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A high frequency, high power CARM proposal for the DEMO ECRH system
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A high frequency, high power CARM proposal for the DEMO ECRH system Francesco Mirizzi a,∗ , Ivan Spassovsky b , Silvio Ceccuzzi c , Giuseppe Dattoli b , Emanuele Di Palma b , Andrea Doria b , Gianpiero Gallerano b , Alessandro Lampasi c , Giuseppe Maffia c , GianLuca Ravera c , Elio Sabia b , Angelo Antonio Tuccillo c , Pietro Zito c , on behalf of the ENEA CARM Task Force a
Consorzio CREATE, Via Claudio 21, I-80125 Napoli, Italy Unità Tecnica Applicazioni delle Radiazioni – ENEA, C.R. Frascati, via E. Fermi 45, I-00044 Frascati, Italy c Unità Tecnica Fusione – ENEA C. R. Frascati, via E. Fermi 45, 00044 Frascati, Roma, Italy b
h i g h l i g h t s • • • • •
ECRH system for DEMO. Cyclotron Auto-Resonance Maser (CARM) devices. Relativistic electron beams. Bragg reflectors. High voltage pulse modulators.
a r t i c l e
i n f o
Article history: Received 25 September 2014 Received in revised form 28 January 2015 Accepted 16 February 2015 Available online xxx Keywords: DEMO ECRH systems CARM device Doppler shift Bragg reflector
a b s t r a c t ECRH&CD systems are extensively used on tokamak plasmas due to their capability of highly tailored power deposition, allowing very localised heating and non-inductive current drive, useful for MHD and profiles control. The high electron temperatures expected in DEMO will require ECRH systems with operating frequency in the 200–300 GHz range, equipped with a reasonable number of high power (P ≥ 1 MW) CW RF sources, for allowing central RF power deposition. In this frame the ENEA Fusion Department (Frascati) is coordinating a task force aimed at the study and realisation of a suitable high power, high frequency reliable source. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The European demonstration fusion reactor DEMO is aimed at producing 500 MW of electrical power, corresponding to a thermal output of about 1500 MW. The DEMO burning plasma characteristics call for a powerful ECRH&CD system operating in the 200–300 GHz frequency range [1]. Suitable high power CW sources for this system are not currently available. Design studies of the Institute of Applied Physics (IAP) in Nizhny Novgorod (Russia) [2] show that a 300 GHz, 1 MW CW conventional cylindrical cavity
∗ Corresponding author. Tel.: +39 0694005259. E-mail address: [email protected] (F. Mirizzi).
gyrotron is feasible. Experimental results on a 300 GHz, 0.5 MW, 2 ms gyrotron are reported in Ref. [3]. In this context a task force including experts of ENEA Fusion and Radiation Application departments in Frascati, and experts from CREATE (Naples) and Plasma Physics Institute of CNR (Milan), is analysing the technological feasibility of a 250 GHz, 500 kW CW Cyclotron Auto-Resonance Maser (CARM). The CARM, which principle was initially proposed by the Nizhny Novgorod IAP [4], is a source of coherent electromagnetic radiation based on the interaction between relativistic electron beams and high frequency TEmn fields in a highly efficient resonant cavity with quality factor Q ≥ 1000. Doppler up-shift of about 2 times ( = relativistic factor) the relativistic electron cyclotron frequency ˝C in the resonator region can generate electromagnetic waves in that frequency range. High quality electron beams
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with longitudinal velocity spread less than 0.5% can effectively transfer significant part of their energy to the electromagnetic fields allowing output powers in the MW range. CARM devices can be then considered promising alternatives to gyrotrons for the realisation of efficient ECRH systems for DEMO fusion reactors. 2. The ENEA CARM The feasibility study in ENEA of a CARM device is the initial step of a long term development programme which final objective is the realisation of a prototype with an output RF power P ≥ 500 kW and a pulse length t ≥ 1 s, for an ECRH system able to match the DEMO foreseen plasma characteristics. In the medium term (about 5 years) two main steps are planned. The first step covers the development and test on dummy load of a CARM device with output power P ≥ 100 kW and pulse length up to 100 s mainly aimed at verifying the feasibility of this source. In the following second step, a CARM pre-prototype with output power P ≤ 500 kW and pulse length t ≤ 100 ms will be developed and possibly tested on FTU, which magnetic field (B ≤ 8 T) and plasma densities (ne ≤ 1021 m−3 in the plasma centre) should allow effective tests. Many experimental scenarios can be envisaged on this machine, from a second harmonic central heating at 4–5 T (frequency range 220–280 GHz) to a first harmonic central high field side heating at 7–8 T (frequency range 200–250 GHz). The not always positive results obtained in the realisation of the early CARMs have been carefully examined and efficient technological solutions have been identified and analysed in order to assure the realisation of a really effective CARM device. 3. CARM basic theory The ENEA CARM is a diode type device (Fig. 1). The electrons emitted by a very hot cathode (T ≤ 1500 ◦ C), following individual helical paths and arranged in a hollow electron beam (beam
Fig. 2. Resonant cavity.
