THE ELECTRON BEAM AND HOW ITS FEATURES ARE RALATED TO THE OTHER IMPORTANT PARAMETERS - PowerPoint Presentation

1. THE ELECTRON BEAM AND HOW ITS FEATURES ARE RALATED TO THE OTHER IMPORTANT PARAMETERSIvan Spassovsky on behalf of ENEA (Frascati) CARM TEAMFrascati, November 3-4, 2016

2. DIFICULTIES TO DEVELOP CARM1) difficult to make mode-selective high-kz cavities (quasioptical or Bragg reflector cavities required)2) requirement for a very low axial velocity spread (e.g., Δpz/pz < 1%) since the high-kz interaction increases the sensitivity to the axial velocity spread), moderate α (<1) pitch factor 3) presence of gyrotron and gyro-BWO oscillations (if waveguide cavities are used) 4) limited experimental track record, consisting mostly of short-pulse experiments with a low quality electron beam produced by field emission.

3. CRUCIAL POINTSELECTRON BEAM1. Axial Velocity Spread2. Pitch Ratio3. Final Bam Radius4. Final Beamm ThicknessVOLTAGE PULSE1. Ripple2. Top FlatnessMAGNETIC FIELD1. Rise Profile2. Flat Top Length3. Flat Top StabilityCAVITY1.Mode Selection2. Thermal Load3. Surface Breakdown 4. Pumping Rate FINAL GOALPoutput – 1 MWEfficiency > 25%Frequency – 250 GHzms pulse

4. SCHEMATICAL DIAGRAM OF THE EXPERIMENT

5. First StepPulsed Modulatori) pulse duration – 1 to 50 microsecondsii) repetition rete – 1 to 10 Hziii)to prove the state of art for different cavity configurations, beam parameters and magnetic field profiles and valuesSecond Step:i) millisecond pulseii) new cw “modulator”(power supply) ii) depressed collector REALIZATION IN TWO STEPS - RELATED TO THE PULSE LENGHT

6. Beam energy – from 500 to 700 KeV (600 KeV most probable solution)Beam Power – 4 MWBeam Current – 6 to 8 AmpsLongitudinal velocity spread – 0.3 to 0.6 percentPerpendicular velocity spread – 1 to 1.5 percentFinal beam thickness – several hundred micronsPitch ratio – 0.5 (proportional to 1/γ)Beam radius – depends on the operating modeELECTRON BEAM

7. Cavity Cross Section – limited downward by the max. power density in cw operationBeam-Wave Interaction Length:i) to keep the diffractive Q-factor for the operating mode much lower in Gyrotron (near-cutoff) regime in comparison to the Q-factor in CARM regimeii) to maintain the ohmical Q- factor good for satisfying the cavity wall thermal load limit of 2 to 3 KW/cm2.Doppler shift – 3 to 4 times ωc :i) above those values the upstream reflector becomes too longii) to reduce the magnetic field valueIii) Less structured modeELECTRODYNAMICAL STRUCTURE

8. Operating mode:i) to minimize the field on the cavity wall for avoiding a surface breakdown and an excessive thermal loadii) to experience near 100 percent reflection from the upstream reflectorPumping issue:Longitudinal and Transversal Slots for both purposes:i) mode selectionii) high pumping rate

9. Set of coils and solenoids to provide:i) Beam transportii) Beam radius compression a) to perfectly couple the beam with the operating modeb) to keep the beam radius varying along the entire upstream mirrorc) to produce the desired pitch ratio and minimize the velocity spreadMAGNETIC FIELD

10. To use the non-adiabatic formation system, where the initial oscillatory velocity is caused by non-adiabatic electric field arising at the beginning of the transportation channelIMPORTANT ISSUES BEING CONSIDERED DURING THE GUN END TRANSPORT LINE DESIGNDiode:1.Gun profile and anode-cathode distance are governed by the limit of 10KV/mm surface field2. Low-Perveance beam – far from the space charge limit. The Perveance should be < 10-8 Perv. (P = I/U3/2)Emitter:Emits electrons freely, without any unwanted effects such as un excessive heating or back bombardment due to reflecting particles (electrons would leak off from it into vacuum easily) 2. Supplying a high current density –usually above 10 A/cm23. Lasts forever > 10 000 h.4. Emits electrons uniformly.Beam Transport:Adiabatical compression and decompression is usedBeam current far bellow the IsclcDispersed uniformly over the beam dump

11. To use the non-adiabatic formation system, where the initial oscillatory velocity is caused by non-adiabatic electric field arising at the beginning of the transportation channelFROM THE EMITTER TO THE CAVITY

