20th Advanced Accelerator Concepts Workshop

6 Nov 2022, 15:00 → 11 Nov 2022, 17:10 America/New_York

Hyatt Regency Long Island

Mark Palmer (Brookhaven National Laboratory) , Navid Vafaei-Najafabadi (Stony Brook University)

The AAC’22 workshop is the 20th in a series of by-invitation biennial fora for intensive discussions on long-term research in advanced accelerator physics and technology. This research supports the development of capabilities for the basic sciences, from photon science to high energy physics, as well as the development of compact accelerators for industrial, medical and security applications.

AAC'22 will be organized into eight working groups covering the following topical areas:

1. Laser-Plasma Wakefield Acceleration 
2. Computation for Accelerator Physics
3. Laser and High-Gradient Structure-Based Acceleration
4. Beam-Driven Acceleration
5. Beam Sources, Monitoring, and Control
6. Laser-Plasma Acceleration of Ions
7. Radiation Generation and Advanced Concepts
8. Advanced Laser and Beam Technology and Facilities

Conference Home:  https://www.aac2022.org/

 

Sunday, 6 November

18:00 → 19:30 Welcome Reception

Monday, 7 November

10:20 → 10:40 Coffee Break

12:10 → 13:30 Lunch

13:30 → 15:00 WG8: Advanced Laser and Beam Technology and Facilities: Session 1

Conveners: Mr Marcus Babzien (BNL) , Stephen Milton

13:30 Working group kick-off

Introduction and discussion of working group goals

13:50 The Argonne Wakefield Accelerator Beam Test Facility for Novel Accelerator Research

The Argonne Wakefield Accelerator (AWA) is a beam test facility at Argonne National Laboratory. It consists of a 65 MeV L-band photoinjector beamline, 3 additional independent photoinjector beamlines, and multiple flexible experimental areas. Its program is composed of three research themes: (1) Advanced Accelerator Concepts (AAC), (2) Beam Manipulation, and (3) Beam Production. The AAC theme focuses on primarily on Structure Wakefield Acceleration but also has a substantial Plasma Wakefield Acceleration program. The Beam Manipulation theme develops several manipulation methods including emittance exchange, flat-round beam transformations, transverse deflecting cavity based shaping and laser shaping. Beam production efforts include the operation of the world’s high charge (100nC) photoinjector (AWA’s Drive Gun) and a recently developed X-band gun demonstrated to operate at 400 MV/m. Research at the AWA operates on a collaborator model and is carried out by both in-house researchers and collaborators drawn from universities, national laboratories and industry from around the world. This talk will present recent research results, research capabilities and planned upgrades of the AWA facility.

Speaker: John Power

20221106-AAC-awafacility.pptx

14:10 Environmental Impact of Future Colliders

We present the results of the Snowmass Implementation Task Force (ITF) analysis of future collider concepts. We consider both the environmental cost of construction (CO2 footprint per meter of tunnel) and the carbon footprint associated with collider power consumption. We discuss strategies to mitigate the power consumption of future high-energy colliders, such as energy recovery, and we discuss the benefits of specific advanced accelerator technologies for reducing power consumption.

Speaker: Spencer Gessner (SLAC)

GreenAccelerators_AAC22_Gessner.pdf

14:30 Development of Coherent Spatially and Temporally Combined Fiber Laser LPA Driver Concept – Progress of the kW-Average and TW-Peak Power System Demonstraton

