1. Ivan Spassovsky On behalf of ENEA CARM Team
DEVELOPMENT of LONG PULSE and HIGH-POWER mm-WAVE SOURCE
Overview on SubTHz radiation sources
Ivan Spassovsky
On behalf of ENEA CARM Team

2. Different RF Sources So Called HPM (High Power Microwaves)

3. Magnetron, Vircator and IOT (Inductive Output Tube)

4. Different Bunching Processes and Fundamental Parameters
Inertial bunching – when the already accelerated beam is bunched in the presence of HF field - most of the comercially available devices
Forced Bunching – When the bunching process takes place in the accelerating gap – Magnetrons, IOTs
Beam Voltage – non relativistic (Gyrotrons, TWT, BWO) , relativistic (CARM, Vircator, HP BWO HP Gyrotron and TWT ), High Relativistic (FEL)
Operating Beam Current – Ph. C, TC, FC or PC
Space Charge Limited Current
Beam velocity spread
Starting Current

5. Most of the Linear and Gyro Devices Experience an Inertial Bunching
Linear Devices :
1. Use linear beam with only longitudinal velocity vparallel
2. The bunching process is longitudinal in density transferred later in velocity bunches
3. The operating modes are TM
4. The frequency is related to the plasma frequency
5. Slow-wave devices vphase < c
6. Operate at the Fundamental mode – the power level depends on the Breakdown limit
7. High-beam compression to increase the particle density
8. High perveance beam - I~U3/2 , High-velocity spread

6. Klystron Schematic

7. Gyro Devices
1. Use the beam transverse velocity (vtransv ) to generate oscillations
2. The bunching process is transversal on the orbit of the SWM
3. The operating modes are TE
4. The frequency is related to the Gyro frequency
5. Fast-wave devices vphase > c
6. Operate at High-order modes using harmonics of the gyro-frequency or significant Doppler shift
7. Less-beam compression
8. Low perveance beam

8. High-Power, Low-Frequency, Short-Pulse Sources
-Klystron (amplifier)– up to 11 GHz(50 MW), high efficiency
-Gyroklystron (amplifier) – 17GHz(10 MW), 34 GHz, several MW, projects
-Magnicon (amplifier)– 11 GHz(50 MW), high efficiency,
-Free electron laser- 17 GHz(20 MW), low efficiency
-High-power, Short-Pulse, Low efficiency Gyrotron oscillator(6-8% eff.), 30-50GHz(50) MW
-High-Power, Cyclotron Autoresonance Maser – CARM; Eff.<10%, 10-50GHz(10-15 MW), osc.& ampl..
-Vircator – High voltage (0.5-2 MV), High-current ( MA), 0.1-3GW, 2-3% eff., no magnetic field.

9. High-Frequency, Low & Mid Power, Long-Pulse Sources
-Gyroklystron – 94 GHz, 10KW average power, high eff.
-Gyro-TWT – from 60 to 90 GHz, several KW RF Power, good eff.
Both devices operate as amplifiers
-Klystron – KW, ms range, at 6-8 GHz, high eff.
-Gyrotron for FUSION - 1 MW, High-Efficiency,, 1s to 1h pulse, cylindrical, qusioptical and coaxial cavity.
-Low-Power, High-Efficiency Cyclotron Autoresonance Maser – from GHz, ms. To operates as an amplifier and an oscillator, as well, PROJECT!!

10. Application of HPM Drivers for RF Colliders Radars Electronic Warfare
Heating Plasma in TOKAMAK
Spectroscopy
Material Treatment
Every application depends on:
Power - from several KW to several GW
Pulse duration – from several ns to CW
Frequency – from several GHz to 1THz

11. CRM - Gyrotron -Operates at cyclotron frequency near cutoff
-High efficiency at nonrelativistic beam energy –from several KV to 100KeV
-Tolerant to poor beam quality and supports high current
– from 1-2 to 100 Amps, high vellocity spread (5-10%)
-High pitch ratio α (Vperp/Vlong.) >1%)
-Needs very high magnetic field for reaching high frequency -28GHz/T

12. Gyrotron

13. Gyrotron Modes

14. CRM - Cyclotron Autoresonance Maser
Operates far from cutoff which reduces the fields at cavity walls
High frequency at low magnetic field
High efficiency due to the Doppler term compensation
Needs high beam quality – velocity spread < 1%
High operating voltage to 3 MV
Low to moderate current – several Amps to 100 A
Pitch ratio α ~ 1/γ