12. Equipotential lines in the gun region The initial oscillatory velocity and the pitch-factor depends on the: i) electric field intensity ii) distance of the beam from the channel wall iii) magnetic field compression ratio. Zaslavsky&ManilovThe region of the lens-diaphragm, where electrons obtain initial oscillatory velocityElectron beamStrong non-adiabatic perturbation of the electric field in the lens-diaphragmv=0v  0Gun electric field and beam formation

13. ELECTRON GUN (TE53 case)(CST Microwave studio)Neck radius46 mm

14. EMITTER RING (TE53 case)(CST Microwave studio)Emitter RingEmitter Radius30 mmEmitter Width1 mmEmitting surface area:i) large enough to reduce the current load to 3-5 A/cm2 for two reasons:to minimize thermal spreadto reduce the surface evaporating rate and roughnessii) Capable to produce a beam current at least twice larger than the projected one2. Emitter radius :i)related to the final beam radius and pitch3. Emitter thickness governs both, the geometrical emittance and the velocity spreadAll parameters are varied while the surface is maintained constant until the best possible beam quality is obtained: optimal current, less velocity spread, prefect coupling with, operating mode, desired pitch ratio Working temp.- 1200deg CSurface area - 2 cm2Current load - max 5 A/cm2

15. GUN COIL

16. SUPERCONDUCTING MAGNET

17. MAGNETIC FIELD FROFILE

18. SC MAGNET

19. RESULTING FIED PROFILE

20. BEAM TRANSPORTTE82TE53

21. 333BEAM SIZE ALONG Z

22. Beam Voltage ~ 600 kV - tunablePitch ratio α ~ 0,5 - tunableCavity Magnetic Field~ 5 T – tunableCavity beam radius – 3.1mm-tunable to fit TE53 modeBeam Current – 5 to10 A (possible to reach 20 A). Longitudinal Velocity Spread < 1%Emitter ring – 1mm wide, 30 mm in radius.A-K distance – 110 mmIsclc ~ 40 A BEAM PARAMETERS

23. The work to be done:Final Gun Profile and Mechanical Design – gun metal barrel, isolating ceramic, central metal rodeDesign of the Cathode Head – emitter nest, temperature reflectors, electrical leadsEmitter Design – diameter and width in relation to the final beam size established by the numerical simulations Final Magnetic Circuit Design – number of magnets with an appropriate bore and length to produce a magnetic field with required strength and profileDesign of the dispersive magnet – for uniformly spreading the beam along the output tapperSUMMARY

24. How we have chosen some of the principal parameters and dimensionsIst ~ Qdif,; Lres; Velocity SpreadQdif. for CARM ~ Reflectivity of the MirrorsQdif. for Gyrotron ~ LresQoh ~ Lres; material propertyPloss ~ Qdif/QohFor CARM to overcome Gyrotron -1. Qdif. CARM > Qdif Gyrotron-2. Lres together with Qdif. to make the Ist. for CARM < Ist. for GyrotronAll together to keep the thermal losses and the Surface field in the accepted limits

25. Cavity Length and DiameterA) To have a Reasonable Qdifr – around 1000( depends on the reflectors). Why?! 1.To keep the thermal load < 2KW/cm2; 2 To handle the Surface Field < 10kV/mmB) Not to be too oversized. Why!? To have enough mode separation at up shift int. pointC) To have a High Qohmical for the operating mode at High frequency and a Low Qohmical for the same mode at low frequency. Why!? To avoid a quick excitation of Gyro and BWO near-cutoff oscillations D) Cavity Length short enough to make a Low Qdid. Why1? To have High starting current oscillations neededPutting all those things together cavity with Rcav =7.5 Lcav. < 100 mm would be a good solution

26. Operating ModeA) To interact with the beam at Doppler shift 3 to 4ωc. Why!? 1. To keep the length of the upstream reflector around 500 mm (higher Doppler shift needs longer reflector); 2. To operate at accetable high kz which reduces the thermal load and the surface fieldB) To be less structured for facilitating the mode selection throughout the longitudinal slots ( more azimuthal variations means more slots-difficult for fabrication; more radial variation means first mode max. far from the wall which increase the reflector length and lower the Isclc.TE82 and TE53 are possible solution

27. Electron BeamA) Low velocity spread. Why!? 1. Reduces the starting current for the higher frequency operating point; Make less possible the excitation of gyro and BWO oscillations (their starting current should be a lot higher due to low Qdif.B) Very thin inside the interaction area. Why1? 1. To perfectly couple the operating mod; 2. Less interception with the adjacent modes.C) Shrinking radius along the upstream reflector!? Why!? For avoiding unwanted low-frequency oscillations all along the upstream reflectorD) Both A and B to make possible High gain at low beam current