Next generation particle accelerators based on laser plasma interactions are a promising path towards achieving GeV gradients in small volumes, thus substantially reducing the size of accelerators needed for both frontier science and practical applications from materials science to medicine. These accelerators will require laser drivers with ultrashort pulses, joule energy levels and 10s kHz repetition rates (100s-kW average power), a trio of requirements beyond current laser technology. We are developing a scalable laser approach based on coherent temporal and spatial combining of large core fiber amplifiers. It is based on coherent pulse stacking amplification (CPSA), a time domain technique that allows an arbitrary number of pulses to be time-domain combined into a single pulse. In conjunction with chirped-pulse amplification (CPA) CPSA allows for near full energy extraction from large core fiber amplifiers, increasing achievable energies by two orders of magnitude compared to traditional fiber CPA. This reduces the number of spatially combined parallel amplification channels used for power and energy scaling by ~100 times – a substantial reduction of the system size and complexity.
We have been validating this approach in a power scalable demonstration system, aiming to achieve ~100mJ and ~1kW coherently combined pulse energies and average powers from a 10-12 parallel-channel spatially combined CPSA system. We demonstrated robust temporal combining in CPSA architecture of 81 pulses to a single pulse with high efficiency and compressibility to approximately 300fs. At present, we have carried out 4-channel coherent spatial combining of 81-pulse CPSA bursts, achieving >25mJ at 2kHz (50W average power) in a combined beam. Pulse energy scaling of individual amplification channels is achieved by using 85µm chirally-coupled-core (CCC) fibers, which store more than 10mJ per fiber and provide high-efficiency extraction at multi-kHz repetition rates. Additionally, we have also demonstrated techniques for controlling gain narrowing, dispersion, and in-burst saturation control for achieving ~100fs duration pulses. Ongoing work is exploring simultaneous operation of coherent spatial and temporal combining at high energies and powers. Further work will extend system size from 4 to 10 channels and will carry out first high-intensity laser-matter interaction experiments with this table-top laboratory demonstration system.

Speaker: Alexander Rainville (University of Michigan)

Development of Coherent Spatially and Temporally Combined Fiber Laser LPA Driver Concept.pptx

15:00 → 15:30 Coffee Break

Tuesday, 8 November

10:00 → 10:30 Coffee Break

12:00 → 13:20 Lunch

13:30 → 15:00 WG8: Advanced Laser and Beam Technology and Facilities: Session 3

Conveners: Mr Marcus Babzien (BNL) , Stephen Milton

13:30 Robust and Efficient Temporal Pulse Combining Enabling Practical Coherent Pulse Stacking Amplification Systems

Practical use of laser plasma accelerators will require drivers with high peak power and high repetition rate. Spatially and temporally coherently combined fiber laser arrays offer one of the most promising pathways to such drivers. Temporal combining of ~100 stretched pulses, implemented as a coherent pulse stacking amplification (CPSA) technique [1], enables near-complete extraction of stored energy in each fiber channel with low nonlinear phase per pulse, and reduces fiber-array size approximately by 100-fold as necessary for making this approach practical. However, stacking a large number of pulses robustly and efficiently is a challenging technical problem.
We report achieving high robustness and efficiency when stacking 81 pulses with multiplexed Gires-Tournois Interferometer (GTI) cavities. This performance was achieved by carrying-out theoretical analysis for finding the required degree of system stability and GTI-cavity alignment accuracy, implementing advanced hardware and fast algorithms for multi-dimensional robust control of this complex system, and developing methods and automated techniques for high accuracy optical alignment of GTI stackers. For large beams in a stacker supporting high energy and power the two critical alignment dimensions are far-field angular, and piston-error alignments of each individual GTI cavity. Far-field angular alignment accuracy depends on the beam size, and for ~6mm diameter beam supporting ~1J stacked pulses it requires better than +-5 µrad precision. Piston errors not exceeding ~1µm (i.e. one optical wavelength) are universally required. We determined metrics for quantitatively tracking the GTI cavity alignment errors, and developed automated alignment hardware and algorithms for achieving and maintaining this required high-degree of alignment precision in real time, which is also crucial for running such coherently combined system in practice. To ensure high degree of stacking stability, we actively locked the “master” 1GHz repetition-rate mode-locked oscillator by referencing it to a rubidium frequency standard whose long-term stability is <1ppb.
Utilizing these techniques we demonstrated highly repeatable day-to-day operation of 81-pulse burst stacking (compatible with ~10mJ per channel) from 85µm core fiber amplifiers, with stacking efficiencies >83% and stacked-pulse peak power stability of <1%. This demonstrates the practicality of the CPSA technique as the key enabler of high energy and power LPA drivers.

