laser delivery to EH1 in progress. To the end of FY20: three EHs are operational, Laser at 25TW)

Targeted reportable goals for the ATF→ATF-II transition: Fully operational multiple EHs. Moving beyond ATF by establishing flagship program.

Approach: Restore all active experiments at ATF-II at earliest opportunity. Continue construction towards completion while unfolding experimental program.

Plan:

i. Move CO2 beyond 10 TW (25 TW)j. Restart laser/e-beam experiments on early stagek. Establish a flagship program and getting first resultsl. Increase productivity by operating multiple experimental halls.

2. ATF unique resources for breakthrough towards Flagship experiments We need to identify ATF’s unique capabilities first:

1. Multi-TW, sub-ps CO2 laser (nobody has).2. CO2 laser combination with e-beam (nobody has)3. Combination of several laser wavelengths (nobody has) 4. Any unique combination of multiple advanced e-beam features and instrumentation

(compressor, deflector, flat beam, masks, dual pulse, pulse burst mode, comprehensive all-stage diagnostic, plasma sources, gas jets, spectrometers, interaction chambers). (Individual features are not unique but nobody has them in combination.)

Which unique scientific opportunities can be opened by those capabilities?1. Multi-TW, sub-ps CO2 laser. 1.1. Ion acceleration: Shock wave acceleration in gas jets practically achievable only with

CO2 due to low critical density. Advantage of shock acceleration – monoenergetic, fast scaling with laser intensity. Byproducts could be THz radiation, neutron production (?), bio/medical studies.

1.2. Bubble LWFA regime: Inherent low plasma density results in plasma bubble 1000 bigger in volume benefiting fundamental LWFA studies (note that field probing will require e-beam); high-charge associated with a big bubble useful for LWFA stand-alone accelerators or to drive all-optical PWA; this leads to all-optical Trojan Horse (will require 1-um laser for ionization injection.

1.3. Ionization and self-focusing in gases: bigger diameter plasma channels for PWA, longer confined propagation through atmosphere.

1.4. HHG: Promising to reach record high harmonics and energies2. CO2 laser combination with e-beam

2.1. Dielectric Laser Accelerator (in vacuum): Our main advantage - accelerating structures scale with lambda.

2.2. LWFA studies in multiple regimes including bubble, quasi-linear, self-modulated, linear; Main advantage – low density result in bigger structures, higher charges. Bigger structures better for conventional linac seeding. E-beam also useful for field probing. Both experiments (DLA and LWFA) require compressed ~10 fs electron bunches.

2.3. IFEL: Two main directions – record-high acceleration and multi-bunch intra-cavity regime for high average power. CO2 proved being advantageous for both application and we shall continue capitalizing on this moving to a new record of 1 GeV/m.

2.4. ICS: main advantages – more photons per Joule combined with efficient interaction geometry (matched beams), easy access to nonlinear regime for the same beam focusing, intracavity regime. Correspondingly several research directions: Record high harmonics and single-shot yields; recirculation including more sophisticated combination IFEL-ICA (need multi-pulse YAG photocathode driver)

3. Combination of several laser wavelengths (actually, we talk about 10m 100TW-class and 0.8 m TW-class).

3.1. Two-color LWFA with ionization seed: 10m light produces a big plasma cavity in quasi-nonlinear regime without self-injection, 0.8m light tightly focused inside cavity provides local ionization (virtual photocathode). Way to low-emittance beams.

3.2. Trojan Horse: Modification of plasma accelerator with some similarity to 3.1, also listed as 1.3. We put it here as Ti:S is important for this scheme.

3.3. Ti:S for plasma diagnostic: Maybe not unique by itself but critically important for any plasma acceleration studies via interference density mapping, Doppler scattering (from shock wave), Thomson scattering. This diagnostic should be a part of flagship plasma experiments.

3.4. Two- color ICS: new, not studied yet phenomenon.4. Any unique combination of multiple advanced e-beam features and instrumentation

Comments on 4: It is unlikely that ATF-II will set any record beam quality. But we should strive that emittance, energy spread, pulse duration/charge, stability of our e-beam satisfies high standards, actually the best that users can find anywhere. Similarly, we cannot pretend that we invent anything unique methods and devices unavailable elsewhere. But we shall insure that we have a set of options to satisfy diverse user requirements. These beam qualities and options could become crucial for successful flagship experiments. Therefore, we mention this (item 4) here in order not to overlook its importance. Properly designed equipment is important part of any experiment. We are not comprehensively equipped to serve flagship experiments with available instrumentation and should work on it (e.g. plasma sources).