current IB ≤ 10 A) by means of suitable static magnetic fields in the gun and cavity regions (Table 2), are accelerated at relativistic velocity through an extremely high cathode-to-collector voltage (VK = 500–700 kV). The electron beam releases part of its kinetic energy to a self excited TEmn electric field in a resonant cavity (Fig. 2) delimited by two Bragg reflectors [4] able to assure a cavity high Q-factor (Q ∼ = 4000). The operational CARM frequency ω, higher than the cut-off frequency of its cavity for the TEmn mode, is obtained by a Doppler up-shift of the relativistic electron cyclotron frequency ˝C in the cavity region by a factor s ≤ 2 , being the relativistic factor, which value is strictly dependent on the cathode voltage VK . For the best performances of the CARM, the ratio between axial and longitudinal electron velocities at the cavity input is set to: ˛=
v⊥ 1 = v||
(1)
The electron beam quality, measured by the electrons longitudinal velocity spread v /v , is crucial for the optimum performances of the CARM. While transferring energy to the TEmn field the electrons lose kinetic energy, hence their relativistic factor decreases and so does their longitudinal velocity v . For limited variation, according to the dispersion relation − k v = ˝C , the consequent relativistic cyclotron frequency increase is compensated by the v decrease, so that the CARM output frequency does not significantly changes (auto-resonance effect). At the cavity output the residual electron beam energy is uniformly distributed on the CARM collector. In the second step of the ENEA CARM development, a depressed collector could be added to the pre-prototype in order to increase its efficiency. 4. The gun region The CARM gun region includes the cathode and the accelerating anode. A diode-type gun has been chosen because it assures a lower velocity spread of the generated electron beam The minimum distance of 110 mm in vacuum between these two geometrically complementary electrodes and their shapes have been accurately optimised [6] by numerical simulation with the commercial code CST in order to limit the resulting electric field, which calculated contour plot is shown in Fig. 3, to a safe maximum value of 8 kV/mm, a value detected around the emitting ring. Simulation work is still in progress to further reduce this value. The start of the electrons gyromotion with cyclotron angular frequency ˝G =
Fig. 1. CARM simplified schematic.
e · BG m
(2)
where BG ≤ 0.05 T is the static axial magnetic field in the gun region and m = m0 the electrons relativistic mass, is determined by an electric field kick close to the input of the drift tube. Along the drift tube, connecting the anode to the resonant cavity, the axial magnetic field is smoothly increased (Fig. 4) up to the value
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Fig. 6. Electron beam shape.
Table 1 Electron beam main parameters. Fig. 3. Electric field in the gun region.
Beam radius (in the cavity) Beam energy Beam thickness Beam current Velocity ratio Longitudinal velocity spread
4.5–5.5 mm 500–700 keV 100–200 m 5–10 A 0.45–0.55 <0.5%
6. The electron beam
Fig. 4. Longitudinal magnetic field qualitative profile.
BC ≤ 7 T it has in the cavity region, thus determining the increase of the transverse energy of the electrons in an adiabatic context. The increasing magnetic field produces also a reduction of the beam diameter. An accurate choice of the frequency Doppler shift factor s ≤ 2 allows a reasonable value of the magnetic field in the cavity region limited to less than 7 T, then available by relatively cheap magnetic coils, provided that the electron beam quality meets the theoretical v /v ≤ 0.5% optimised value. 5. The emitting ring The emitting ring is an integrating part of the cathode (Fig. 5) and lays in the upper flat region of this electrode, so that all the emitted electrons are subject to the same accelerating electric field value and follow paths with similar lengths. Consequently the electrons longitudinal velocity spread is limited, as confirmed by the simulation results [7], to less than 0.5%, then assuring a high efficiency energy transfer to the electromagnetic field in the resonant cavity. Rare earth hexaborides, with an average work function of 2.4 eV in the 1000–1500 ◦ C temperature range, can be efficiently used for the realisation of the circular, 50 mm diameter, 1 mm wide emitting ring. The expected emitter surface roughness (≤1 m) does not influence the velocity spread. The operational temperature of the emitter will be limited to a maximum of 1300 ◦ C in order to extend its lifetime to a minimum of 10,000 h and to reduce its evaporation rate.