Speaker: Yanwen Jing (University of Michigan)

Robust and Efficient Temporal Combining_final.pptx

13:50 Nonlinear Coherent Pulse Stacking enabling energy scalable several optical cycle pulses for the next generation drivers of laser plasma accelerators

Laser-wakefield plasma accelerators (LWFA) promise compact sources of highly energetic electrons and photons, but for their practical use they need efficient and high repetition rate laser drivers. The current standard is the Ti:sapphire CPA system, which can produce multi-J pulses with bandwidths supporting ~30 fs pulses, but it has low wall plug efficiency (WPE) and ~Hz repetition rates. Fiber laser systems can operate with high WPE at 10's of kHz and are scalable to high energies and powers using spatial and temporal coherent combining but have bandwidths sufficient for only 50-100fs pulses. Additional spectral combining can extend this bandwidth, but by increasing overall complexity of the fiber laser driver. We propose a Nonlinear Coherent Pulse Stacking (N-CPS) technique, which could enable achieving several cycle pulses comparable to those of Ti:sapphire, while maintaining multi-kW power and the multi-J energy scalability of coherently combined fiber laser arrays with only a minor increase in the overall complexity of the system.
Coherent Pulse Stacking Amplification (CPSA) is critical for reducing spatially-combined fiber laser array sizes by approximately two orders of magnitude. In demonstrated CPSA systems [1] a stacking-burst of stretched pulses extracts nearly-all stored energy from the final amplification stage and is temporally combined (using GTI cavities) into a single stretched pulse for compression to the bandwidth-limit at the system output.
We show that N-CPS can extend CPSA by compressing the amplified stacking-burst first, then spectrally broadening each individual compressed pulse in, for example, a Herriott-cell gas chamber [2], and only then stacking the burst into a single pulse, which is subsequentially compressed using chirped mirrors to durations much shorter than fiber gain bandwidth supports. Stacking-burst in this case allows both near-complete energy extraction during amplification and overcoming individual pulse energy limitations (~10-20mJ) of the spectral broadening step. We also show via numerical simulations that N-CPS needs only a minor increase in a CPSA system complexity, because it can reshape unequal-amplitude saturated bursts from final amplifiers into equal amplitude burst necessary for spectral broadening using only a couple of additional GTI stages. We will report on the experimental progress demonstrating this new N-CPS technique.

Speaker: Tayari Coleman

14:10 Coherent temporal stacking of tens-of-fs laser pulses towards plasma accelerator applications

A laser-plasma accelerator (LPA) could reach high energies with an accelerating length orders-of-magnitude shorter than in conventional RF accelerators. Compact LPAs will enable high-impact applications in science, medicine, security, and industry. As LPA applications will require new driver lasers with kHz to 10s kHz repetition-rates at high energy and efficiency [1], one promising laser approach is to combine many ultrashort pulses from high-power, high-efficiency fiber lasers, in space, time, and spectrum [2].

Coherent pulse stacking (CPS) temporally stacks many amplified laser pulses into a high energy pulse using cascaded reflecting cavities [3]. This largely reduces the number of spatial fiber channels needed, e.g. to a practical 100 level for Joule pulse energies. However, while LPA needs 30-100fs driver laser pulses, CPS has only been demonstrated experimentally with ~300fs pulse lengths and longer [3,4]. Thus, we propose to validate and demonstrate broadband CPS that can stack tens-of-fs laser pulses with high fidelity, critical for showing CPS is applicable to driving LPAs.

The main challenge associated with broadband CPS, limiting its stacking fidelity, is the unmatched dispersions accumulated for pulses undergoing different cavity roundtrips upon stacking. However, we validated via simulation that by using very low dispersion dielectric mirrors for stacking cavities, high fidelity CPS could be achieved for : 1) 4-cavity stacking of ~9 pulses at 30fs pulse lengths; 2) 8-cavity stacking of ~81 pulses at 50fs pulse lengths.