3. Flagship experiments in detail

(Planning for selected high-impact experiments driven by unique ATF capabilities)

1. Trojan Horse plasma accelerator (including all optical TH)General significance and motivation; community impact. This plasma wakefield accelerator (PWA) scheme is one of most advanced and attracts prime attention because of the possibility to attain a low emittance. Long plasma channel is produced by laser ionization (axicon focus), Electron bunch produces a wake and a solid state laser produces seed ionization (virtual cathode). ATF advantage. First experiment is just completed at SLAC; it did not measure emittance. People who did experiment at SLAC are looking already at the ATF direction just by a fact that

CO2 laser can produce a wider plasma column by axicon focusing that is more convenient for experiment. More importantly, they want to use CO2 produced high-charge bubble for driving a plasma wake in this channel. Is opens a gate for conceptually new all-optical Trojan Horse. Status and plan to proceed. Collaborators. Users from Strathclyde Univ. and RadiaBeam (both participated in SLAC experiment) already approached us with this idea and wish to submit a proposal. First, they will produce a plasma column in hydrogen. We already have experience with this after Inverse Cherenkov acceleration experiment. Second, they can explore generating a wake inside this plasma with the ATF electron beam and seed a second probe bunch with the same linac. Third, they can seed a bunch optically with a strong-field laser placed in ATF EH. Final stage of all-optical TH will be done at ATF-II after laser upgrade.

Expected ATF breakthrough. If successful, the experiment may become possibly the most intricate and conceptually significant plasma acceleration experiment. Will put ATF among the leaders in plasma acceleration. Possible byproduct of low-emittance beam could be FEL studies, coherent Compton scattering. This may evolve in a novel research program at ATF-II.

2. Two-color LWFA. General significance and motivation; community impact. Two groups independently proposed this LWFA scheme that shares a lot of similarity with TH as leading to super-low emittance as well. Impact could be similar to the discussed TH.

http://journals.aps.org/prab/abstract/10.1103/PhysRevSTAB.17.061301

http://journals.aps.org/prab/abstract/10.1103/PhysRevSTAB.17.101301

ATF advantage. Method is designed for ATF as we are the only facility to do this.

0.4 μmao=0.01

10 μmao=1.4

εn=50 nm

Status and plan to proceed. Collaborators. Presentations by LBNL and Tsinghua were given at last meeting as an opportunity for ATF-II. Experiment does not need e-beam and can be started as soon as necessary laser power is achieved. 10 TW will be sufficient.

Expected ATF breakthrough. Very similar to the previous TH PWA. This will be the first demonstration of conceptually significant new plasma acceleration configuration. Together with HP, will mark establishing a strong plasma acceleration program setting ATF among the leaders in plasma acceleration. Possible by-product of low-emittance beam could be FEL studies, coherent Compton scattering.

3. LWFA with linac seed.

Plasma accelerator program highlighted by two previously discussed experiments will include also experiments that use linac’s femtosecond e-beam for probing different LWFA regimes including bubble, quasi-linear, self-modulated, linear. Directed with a laser, bunch will be accelerated to investigate accelerating fields, phasing, staging. Directed perpendicular, e-beam will map wake fields on a screen.

General significance and motivation; community impact. Never attempted previously only because of lack of proper facility, these studies important for understanding fundamentals of LWFA process, optimizing LWFA accelerators, and researching staging for collider.

ATF advantage. ATF not just will have combination of laser and e-beam, but CO2 provides bigger plasma structures that possible to investigate by this method.

Status and plan to proceed. Collaborators. Initial experiment from this program is in preparation by a collaboration (SUNY leads). DiMauro and Najmudin also have MURI grant for ATF experiment. Not clear what is their plan. To maintain successful plasma program, we need a plasma source, 10-20 fs, well focused and controlled e-beam, proper e-beam diagnostic to measure low-emittance beams

Expected ATF breakthrough. Altogether, the proposed 3 experiments will represent the world-wide most diverse and innovative program of LWFA studies. It is not targeted to set acceleration records but to get a superior experimental insight into the physics of LWFA and investigate new, unexplored mechanisms that may open routs to producing better quality beams.