Fig. 5. Cathode and emitting ring.
The electron beam shape (Fig. 6) has been evaluated and optimised by simulation [7] with the CST Particle Studio (Stationary Particle Tracking Solver) code. The resulting main parameters of the optimised beam are given in Table 1. This set of parameters will be used to simulate the beam–wave interaction through both commercially available and homedeveloped particle-in-cell (PIC) codes. In parallel a purely analytical optimisation work is presently in progress [8]. 7. The high Q resonant cavity The resonant cavity (Fig. 2) is obtained by delimiting a straight section of a smooth circular waveguide, which cut-off frequency is significantly lower than the CARM operational frequency, by two high efficiency Bragg reflectors [5]. The waveguide diameter and length are set by an acceptably safe power losses density limit PD ≤ 2 kW/cm2 on the cavity walls at maximum performances, while the operating mode is determined by the cavity magnetic field, the Doppler shift factor and by the beam radius, energy and quality. The main geometric parameters of a TE92 resonant cavity have been preliminarily optimised [9] according to the coupled mode theory formulation given in Ref. [10]. In Fig. 7 the computed reflectivity for five modes of the up-stream reflector, with a basic waveguide diameter D = 15 mm, sinusoidal corrugation amplitude
Fig. 7. Reflectivity of the up stream Bragg reflector.
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4 Table 2 Magnetic coils essential parameters.
Inner radius (mm) Length (mm) Magnetic field (T)
10. Conclusions Gun coil
Cavity coil
450 150 ≤0.05
60 500 ≤7
A = 25 m, period T = 651 m and length L = 450 mm is shown. With these geometrical parameters an optimum reflectivity of 96% and a 3 dB bandwidth of about 0.8 GHz for the TE92 mode have been evaluated. The nearest modes in competition, TM91 and TM92 , are safely distanced of about 2 GHz from the central TE92 mode. In any case an accurate dimensioning of the resonant cavity and a pretty good quality of the electron beam should not allow the generation of these spurious modes. Optimisation work is still in progress mainly to reduce the overall length of the corrugated section of the two Bragg reflectors by investigating different corrugation profiles and parameters. A cold test of the real cavity is also foreseen in order to verify its oscillation frequency and mode. Adjustments will be made in case of discrepancy from the expected ones. 8. The magnetic coils The two main CARM magnetic coils: the gun and the cavity coils, have been very preliminarily dimensioned according to the foreseen CARM performances and dimensions. Their resulting main parameters are summarised in Table 2. A more accurate analysis is presently in progress for a detailed definition of all their electromagnetic and mechanical parameters. 9. The high voltage modulator unit For the first step of the CARM development schedule, this unit will have an output voltage in the range 500–700 kV with a pulse length up to 100 s, rise time R ≤ 1 s, voltage overshoot less than 2%, voltage drop, ripple and stability less than 0.1% during the pulse flat top. The maximum energy delivered in case of a CARM inner electric breakdown is limited to 10 J. The analysed unit [11] is made by an AC/DC, voltage regulated, voltage converter with an input voltage of 20 kV ac, 50 Hz, three phases. A two secondary windings, star and delta connected, step down transformer (20 kV/2 × 1650 V AC) feeds two, series connected, thyristor bridges delivering a maximum output voltage of 4.3 kV DC. This converter feeds the DC/DC pulse modulator made by eight IGBT modules connected to the eight primary windings of a pulse transformers (transformer ratio 4/87.5) which secondary windings are series connected for generating a maximum overall output voltage of 700 kV. The required accuracy of the pulsed output voltage is obtained by a PID digital controller based on the pulse width modulation (PWM) technique. The configuration, layout and parameters of this unit have been preliminarily optimised by a MATLAB/Simulink model. The chosen configuration allows high flexibility of pulse amplitude and length.