We experimentally demonstrated stacking of ~10 pulses with broad bandwidth supporting ~50fs transform-limited pulse lengths. The broadband CPS setup consists of a nonlinearly broadened source, pulse modulation and amplification, 4 stacking cavities, and diagnostics. With one cavity, we achieved stacking of ~3 broadband pulses with a pre-pulse contrast of 40:1. Using four cavities, we stacked ~10 broadband pulses with a pre-pulse contrast of 8:1, which is expected to be further improved after ongoing optimization of cavity alignment and phase control.

[1] Basic Research Needs Workshop report, 2019
[2] Brightest Light Initiative report, 2019
[3] Optics Express 23(6), 2015
[4] ASSL (OSA) AW4A.4, 2017

Speaker: Lauren Cooper

14:30 Ultra-broadband spectral combination of fiber lasers with synthesized pulse shaping to reach short pulse lengths for plasma accelerators

Laser-plasma accelerators (LPA) can significantly reduce the large sizes of conventional accelerators, showing great potential, but they are challenged by today’s low operation repetition-rates (Hertz class). Achieving kilohertz repetition-rates is necessary to enable high impact applications in science, security, and medicine [DOE Basic Research Needs Workshop report, 2019].

One recognized laser technology towards kilohertz LPA drivers is multidimensional fiber laser combination [DOE Brightest Light Initiative report, 2019], due to its advantages in wall-plug efficiency and thermal management. While achievable pulse lengths from fiber lasers are limited by gain narrowing and high order fiber dispersions in high energy systems, coherently combining multiple fiber output spectra has been demonstrated to generate shorter pulses, with a shortest 97fs at one-micron wavelength [Proc. SPIE 9728, 2016]. However, LPAs need driver lasers with pulse lengths much shorter than ~100fs.

Thus, we propose ultra-broadband spectral combining of fiber lasers to reach short pulse lengths beyond the state of the art, using spectrally synthesized pulse shaping. We demonstrated a 54-fs, two-channel spectrally-combined fiber laser system, with two pulse shapers operating at different but partially-overlapped spectrum in each channel to control the spectral intensity and phase. Coherent synthesis of the two shapers was achieved by phase-synchronizing the two channels at the overlapped spectrum. 94.5% combining efficiency was obtained through an SPGD-based feedback loop.

To the best of our knowledge, 54fs is the shortest pulse length from a spectrally combined fiber system at one-micron wavelength, and we achieved the first demonstration of coherent spectral synthesis of two pulse shapers [accepted by Advanced Solid State Lasers (Dec 2022) and Photonics West (Jan 2023) as oral presentations]. We recently completed a three-spectral-channel combination setup, with a fiber amplifier in each channel and two synthesized pulse shapers covering all three spectra. We have demonstrated an 80nm spectrum combined from three channels, with a flat spectral phase, corresponding to <40fs transform-limited pulse length. Phase synchronization of all channels is in progress.

This ultra-broadband, energy-scalable approach of spectral combination with synthesized pulse shaping paves the way to high energy, tens-of-fs fiber lasers for driving plasma accelerators.

Speaker: Siyun Chen (Lawrence Berkeley National Lab)

ultra-broadband spectral combining for AAC2022 FINAL.pdf

14:50 Discussion

15:00 → 15:30 Coffee Break/Exhibits

Wednesday, 9 November

10:00 → 10:30 Coffee Break/Exhibits

12:00 → 13:20 Lunch

Thursday, 10 November

10:00 → 10:30 Coffee Break/Exhibits

10:30 → 12:00 WG8: Advanced Laser and Beam Technology and Facilities: Session 7

Conveners: Mr Marcus Babzien (BNL) , Stephen Milton

10:30 Advanced Lasers for accelerators at Colorado State University: advances in kW average power cryogenicallycooled ultrafast Yb:YAG lasers