4. Next-generation IFEL research programGeneral significance and motivation; community impact. IFELs are most advanced vacuum-based laser accelerators. Recent successful RUBICON experiment done at ATF by a UCLA group demonstrated two world records for IFELs: 100 MV/m energy gradient and 55 MeV net acceleration. Two directions to progress: 1-GeV IFEL with 100 TW CO2 laser, and a high-repetition IFEL with recirculating beam and electron bunch train. Extension of this idea is combining IFEL with ICS in circulation scheme STINGRAY.

ATF advantage. Long wavelength advantage in alignment and synchronization, especially for micro-bunched beams necessary for monoenergetic acceleration. These practical advantages proved being crucial as demonstrated by successful history of ATF IFEL experimentation from STELLA to RUBICON.

Status and plan to proceed. Collaborators. ATF has unsurpassed experience in IFEL. Active studies continue now. 1 GeV UCLA is in 2013 ATF Upgrade Proposal already. Recirculating IFEL is submitted for SBIR by Radiabeam.

Expected ATF breakthrough. After STELLA and RUBICON, ATF is already a leading destination for IFEL research. New results will add significantly higher visibility and value to the IFEL technique as an option for compact accelerators for practical applications, including DoD. This may also open additional sources of funding towards development of more advanced high repetition CO2 lasers.

5. Dielectric Laser Accelerator, DLA (in vacuum)General significance and motivation; community impact. DLA concept since 1982. CO2 advantages have been recognized early as well, have been promoted by R. Palmer. His Grating Linac proposal was among main motivations for selecting CO2 for ATF. Experiment was never done as other interesting options were investigated instead (such as IFEL, inverse Cherenkov). Although DLA research continued elsewhere, there were no really high-impact developments until last two years when interest to DLA was further boosted by a) experiment on energy modulation in SLAC, b) progress of micromachining technology, c) initiating new projects (Accelerator on a Chip SLAC, AXIS UCLA, and worldwide) and papers towards future colliders, compact light sources, medical accelerators. Presently, it is the second leading direction in advance accelerators after plasma methods. Remarkably, all the 25+ years old Ideas about CO2 laser use are still applicable and ATF with its CO2 laser and e-beam is uniquely positioned for demonstrating the next breakthrough in DLA development within our present day capabilities.

ATF advantage: Accelerating structures scale with lambda. This means: Easier fabrication Much higher charge in bucket (x1000!) Easier to produce and maintain microbunches necessary for monoenergetic acceleration Easier to maintain phase synchronism between particles and field Easier to fit e-beam to uniform portion of accelerating field for monoenergetic acceleration.

Status and plan to proceed. Collaborators. Dielectric structures have been studied at ATF with e-bam only and several active experiments are still in portfolio. One DLA from Austin Un. (G. Shvets) is an active experiment, but approach is complex, progress not impressive, outcome is uncertain. The SLAC demonstration and next experiments elsewhere use experimentally easier realizable open Foxhole structure similar to what has been manufactured for ATF 25 years ago at Instrumentation. I am in contact with T. Rao; they at Instrumentation Dept. are ready to build such structure again (may not find the old one). To make it a user experiment, I talked to W. Kimura (STI Optronics). He is enthusiastic to submit a proposal for next SBIR solicitation this fall. We do not need more that our CO2 regen (~mJ) to produce several MeV from 1-cm structure. Other innovative features associated with this experiment will be microbunching for obtaining monochromaticity and structure transverse “sweeping” with a picosecond laser pulse. Note that other groups might be also interested. We already had a proposal from LANL in 2014 that was not well prepared and not accepted by APAC. Radiabeam and UCLA might be equally interested. I just approached STI first as Wayne has certain credentials to do this at ATF.

Expected ATF breakthrough. ATF with its present capabilities is positioned to demonstrate the first real quasi-monoenergetic DLA instead of scattered few electrons seen at SLAC, make it to orders of magnitude higher energies, make it for a measurable charge instead of a few electrons at SLAC.

6. Shock wave Ion acceleration, SWA:

General significance and motivation; community impact. This is already a flagship for ATF and quite visible in community due to its monoenergetic feature. However, achieved so far low proton energies <3 MeV narrow the community interest. Due to fast scaling with laser intensity, the experiment shall benefit from laser upgrade to allow >10 MeV in near future with medical applications in more distant prospects. 10 MeV may still produce a limited interest unless we figure out some applications, extensions, enhancements, byproducts. Byproducts/applications could be THz radiation, neutron production (?), bio/medical studies. Extension could be adding beam transports showing practically deliverable beam. Enhancements include more in-depth study of the process with ultra-fast diagnostic, etc.