A CARM device, a promising high power source in the ECRH frequency range required for DEMO, is in an advanced phase of study in ENEA. The attained simulation results show the concrete feasibility of this source. Preliminary informal contacts with worldwide leader factories are in progress to evidence any possible technological difficulties and to search for possible solutions. Acknowledgments This work has been performed in the frame of the Enabling Research of the EFDA Work Programme 2014 for the implementation of the Fusion Roadmap. The authors would like to acknowledge the ENEA CARM task force: L. Amicucci, M. Aquilini, A. Bruschi (IFP, CNR, Milano), P. Buratti, E. Campana, M. Carpanese, F. Causa, S. Ceccuzzi, F. Ciocci, G. Dattoli, D. De Meis, A. De Stefano, S. Di Giovenale, L. Di Pace, E. Di Palma, D. Di Girolamo (CREATE, Napoli), A. Doria, A. Fastelli, G.P. Gallerano, S. Garavaglia (IFP, CNR, Milano), E. Giovenale, A. Lampasi, G. Maffia, L. Mezi, F. Mirizzi (CREATE, Napoli), A. Petralia, P. Petrolini, B. Raspante, G.L. Ravera, G. Rocchi, E. Sabia, G. Schettini (Università Roma Tre, Roma), A. Simonetta (IFP, CNR, Milan), I. Spassovsky, A.A. Tuccillo, P. Zito for its effective contribution to the preliminary analysis of the proposed CARM device. References [1] E. Poli, G. Tardini, H. Zohm, E. Fable, D. Farina, L. Figini, et al., ECRH efficiency in DEMO plasmas, Nucl. Fusion 53 (2013) 013011. [2] V.E. Zapevalov, M.A. Moiseyev, N.A. Zavolsky, Numerical simulation of processes at the cavity of high power 300 GHz gyrotrons, in: Proc. of the IRMMW-THz Conference, Mainz, Germany, 2013. [3] K. Sakamoto, Y. Oda, T. Kariya, R. Minami, R. Ikeda, K. Kajiwara, et al., Preliminary results of a 300 GHz, short pulse high order mode gyrotron, in: Proc. of the IRMMW-THz Conference, Tucson, AZ, USA, 2014. [4] V.L. Bratman, N.S. Ginzburg, G.S. Nusinovich, M.L. Petelin, P.S. Strelkov, Relativistic gyrotrons and cyclotron autoresonance masers, Int. J. Electron. 51 (1981) 541–567. [5] C.K. Chong, D.B. McDermott, M.M. Razeghi, N.C. Luhmann Jr., J. Pretterebner, D. Wagner, et al., Bragg reflectors, IEEE Trans. Plasma Sci. 20 (June (3)) (1992). [6] G. Dattoli, L. Giannessi, J.V. Rau, A. Petralia, M. Artioli, M. Quattromini, et al., High voltage electron gun for high average power FEL, in: Proc. of the 19th Int. Conference on High Power Particle Beams, Karlsruhe, Germany, September 30–October 4, 2012. [7] I. Spassovsky, E. Di Palma, V. Surrenti, G. Dattoli, A. Doria, G.P. Gallerano, et al., Preliminary design of a thermionic gun to generate a relativistic beam for driving a high power millimeter wave oscillator, J. Vac. Sci. Technol. B (2015) (in press). [8] G. Dattoli, E. Di Palma, A. Doria, E. Sabia, I. Spassovsky, S. Ceccuzzi, et al., The Gyrotron, CARM and FEL devices: an analytical unified formulation for the small signal analysis, in: Proc. of the IVECS Conference, Saint-Petersburg, Russia, June 30–July 04, 2014, 2015 (in press). [9] S. Ceccuzzi, P. Buratti, G. Dattoli, E. Di Palma, A. Doria, G.P. Gallerano, et al., Initial steps towards a Bragg reflector for a 250 GHz CARM experiment, in: Proc. of the 9th Int. Workshop on Strong Microwaves and Terahertz Waves, Nizhn Novgorod, Russia, 24–30 July, 2014. [10] V.L. Bratman, G. Denisov, N.S. Ginzburg, M.L. Petelin, FEL’s with Bragg reflection resonators. Cyclotron Autoresonance Masers versus Ubitrons, IEEE J. Quantum Electron. 19 (3) (1983). [11] P. Zito, G. Maffia, A. Lampasi, Conceptual design of a pulsed high voltage, high precision power supply for a Cyclotron Auto-Resonance Maser (CARM) for plasma heating. Proc. of the 28th SOFT, Sept. 29th – Oct 3rd 2014, S. Sebastian, Spain (in press).
