Answers to FACET Committee Questions FACET Team

Questions 1. The review team wants to see a more prioritized approach to reaching a multi-TeV machine. Please give a list of prioritized sub-projects which will get us to the goal of a design for a multi-staged wakefield accelerator. 2. The present plan will take 10 years to implement and the multi-staging will hardly be investigated by then. At that point FACET must stop for LCLS II. Isn't this a "Show Stopper"? What are the plans for FACET II? 3. The review team needs more details of the operations budget. Provide these details by explaining the components of the operations plan and the budget for each component. 4. Most of the experiments in the ASF can be done with 10 GeV electrons. Could you design the facility at lower energy, save money in operations and focus on beam quality and beam parameters instead of energy?

Questions 5. All the work at ESA can be done with the LCLS beam. This option is much cheaper. Why isn't the lab pursuing this option? Please provide more budget details for the FACET plan to provide a beam to ESA and the alternative of borrowing the LCLS beam. 6. The review team wants to hear more about the simulation. How good is the agreement between with experiments? How thoroughly have parameters been investigated? 7. Make a better argument for doing positrons now. Is the positron work of the highest priority or could it be done after the work on witness beams?

Q.1 Path to a TeV Collider Prioritized list of sub-projects at FACET Beam load –2nd bunch with 33% of particles –Small energy spread –Energy double: 25 --> 50 GeV (if required, examine additional focusing to eliminate head erosion) Replicate for positrons –Demonstrate high gradient e+ (on e- and/or e+ wake) –Extend to max energy in uniform plasma (less than double due to small R in e+ wake) –Explore hollow channel to increase R, gradient Emittance preservation –Study hosing tolerances –Remove upstream foils/windows to study matching of lowest possible emittance beams –Study radiation losses as a function of spot size and compare to models Beyond FACET –Staging, drive train development, feedback control, pointing, etc. –Working group already studying issues and aimed towards a workshop this summer that will expand on our early concepts

Question 2: Potential Interference between FACET and LCLS-II From J. Galayda: LCLS-II, which may use the upstream 2/3 of the linac, is now just a concept and has not been worked even to first order. The most likely scenario would be a new RF gun at the front end and an extraction line in Sector leading to a bypass line to the end of the linac and undulator. Thus, the layouts of FACET and LCLS-II would likely be very similar. With planning in advance the two projects could be made compatible. Although the operation of the two facilities would probably be not simultaneous.

Question 2: Potential Interference between FACET and LCLS-II Another alternative might be to emerge from the expanded high-gradient effort. This could lead to a 3-6 GeV demonstration project on the time scale of the middle of the next decade, which would then serve as a suitable frontend for an expanded FACET II effort. If FACET I points to PWFA as a viable transformative technology, we believe that HEP will want to invest in such an expanded capability. FACET II would require a redesigned system that more closely matched the PWFA capability, e.g., beam merging and staging, and not be a straightforward extension of the existing facility.

Question 4: Why not 10 GeV e-? With the present SLAC linac hardware, shorter bunches come from more linac energy due to the bunch compression process. The capability of make 23 to 30 GeV beams in FACET should be maintained for future possibilities. However, one could run the linac such that the beam drifts through the middle 1 km of the linac delivering 10 GeV to FACET. This will save power. The estimated power-cost savings of not running the middle 1 km of the linac is: –Power = 50 kW/tube*30Hz/120Hz*2(eff)*80 tubes = 2 MW –Cost = 2 MW*65$/MWhr*4 month*30 day*24 hr=374 k$

Question 5: Why not use LCLS beam in ESA The LCLS beam already has been sent to the ESA hall for ILC experiments and this capability could be maintained in the future. –Low emittance source is an attraction for some applications, although charge limit (<1nC) and lack of long pulse train capability are a limitation –The capability to send LCLS beam to ESA will be maintained using pulsed kickers in the Beam-Switch-Yard (BSY); some development likely would be required to improve performance (to ensure no impact on LCLS forward beams) or accommodate full energy ($ M) –Would still need to upgrade ESA PPS system ($0.4M) and develop the hadron production facility ($0.6M) –Beam energy for LCLS could vary between 4.3 and 13.6 GeV depending on requirements for the experimental program

