1. High gradient acceleration Kyrre N. Sjøbæk * FYS 4550 / FYS 9550 – Experimental high energy physics University of Oslo, 26/9/2013 *k.n.sjobak(at)fys.uio.no CERN & University of Oslo

2. Outline Uses of high gradient acceleration  Particle physics  Other uses Techniques for high gradient acceleration  Superconducting  Plasma-wakefield  Normal-conducting RF

3. The next big particle physics machines If the LHC discovers something new (apart from the Higgs)  heavy & low cross section => high energy Want a precision experiment  Electron accelerator Circular machines limited by synchrotron radiation  Use a linear accelerator X-rays

4. CLIC and ILC Two projects to build the next big particle physics machine  TeV-scale electron- positron colliders ILC superconducting, CLIC normal conducting Share detectors etc. CLICILC Energy [GeV] 500 – 3000200 – 500 Luminocity [10 34 cm -2 s -1 ] Peak 1% : 1.4 – 2.0 full spectrum: 2.3 – 5.9 2 Accelerating gradient [MV/m] 80 – 10031.5 Bunches/ train 234 – 3121000 – 5400 Particles/ bunch [10 10 ] 6.8 – 3.710 – 20 Bunch spacing [ns] 0.5180 – 500 Wall-plug power [MW] 272 – 589230 Site length [km] 13.2 – 48.331 Data from ILC Reference Design Report and CLIC Conceptual Design Report

5. High energy and high gradient In a linear accelerator: Energy = L * q * E z * f  f is the “fill factor”, fraction of L which are accelerating structures Need to reach very high energies, O(energy) = 1 TeV  Assuming f = 1, 1 TeV: E z = 20 MV/m (SLC)=> L = 50 km E z = 30 MV/m (ILC)=> L = 33 km E z =100 MV/m (CLIC)=> L = 10 km Gradient is critical!

6. CLIC – how compact is it? CLIC gradient = 100 MV/m ILC superconducting ~30 MV/m “Laser straight” tunnel

7. Other uses for high gradient Free electron lasers: Very bright UV/X-ray lasers with defined time structure  Used for research into materials  Captures “snapshots” of atomic structure with ~ 1nm and ~10fs resolution (LCLS)  Multiple proposals to build with “CLIC” technology: More compact & cheaper Hadron therapy  Smaller & cheaper

8. Superconducting high gradient Superconducting cavities have extremely large Q- factors ~10 10  Can store field for a long time  Ideal for circular accelerators Superconductivity breaks at high magnetic field  Peak surface field proportional to gradient  Design cavities to minimize constant of proportionality Even with optimal cavities, gradient is limited to ~40 MV/m Need to keep at cryo temperatures, liquid Helium necessary H c (T) T TcTc H0H0 Meissner state (superconducting) H Normal state

9. Plasma-wakefield acceleration Capable of extremely high gradients, ~100'000 MV/m Drive beam or laser pushes away electrons in plasma  Steep charge density gradient => huge fields Still relatively unproven technique

10. Normal-conducting high gradient structures Able to reach ~100 MV/m Gradient limited by vacuum arc “breakdowns”  Breakdown probability determined by gradient, pulse length, material, and structure shape  Breakdown phenomena not completely understood Baseline for CLIC

11. Normal conducting breakdowns Breakdown = vacuum arc Spontaneous formation of plasma on the surface Breakdowns are bad:  Surface craters  Deflects the beam  Reflects RF power Scanning Electron Microscope image by Markus Aicheler Measurement by Andrea Palaia Beam Kicked beam Normal

12. “Standard model of vacuum arcs” Phases: 1.Field emission from tip 2.Ionization of neutrals 3.Creation of plasma sheath => Enhanced emission 4.Sputtering of neutrals 5.Growth 6.Saturation of energy supply 7.Extinction Extremely high current densities on the order of 10 6 A/cm^2 ≈ 10 25 ions/cm^2/s From pA to kA and Ångstrøm -> 100 µm in a few ns Creating plasma densities on the order of 10 20 ions / cm 2

13. Particle- and density plots

14. Potential & field

15. Scaling laws Tested large number of structures with different designs Approximate scaling law: Constant differs between structures...

16. Electric field Scaling law – predicting the constant local complex power flow global power flow Different designs have different field patterns Field magnitude proportional to gradient Surface fields proportional to gradient Scale Esurf, sqrt(S c ), and sqrt(P/C) to same gradient and pulse length Find that sqrt(S c ) and sqrt(P/C) clustered above some limit for all structures

17. Structure design Vary the shape of the structure In each geometry, calculate field pattern Minimize ratios of surface fields/gradient  Avoid concentrating fields Use scaling laws to predict which design yields the lowest BDR for any gradient

18. Summary High gradient acceleration technology needed for next big particle physics machines  Useful for other projects as well Normal conducting technology can reach ~100 MV/m accelerating gradient Gradient limited by vacuum arcs Avoiding these is an ongoing research topic

19. Backup

20. TLEP Circumference = 80 km Energy = 350 GeV (LEP: 209 GeV)

21. Normal conducting breakdowns

22. Simulation #1: Particle- and particle density plots Phases visible:  Emission  Ignition  Spreading Also see powerful oscillations which some ions “surf”  Electrostatic oscillations  May be a numerical instability...

23. Simulation #1: Potential and field plots