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

Different RF Sources So Called HPM (High Power Microwaves)

Magnetron, Vircator and IOT (Inductive Output Tube)

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

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

Klystron Schematic

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

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 (0.01-1 MA), 0.1-3GW, 2-3% eff., no magnetic field.

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 – 100-500 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 250-300GHz, 1MW, @several ms. To operates as an amplifier and an oscillator, as well, PROJECT!!

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

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

Gyrotron

Gyrotron Modes

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 - 0.5 to 3 MV Low to moderate current – several Amps to 100 A Pitch ratio α ~ 1/γ

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

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

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) - 1.1 MW Efficiency < 46 % Klystrons – 4 CW operatyng Klystrons at 8GHz with 250 KW of RF power each Freq. stability – several KHz

High-frequency Gyrotrons for Electron Cyclotron Plasma Heating (ECRH) - frequency -140 GHz - output power capability - 500 KW each - pulse duration - 0.5 s - Frequency Stability - 1MHz - Frequency Fluctuation - 5-10MHz

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)

CARM Project Assembly

Schematic Diagram of the Experiment

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.

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

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

Surface Electric Field Along the Blue Line

Beam Size Along Z

Pitch Ratio along Z α= V / Vz mm

Longitudinal Velocity Spread ∆v/v mm

Brillouin Diagram

Brillouin Diagram

TE9,2 TE10,2 TE8,2 TE11,2 TE12,2

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

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

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

RESULTS

ITER 1 MW, 170 GHz, 1h, Gyrotron

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 300-500 KE SOFTWARE CODES 200KE MAJORE COMPONENTS COMPONENTS COST HIGH VOLTAGE MODULATOR 1000KE SUPPERCONDUCTING MAGNET 350KE ELECTRON GUN 500KE HIGH VOLTAGE POWER SUPPLY 50 KE