ATF advantage: Acceleration in gas jets practically achievable only with CO2 due to its low critical plasma density.

Status and plan to proceed. Collaborators. Two user groups presently joined efforts: Imperial College and NRL. They are interested to continue explorations as we will increase the laser intensity. They also adding more diagnostic for in-depth study. We need to work with users to expand the program as is mentioned above (applications, extensions, enhancements, byproducts). UCLA is willing to join as we develop multi-TW capability. Our immediate technical problem is to avoid reflections from plasma that damage amplifier optics. This presently limits our ability to deliver even power we have. We are addressing this now.

Expected ATF breakthrough. If progressing as anticipated, our proton source may become a leading experiment in the field with application to cancer therapy. This experiment site may easily become highly acclaimed facility by itself with several experimental stations for several groups.

7. Inverse Compton Scattering, ICS:

General significance and motivation; community impact. Several research directions: Record high harmonics and single-shot yields in conventional ICS; recirculation including more sophisticated combination IFEL-ICA (need multi-pulse YAG photocathode driver). Still coming with strongest interest for advanced accelerator community: All-optical Compton source based on e-beams produced in ATF plasma acceleration program and, if low emittance demonstrated, studies towards coherent ICS.

ATF advantage: Due to more photons per Joule combined higher charge in LWFA bubble, strongest ever all-optical ICS is expected. Compton FEL is contemplated with CO2 lasers (L. Serafini).

Status and plan to proceed. Collaborators. ICS is traditionally strong at ATF. So far – highest photon yields, highest ICS harmonics seen, the only multi-bunch recirculation ICS. UCLA has a program on further investigating nonlinear ICS, including two-color. All optical ICS and coherent ICS will come in play as LWFA program progresses.

N o z z l e

Hydrogen jet

10

Expected ATF breakthrough. Success in combining two programs on LWFA and ICS might show a significant step towards compact light sources.

Ti:S

CO2

CO2

ComptonFEL

PWFA(low )

LWFA

Conclusions

We identified here seven strong candidates for game-changing flagship experiments that can reach full productivity at ATF within 1-2 years upon completing ATF-II Phase 2.

In next two tables, we summarize the required capabilities for each experiment, the period of performance and expected results. Not that in several cases we give a qualitative performance as dedicated simulations are needed to give more concrete predictions.

Table 1 of ATF Flagship experiments*Experiment e-beam CO2 Other resource Start Results CommentsTrojan Horse PWA (x) x Ti:S, H2 cell FY17 FY18-FY20 including all opticalTwo-color LWFA x Ti:S, jet FY18 FY18-FY20LWFA in e-beamline x x jet FY17 FY17-FY20 Starts off lineIFEL x x FY18 FY18-FY19 SBIR?DLA x x structure FY17 FY17-FY19 SBIR?SWA x jet ongoing FY16-FY19ICS x x (Ti:S) ongoing FY17-FY20 Including two-color,

possible LWFA extension

Table 2 of ATF Flagship experiments*Experiment e-beam 1 TW(ps) 10TW(ps) 25TW(fs) 100TW(fs)Trojan Horse PWA (100fs) Plasma source Standard scheme

setup700 MeV, 3 nC

All-opt. TH3 GeV, 10 nC

Enhanced A-O THTwo-color LWFA NA Plasma source 1st results Stronger accel. Higher chargeLWFA in e-beamline 10 fs setup Seeding test Staging test In-depth studyIFEL 1-0.1ps 100 MeV/m 300MeV/m 500 MeV/m 1 GeV/mDLA 1-0.1ps 1 GeV/m @10GW NA 2 GeV/m@40GW NASWA (protons) NA 2 MeV 20 MeV 50 MeV 200 MeVICS 1-0.1ps 3rd harmonic Multiple harmon.

Two-color ICSIFEL/ICS

Chirped-pulse ICSCoherent ICS

There is also a number of other potential directions that may acquire a similar flagship status provided they are chosen for execution. This is related to THz radiation as a byproduct of ion acceleration setup, HHG, long-range CO2 confinement in air.

Appendix.