The petawatt-class multi-Hz Ti:Sa laser ALEPH developed at Colorado State University has recently enable major advances in laser wakefield acceleration [1]. However, progress on laser driven-particle accelerators for applications depends on the development of compact, more efficient lasers capable of producing of high energy ultrashort laser pulses at greatly increased high repetition rate. A promising laser gain material for such lasers is Yb-YAG, which has the advantages of millisecond upper level lifetime, low quantum defect, and high thermal conductivity that in combination facilitate high repetition rate operation. We have demonstrated a kW average power laser that generates picosecond pulses with energy up to 1.1 J at 1 kHz repetition rate [2]. The system uses cryogenically cooled Yb:YAG active mirror amplifiers to generate pulses >1.2 J energy at a repetition rate of 1kHz. After compression 1.1J pulses with < 4.5ps duration are obtained with good beam quality and shot-to-shot stability. To shorten the pulse we are investigating spectral broadening in a gas-filled hollow core fiber. In addition, we demonstrated the efficient generation of Joule-level λ=515nm ns laser pulses at 1kHz repetition rate by frequency doubling in LBO crystals [3], a result that is of interest for pumping high average power femtosecond lasers. The seed pulses are generated by an arbitrary-waveform laser which can be programmed to produce square pulse shapes at the end of the amplifier chain for efficient doubling. The total 515 nm average power reached 1.04 kW (1.04J pulses at 1 kHz) in a beam with a measured M^2= 1.3-1.4.

[1] B. Miao, J.E. Shrock, L. Feder, R.C. Hollinger, J. Morrison, R. Nedbailo, A. Picksley, H. Song, S. Wang, J.J.Rocca, and H.M. Milchberg, "Multi-GeV Electron Bunches from an All-Optical Laser Wakefield Accelerator," Physical
Review X, 12, 10.1103, (2022)
[2]Y. Wang, H. Chi, C. Baumgarten, K Dehne, A. Meadows, A. Davenport, G. Murray, B. Reagan, C.S. Menoni, and J.J. Rocca “1.1 J, 1 kHz repetition rate, Yb:YAG ps laser,” Opt. Lett. 45, 6615 (2020).
[3] H.Chi, Y.Wang, A. Davenport, C.S. Menoni and J.J. Rocca, "Demonstration of a kilowatt average power, 1J, green laser," Opt. Lett. 45, 6803, (2020).

Speaker: Jorge Rocca (Colorado State University)

10:50 Diode-Pumped Tm:YLF Lasers for Advanced Accelerators

High peak power laser systems with architectures that are scalable in average power are essential to drive the next generation of advanced, compact electron accelerators. For this purpose, the Big Aperture Thulium (BAT) laser concept is designed to simultaneously operate at PW-class peak powers and multi-100kW average powers through the use of an energy extraction regime that scales in efficiency with repetition rate. The gain material Tm:YLF exhibits a long radiative lifetime of 15 ms and can be directly pumped with peak power limited CW diodes, while efficiently amplifying broadband pulses at low fluences. In this work, we report on the current status of Tm:YLF laser development, including energy extraction demonstrations of pulse energies >21J in 20ns (>1GW peak power) in a 4-pass amplifier, as well as 108J pulse energies in a long duration pulse using a 6-pass configuration of the same amplifier. Additionally, we describe upcoming experimental demonstrations, including high energy chirped pulse amplification of ultrashort pulses in Tm:YLF, to support the high peak and average power potential of the BAT laser concept.