15. Some limits for CW operation
Cavity Cross Section:
– limited downward by the max. power density in cw operation
Power density – 500 KW/cm2
Power losses
ΔP/ΔS ~ P (Qdif/QΩ)/(2πRcLc)
Ohmical Q-factor
QΩ = (Rc/δ) (1 − m2/ν2)
Acceptable power losses
ΔP/ΔS) = 2−3 kW/cm2

16. Limits for Max. Surface Field
Operating mode:
i) to minimize the field on the cavity wall for avoiding a surface breakdown and an excessive thermal load
ii) to experience necessary reflection from the cavity mirrors
Surface field for CW operation is 10KV/mm

17. Here is what we do have at ENEA
Low-frequency Gyrotrons and Klystrons
Cavity Oscillation Mode -TE511
Nominal Output Frequency -8GHz
Frequency Stability + / - 1MHz
Output Power (TE01) MW
Efficiency < 46 %
Klystrons – 4 CW operatyng Klystrons at 8GHz with 250 KW of RF power each
Freq. stability – several KHz

18. High-frequency Gyrotrons for Electron Cyclotron Plasma Heating (ECRH)
- frequency -140 GHz
- output power capability KW each
- pulse duration - 0.5 s
- Frequency Stability - 1MHz
- Frequency Fluctuation MHz

19. Here is what we are plenning to have (to build) at ENEA
1. Design and fabrication of 1 MW, 250 GHz 5 us Short Pulse, 100 Hz Rep. Rate Oscillator
2. Design and fabrication 1-2 ms Long Pulse Osc. with the above characteristics.
3. Realization of 1h CW operation of the same oscillator
4. Realization of an Amplifier version for plasma diagnostic, application in High-gradient RF accelerators and RF Undulators for FEL (1 KHz Freq. stability)

20. CARM Project Assembly

21. Schematic Diagram of the Experiment

22. Difficulties to develop CARM
1) difficulties in making 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%), moderate pitch ration (<1) beam, since the high-kz interaction increases the sensitivity to axial velocity spread
3) stability of gyrotron and gyro-BWO modes (if waveguide cavities are used)
4) limited experimental track record, mostly short-pulse oscillators with efficiency 5–10%.
A new experimental program to explore the capabilities of a CARM driven by an advanced low-velocity-spread MIG could test the ability of CARMs to compete with gyrotrons and gyroklystrons.

23. Main Project Parameters of the E. Gun
Anode-cathode distance (Da-Dg)/2
200mm
Magnetic field along the cavity
5-7 T
Pitch ratio α = v/vll
<0.5
Axial and transverse velocity spread
<0.1÷0.3 %
Cathode potential
500÷700 kV
Electric field at the cathode surface (Ec)
<10kV/mm

24. Electron Gun and Beam Transport
Neck radius
46 mm
(CST Microwave studio)

25. Surface Electric Field Along the Blue Line

26. Beam Size Along Z

27. Pitch Ratio along Z
α= V / Vz mm

28. Longitudinal Velocity Spread
∆v/v mm

29. Brillouin Diagram

30. Brillouin Diagram

31. TE9,2
TE10,2
TE8,2
TE11,2
TE12,2

32. Maryland Gyroklystron Gun 500kV 1000A 4μs rep.rate 10Hz

33. The Experiment Might Look Like This
3 m long, 1,5 m high (excluding the anechoic chamber)

34. FOM-Instituut voor Plasmafysica “Rijnhuizen” FEM(Free Electron Maser) >200GHz
FOM FEM Project
Electrostatic accelerator 2MeV
f 160÷260 GHz,
P 1 MW
T 2 s
To increase the efficiency a 99,99% beam recovery is needed

35. RESULTS

36. ITER 1 MW, 170 GHz, 1h, Gyrotron

37. Costs for Design and Assembling 5us pulse
HOME DESIGNED COMPONENTS
COMPONENTS
COST
ELECTRON GUN
500 KE
GUN COIL
50 KE
CAVITY
200 KE
RF OUTPUT SYSTEM
DEPRESSED COLLECTOR KE
SOFTWARE CODES
200KE
MAJORE COMPONENTS
COMPONENTS
COST
HIGH VOLTAGE MODULATOR
1000KE
SUPPERCONDUCTING MAGNET
350KE
ELECTRON GUN
500KE
HIGH VOLTAGE POWER SUPPLY
50 KE