1. Group Meeting October 15 th 2012 Lee Carver

2. Outline Multi-Harmonic Cavity (MHC) Design Project - Cavity Designs, Couplers, HOM analysis Electron Two Beam Experiment (ETBA) - Theory, Experimental Setup, Expected Results, Simulation Outline. Lee Carver - Group Meeting15/10/20122

3. Acknowledgements Many slides and figures have been stolen from a presentation given by Yong Jiang, a postdoc at Yale University. Due credit also has to go to J. Hirshfield, S. Kuzkiov, S. Kazakov, R.M. Jones. Lee Carver - Group Meeting15/10/20123

4. Multi Harmonic Cavity Lee Carver - Group Meeting15/10/20124

5. Why two modes? Lee Carver - Group Meeting Superimpose two or more harmonically related eigenmodes. Reduces the exposure time to intense RF fields, which could potentially raise the threshold for both rf breakdown and pulsed surface heating. This would allow for much higher gradients to be sustained, thus reducing the length of any future Linac. 15/10/20125

6. Project Outline Lee Carver - Group Meeting To design a cavity structure for high power beam testing that is a capable of sustaining two harmonically related eigenmodes that can operate at a high gradient but with a lower breakdown probability. At the Beam Physics Lab at Yale University they have a dual frequency multi-MW phase synchronous source available, with output frequencies of 2.856 GHz and 5.712 GHz. The cavity structure will need to have an optimised shunt impedance for both the TM010 and the TM020 modes, phase advances that result in both modes superimposing simply predictably, well matched ports and couplers, Q optimisation, consideration of HOM’s and probably many more. 15/10/20126

7. Potential Design Lee Carver - Group Meeting $L: $Lambda/3; $Fo: 0.05 cm; $w: $Li-$hi; $Ri: 0.5 cm; $ho: 0.5 cm; $Ro: 0.25 cm; $Li: $L-$Fo+0.55cm; $Rb: $Lambda*0.18; $Rc: 4.7887 cm; $hi: 2.6216 cm; Rc hi w Ri Ro ho Rb L Fo Li Three potential cavities, a/λ=0.1, a/λ=0.15, a/λ=0.18. The curved edges are for Q optimisation and it also allows the surface fields to be swept along the surface during one rf oscillation, rather than focus on a fixed point. 15/10/20127

8. Comparison to previous cavity Lee Carver - Group Meeting Simplified geometry with smaller iris radius. a/λ =0.075. R/Q of 45 Ohms, which is better than the design on the previous slide as it is yet to be optimised for the shunt impedance. There are many geometric similarities, but due to the low value for a/λ it was an unfeasible cavity to be fabricated. 15/10/20128 Units MHCPrevious TM01 f0GHz2.78582.9994 fπGHz2.90353.0003 fsyncGHz2.87553.0001 ϕsync-2π/3 Q-102937057 k loss V/pC0.179570.43310 R/QΩ19.8741045.94990 KV/pC0.085140.20678 κ%4.133320.02767

9. Current Coupler Design Lee Carver - Group Meeting S-Band Input C-Band Input 15/10/20129

10. Coupler Kicks Lee Carver - Group Meeting For single mode cavities, the couplers are usually symmetric on either side of the cavity. This is to counteract any asymmetry in the field that the coupler induces. However, as there are two different couplers being fed into the cavity, then there is going to be an asymmetric field that is caused by the couplers that is going to have an effect on the beam. These coupler kicks will need to be simulated and their effect minimised. 15/10/201210

11. HOM Analysis of a/λ=0.18 Lee Carver - Group Meeting Dispersion curves and simulation data for the first five monopole modes for the MHC. Units HFSSkn7c Equivalent Pillbox TM01TM02TM01 f0GHz2.78585.39992.8133 fπGHz2.90355.93102.8814 fsyncGHz2.87555.73742.8562 ϕsync-2π/3 Q-1029320124- k loss V/pC0.179570.050970.69810 R/QΩ19.874102.8280377.80134 KV/pC0.085140.012120.33353 κ%4.133329.353432.39342 Dipole modes are currently ongoing… 15/10/201211

12. Longitudinal Wakefield Lee Carver - Group Meeting Blue solid line is the wakefield for the MHC and the dashed line is the wakefield for an equivalent pillbox cavity simulated in kn7c. Due to the fact that the cavity has not been optimised for the shunt impedance, the wakefield is considerably smaller in magnitude than for a pillbox with the same frequency for the TM010 mode. 15/10/201212