Speaker: Leily Kiani (LLNL)

20221111 AAC talk draft_upload to AAC.pdf

11:10 Raman-based wavelength conversion for seeding and optical pumping of CO2 laser amplifiers

The long wavelength of long-wave infrared (LWIR) lasers suit them to applications relying on ponderomotive interactions, such as laser wakefield acceleration and high harmonic generation. The workhorse source of such wavelengths is the CO2 amplifier, providing the ability to reach TW peak powers and sub-ps pulse lengths. Two pathways to improve the performance of these amplifiers are to increase the energy by increasing the energy of the seed and to increase the repetition rate by optically pumping the gas instead of pumping with an electrical discharge. The wavelengths required, 9.2 um for the seed and 4.3 um for the pump, are outside the range of conventional millijoule-class, nanosecond laser sources, and must be obtained through nonlinear wavelength conversion processes. One such process is known as stimulated Raman scattering (SRS), where photons are inelastically scattered by a coherent excited state of a material. In principle, ionic liquids (ILs), artificial salts that are liquid at room temperature, are an excellent choice of material for SRS, as the vibrational modes and optical properties can be tailored with the choice of ions. Relatively efficient difference frequency conversion from visible/near-infrared to the seed and pump wavelengths can be achieved with Raman shifts of 1087 cm-1 and 2200 cm-1 respectively. We find that calcite crystals offer high efficiency conversion corresponding to the first Raman shift, from 800 nm Ti:Sa to 876 nm, and an ionic liquid, 1-ethyl-3-methylimidazolium dicyanamide (EMIM-DCA), provides the second, from 532 nm Nd:YAG to 603 nm. As a proof of principle, we measure a conversion efficiency in EMIM-DCA three times higher than that of water. Future work will consist of measuring conversion efficiencies in a variety of other ILs. Beyond upgrading the CO2 amplifiers, Raman shifting in ILs provide a pathway for efficient, simple, alignment-tolerant high-energy wavelength conversion.

Speaker: William Li (Brookhaven National Laboratory)

raman_aac_2022.pptx

11:30 Status and prospects of optically pumped high-pressure CO2 amplifiers

Multiple advanced accelerator concepts such as electron and ion acceleration from plasmas, inverse FEL’s, and Compton sources would benefit from the development of high-repetition-rate and short-pulse but high-energy mid-IR lasers. However, this intense-field mid-IR is still extremely difficult to access, since solid-state laser sources in this spectral region are limited in power. CO2 lasers systems are currently the most promising strategy towards the generation of such pulses, as the CO2 molecule is capable of storing Joules of energy for 10 µm amplification and does not face the damage threshold limitations that inhibit optical parametric amplifiers from reaching high peak powers at these wavelengths. Picosecond CO2 gain modules are typically pumped with an electric discharge, however, the voltage required at the high pressures (>10 atm), needed for a smooth gain bandwidth, is prohibitively high and it is extremely difficult to maintain a stable electric discharge in large volumes.
Funded by the DOE Accelerator Stewardship grant, a UCLA/BNL/UAB team launched a program towards development of a compact multiatmopshere CO2 amplifier optically pumped by pulses of a 4.3 m Fe:ZnSe laser. Switching from the traditional electrical discharge pumping to an optical pumping of a high-pressure CO2 amplifier could drastically decrease the size of a CO2 MOPA system and the pulse length to ~ 300-500 fs, simultaneously increasing the repetition rate to 10-100 Hz. To this end, a novel Fe:ZnSe laser generating ~50mJ pulses around 4.1-4.8 µm [1] was tested as a pump source. In this scheme of optical pumping, the upper laser level in the 10 m lasing channel 001-100 of a CO2 molecule is pumped directly from the ground state and in recent proof-of-principle experiments lasing was observed up to 15 atm pressure and the optical-to-optical conversion efficiency reached 10% at ~10 atm [2]. Simulations of amplification of a 10 uJ seed showed possibility to reach a few GW power level in a palm-size regenerative amplifier [3]. Current activities and future prospects will be discussed.

1. V. Fedorov et al, Opt. Express 27, 13934-13941(2019).
2. D. Tovey et al, Opt. Express, 29, 31455-31464(2021).
3. D. Tovey et al, Appl. Opt. 58, 5756-5763(2019).

Speaker: Dr Sergei Tochitsky

12:00 → 13:20 Lunch

15:00 → 18:00 Afternoon at Leisure

Friday, 11 November

10:00 → 10:30 Coffee Break

12:00 → 13:00 Lunch

14:40 → 15:00 Coffee Break