Active ATF Experiments

AE39 - DWA - High Gradient, high field dielectric wakefield acceleration experiments at the ATF. Spokesperson: J. Rosenzweig, UCLA (2010)

AE43 - PWFA Holography. Spokesperson: M. Downer/P. Muggli (2009) AE48 - Experimental study of electron-beam microbunching dynamics. Spokesperson:

A. Gover, Tel-Aviv U. (2010) AE52 - Beam Manipulation by Self-Wakefield at the ATF. Spokesperson: S. Antipov/A.

Kanareikin (2012) AF58 - ERL BPM Test. Spokesperson: R. Michnoff, BNL (C-AD) (2013) feasibility study AE59 - Inverse Compton Source for Extreme Ultraviolet Lithography, Spokesperson: A.

Murokh, Radiabeam (2013) AE60 - Ultrafast High-Brightness Electron Source. Spokesperson: J. Park, Advanced 

energy systems (2012) AE62 - Sub-femtosecond beam line diagnostics, PI: G. Andonian, UCLA (2014) AE63 - Stony Brook Accelerator Laboratory Course, CASE@ATF, PI: Litvinenko, Stony

Brook (2014) AE64 - Surface Wave Accelerator and Radiation Source Based on Silicon Carbide, PI:

G. Shvets, U. Tex. (2010) AE65 - NOCIBUR: an inverse free electron laser decelerator experiment, PI: P.

Musumeci, UCLA (2014) AE66 - Modification of Gas Jet Density Profile with Hydrodynamic Shocks for CO2

Laser Ion Acceleration Experiment,  PI: A. Ting/Z. Najmudin, NRL/Imperial College (2014)

AE67 - Space Radiation Effects Experiments,  PI: Wousik Kim, NASA (2014) AE68 - Ramped Beam Generation Using Dielectric Wakefield Structures,  PI: G.

Andonian, RadiaBeam (2014) AF69 - Key physics study of LPI with NCD plasma using laser machined plasma

structure, PI: Wei Lu, Tsinghua Univ., China (2014) feasibility study AE70 - Nonlinear Inverse Compton Scattering, PI: J. Rosenzweig, UCLA (2014) AE71 - CO2-laser-driven GeV wakefield accelerators with external injection / Key

Physics Study of Laser Wakefield Acceleration Utilizing Ultrafast CO2 Laser and Electron Beam, Principle Investigators: V. Litvinenko/W. Lu, SUNY SB/Tsinghua Univ. (2014)

AE72 - Interaction Physics of Pico-second far-IR Terawatt Laser with Materials, PI: A. Ting, NRL (2015) proprietary research

AE73 - Energy Chirp Compensation in Plasma, PI: J. Osterhoff, DESY (2015) AE74 - Self-Channeling of CO2 Laser in Air, PI: S. Tochitsky, UCLA (2015) AE75 - MEMS Undulator for EUV Lasers, PI: I. Gadjev, UCLA (2015) AE76 - High Duty Cycle IFEL, PI: A. Murokh, Radiabeam (2015)

Active ATF Experiments

Experiment e-beam CO2 both

SFL YAG x-band

AE39 - DWA UCLA (2010) x (x)AE43 - PWFA Holography. UT (2009) x x

AE48 - Microbunching dynamics. UCLA/Tel-Aviv U. (2010) x

AE52 - Beam Manipulation by Self-Wakefield. Euclid (2012) x (x)

AF58 - ERL BPM Test., BNL (C-AD) (2013) x (x)

AE62 - Sub-femtosecond beam line diagnostics, UCLA (2014) x x

AE64 – SiC Accelerator and Radiation Source UT (2010) x x

AE65 - NOCIBUR, UCLA (2014) x (x)AE66 Ion acceleration NRL/IC (2014) x x

AE67 - Space Radiation Effects NASA (2014) x (x)

AE68 - Ramped Beam Using DWF, RadiaBeam (2014) x x

AF69 - L aser machined plasma Tsinghua (2014) (x) x

AE70 - Nonlinear ICS UCLA (2014) x xAE71 - LWFA, SUNY SB/Tsinghua Univ. (2014) x x x

AE73 - Energy Chirp Compensation in Plasma, DESY (2015) x x

AE74 – Laser Self-Channeling Air, UCLA (2015) x

AE75 - MEMS Undulator, UCLA (2015) x

AE76 - High Duty Cycle IFEL, Radiabeam (2015) x x