Question 5: Why not use LCLS beam in ESA The LCLS is a major new facility for Photon Science in the US, representing a $400M investment, and we expect it will enable a set of unique science opportunities. –This will be a challenging machine to commission and operate. If lasing is very sporadic and unreliable, then development and operations will clearly be focused on delivering the LCLS science program. ESA users might get beam only when the LCLS downstream section is in maintenance mode, although access to the linac may severely limit even this time. –If the reproducibility of beam lasing in the LCLS undulator is not affected by pulsed kicking of beam, then a few Hz of the LCLS pulse rate could be sent to the ESA hall with little adverse affect to the photon science community. However, the science opportunities will be involve 4 and soon afterwards 6 experimental stations and, on the timescale of , a second undulator. The science opportunity will be pulse limited long-term, in a situation which is highly competitive, e.g., XFEL. Issues related to building and maintaining an effective user test facility –Reliability of beam conditions, experimental support, and schedule are all important elements to attracting and maintaining a user base –It is clear that the LCLS linac as a source will be prioritized on the basis of operational or science needs for the photon science program; leading to the potential for significant uncertainty in availability

Q.6 How good are simulations? predictive PIC methods with low level of approximation, validated against numerous experiments and benchmarked against each other Discrepancy between simulation and experiments can indicate a new phenomenon (e.g., synch rad needed for agreement, trapped particles etc...) or a misunderstanding in the beam input or plasma conditions. 12 peer-reviewed publications on simulations, 7 Ph.D theses: a substantial effort in this area Examples in the slides that follow

Refraction of an Electron Beam: Interplay Between Simulation & Experiment Laser off Laser on 3-D OSIRIS PIC Simulation Experiment (Cherenkov images) 1st 1 to 1 modeling of meter-scale experiment in 3-D! (128 processors at NERSC, 5000 cpu hours) P. Muggli et al., Nature 411, 2001

Modeling self-ionized PWFA experiment with QuickPIC E164X experiment QuickPIC simulation Located in the FFTB 25 m FFTB

H EAD E ROSION Length, energy gain limit for large  beams Solution: lower  beams

T RANSVERSE D YNAMICS e + c) a)  x0 =  y0 =25µm,  Nx =390  10 -6,  Ny =80  m-rad, N=1.9  e +, L=1.4 m Downstream OTR Excellent experimental/simulation results agreement! Simulation Experiment

Positrons Loss ≈ 50 MeV Gain ≈ 75 MeV B. Blue et al., Phys. Rev. Lett R. Bingham, Nature, News and Views 2003 Acceleration Of Electrons & Positrons: E-162 Data OSIRIS Simulation

Simulation Data Vs. Experiment n e =0.7  cm -3 Head E  n e =2.3  cm -3 Head E  Results after propagating simulation data through experiment diagnostics Experiment Results *Use low n e events as “plasma off”

Q.7 Why positrons now? Highest priority? Highest priority IS the witness beam work NEXT highest is e+ Follows the “Path to a TeV collider” slide (answer to Q.1) Short window of opportunity, minimal cost

Question 8: Additional Accelerator Physicists FFTB Collaborations has attracted many SLAC accelerator systems physicists: Ralph Assmann, Paul Emma, Franz-Josef Decker, Rick Iverson, Patrick Krejcik, Pantaleo Raimondi Currently working with Tor Raubenheimer, Andrei Seryi, and Peter Tenenbaum to develop PWFA-LC concepts There are other accelerator physicists at SLAC who could help with the FACET accelerator and experiments; there is depth in the community, which will be attracted as the project becomes real. –These physicists could come from a variety of areas: PEP-II, LC, ARD, SPEAR-3, LCLS. –FACET proposal authors were directly engaged in preparing the document; we expect many of the following incomplete list to be engaged as the project develops: F-J. Decker, Uli Wienands, M. Sullivan, W. Wittmer, G. Yocky, D. Dowell, N. Phinney, P. Tenenbaum, J. Sheppard, J. Nelson, A. Seryi, Y. Cai, A. Fisher, A. Novokhatski, J. Safranek, M Woodley

Q.9 Consider Linear Regime? Lower Density? We think that for e- acceleration, nonlinear regime is better – higher gradient implies shorter propagation length and blowout implies more linear focusing and easier emittance preservation Linear regime is something of interest for e+ if nonlinear regime proves too difficult. B. Blue et al., PRL is in this regime. Lower density is of interest in broad parameter study – tradeoffs –Lower gradients but… –Longer wavelengths make phase tolerances easier –Wider waves mean more particles per bunch can be accelerated but requires proportionally higher drive bunch charge too!