13. Dipole Modes Lee Carver - Group Meeting Will also need to investigate the dipole modes of the MHC. Once the transverse wakefield has been simulated, an exploration of the beam dynamics of the test bunch will need to occur. Mainly to simulate the emmittance dilution, and also to see if Beam BreakUp (BBU) will be an issue. 15/10/201213

14. Electron Two Beam Accelerator Lee Carver - Group Meeting15/10/201214

15. General Idea Lee Carver - Group Meeting A high current low energy electron drive beam of β=1 enters a cavity that is detuned slightly away from resonance. This excites a wakefield, which then decelerates the following drive bunch. The rf power that is released from the deceleration of the second drive bunch causes high fields within the cavity, and the peak of these fields can be phased with the arrival of a low current test bunch. This can allow for a very high Transformer Ratio (ratio of energy gained by test beam / energy lost by drive beam), with the added benefit of not requiring any transfer elements (like CLIC) making the process much more efficient. This idea can be extended to use a drive beam of β=1 with a test beam of β<=1. This can be achieved through the use of fixed detuning or alternate detuning to achieve phase synchronisation between the drive and test beams (see next slide). 15/10/201215

16. Fixed v Alternate Detuning Lee Carver - Group Meeting Fixed Detuning Alternative Detuning: Every other cavities, detuning flips sign integer m=0Good for electron accelerator m=0Good for proton or heavy ion accelerator +∆ F -∆ F +∆ F -∆ F +∆ F = ≤|| Cavity period Λ +∆ F 15/10/201216

17. Nice animation Lee Carver - Group Meeting15/10/201217

18. Project Outline and Current Status Lee Carver - Group Meeting Aims: To test this theory!! There is an experiment that is due to begin soon, that consists of two parts. Part 1: To measure the Transformer Ratio. Part 2: To measure the energy deposition of a test bunch of length λ. At present, the cavity structure has been finalised and is currently being fabricated. At some time in the coming weeks, a cold test will begin on a single cell of the cavity structure. Part 1 should hopefully begin at some point early in the new year. 15/10/201218

19. To measure the Transformer Ratio by measuring the phase relationship between the drive bunch and the excited wakefield in a detuned structure. If the drive bunch is located near the node of the electric field, then it will only experience a small decelerating field and deposit a small amount of energy per particle to the wakefield, which will result in a high transformer ratio. Part 1: Transformer Ratio Lee Carver - Group Meeting15/10/201219

20. To run a series of drive bunches through the cavity structure (fixed detuning, same structure as before), and then to send in a test bunch of length λ. Then, quite simply, a measurement of the energy deposition of the test bunch. Part 2: Energy Measurement Lee Carver - Group Meeting15/10/201220

21. Experimental Setup Lee Carver - Group Meeting The DC gun that will act as the test beam is a 300kev 30A. It’s currently setup for use in another experiment. But with minor adjustments by adding a collimator to the end of the collected it can extract enough beam current (several mA) to act as a suitable test beam. 15/10/201221

22. Cavity Structure Lee Carver - Group Meeting A slightly elliptical pillbox cavity structure. 4-cells each operating in a π mode, the curved surfaces are to optimise the Q and the slight squeeze of the cavity shape is to increase the shunt impedance. Cavity operates at 2.855 GHz (fixed detuning of 1MHz). This cavity will require a full analysis of the HOM’s in order to understand exactly what each of the modes are doing. 15/10/201222

23. Simulation Studies Lee Carver - Group Meeting Full analysis of the longitudinal and dipole modes. Calculate the energy spread induced by the longitudinal wakefield to attempt to show that it is negligible (energy spread is only really an issue for many structures). However, if it is not, is there a way to correct for the energy spread to try and improve the clarity of results? Use TRNSVRS and kn7c to try and find an equivalent pillbox geometry that best suits the modes of the structure. Then, if it can be shown that the wakefields agree to a reasonable level, then they can be used to for the first 300 synchronous modes. This will allow great detail in the short-range wakefield, which is dominated by the high frequency synchronous modes. 15/10/201223

24. Implications Lee Carver - Group Meeting Should this experiment work, it could pave the way for a multi-TeV electron-ion collider, as it can be used to accelerate protons or other heavy nuclei. The overall aim is to try and combine this with the MHC project, to create a cavity structure capable of sustaining very high gradients by exciting two modes, with a high transformer ratio and efficiency due to the two beam setup. Essentially to combine both of these projects into one. 15/10/201224