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A high frequency, high power CARM proposal for the DEMO ECRH system Francesco Mirizzi a,∗ , Ivan Spassovsky b , Silvio Ceccuzzi c , Giuseppe Dattoli b , Emanuele Di Palma b , Andrea Doria b , Gianpiero Gallerano b , Alessandro Lampasi c , Giuseppe Maffia c , GianLuca Ravera c , Elio Sabia b , Angelo Antonio Tuccillo c , Pietro Zito c , on behalf of the ENEA CARM Task Force a
Consorzio CREATE, Via Claudio 21, I-80125 Napoli, Italy Unità Tecnica Applicazioni delle Radiazioni – ENEA, C.R. Frascati, via E. Fermi 45, I-00044 Frascati, Italy c Unità Tecnica Fusione – ENEA C. R. Frascati, via E. Fermi 45, 00044 Frascati, Roma, Italy b
h i g h l i g h t s • • • • •
ECRH system for DEMO. Cyclotron Auto-Resonance Maser (CARM) devices. Relativistic electron beams. Bragg reflectors. High voltage pulse modulators.
a r t i c l e
i n f o
Article history: Received 25 September 2014 Received in revised form 28 January 2015 Accepted 16 February 2015 Available online xxx Keywords: DEMO ECRH systems CARM device Doppler shift Bragg reflector
a b s t r a c t ECRH&CD systems are extensively used on tokamak plasmas due to their capability of highly tailored power deposition, allowing very localised heating and non-inductive current drive, useful for MHD and profiles control. The high electron temperatures expected in DEMO will require ECRH systems with operating frequency in the 200–300 GHz range, equipped with a reasonable number of high power (P ≥ 1 MW) CW RF sources, for allowing central RF power deposition. In this frame the ENEA Fusion Department (Frascati) is coordinating a task force aimed at the study and realisation of a suitable high power, high frequency reliable source. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The European demonstration fusion reactor DEMO is aimed at producing 500 MW of electrical power, corresponding to a thermal output of about 1500 MW. The DEMO burning plasma characteristics call for a powerful ECRH&CD system operating in the 200–300 GHz frequency range [1]. Suitable high power CW sources for this system are not currently available. Design studies of the Institute of Applied Physics (IAP) in Nizhny Novgorod (Russia) [2] show that a 300 GHz, 1 MW CW conventional cylindrical cavity
∗ Corresponding author. Tel.: +39 0694005259. E-mail address: [email protected] (F. Mirizzi).
gyrotron is feasible. Experimental results on a 300 GHz, 0.5 MW, 2 ms gyrotron are reported in Ref. [3]. In this context a task force including experts of ENEA Fusion and Radiation Application departments in Frascati, and experts from CREATE (Naples) and Plasma Physics Institute of CNR (Milan), is analysing the technological feasibility of a 250 GHz, 500 kW CW Cyclotron Auto-Resonance Maser (CARM). The CARM, which principle was initially proposed by the Nizhny Novgorod IAP [4], is a source of coherent electromagnetic radiation based on the interaction between relativistic electron beams and high frequency TEmn fields in a highly efficient resonant cavity with quality factor Q ≥ 1000. Doppler up-shift of about 2 times ( = relativistic factor) the relativistic electron cyclotron frequency ˝C in the resonator region can generate electromagnetic waves in that frequency range. High quality electron beams
http://dx.doi.org/10.1016/j.fusengdes.2015.02.026 0920-3796/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: F. Mirizzi, et al., A high frequency, high power CARM proposal for the DEMO ECRH system, Fusion Eng. Des. (2015), http://dx.doi.org/10.1016/j.fusengdes.2015.02.026
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with longitudinal velocity spread less than 0.5% can effectively transfer significant part of their energy to the electromagnetic fields allowing output powers in the MW range. CARM devices can be then considered promising alternatives to gyrotrons for the realisation of efficient ECRH systems for DEMO fusion reactors. 2. The ENEA CARM The feasibility study in ENEA of a CARM device is the initial step of a long term development programme which final objective is the realisation of a prototype with an output RF power P ≥ 500 kW and a pulse length t ≥ 1 s, for an ECRH system able to match the DEMO foreseen plasma characteristics. In the medium term (about 5 years) two main steps are planned. The first step covers the development and test on dummy load of a CARM device with output power P ≥ 100 kW and pulse length up to 100 s mainly aimed at verifying the feasibility of this source. In the following second step, a CARM pre-prototype with output power P ≤ 500 kW and pulse length t ≤ 100 ms will be developed and possibly tested on FTU, which magnetic field (B ≤ 8 T) and plasma densities (ne ≤ 1021 m−3 in the plasma centre) should allow effective tests. Many experimental scenarios can be envisaged on this machine, from a second harmonic central heating at 4–5 T (frequency range 220–280 GHz) to a first harmonic central high field side heating at 7–8 T (frequency range 200–250 GHz). The not always positive results obtained in the realisation of the early CARMs have been carefully examined and efficient technological solutions have been identified and analysed in order to assure the realisation of a really effective CARM device. 3. CARM basic theory The ENEA CARM is a diode type device (Fig. 1). The electrons emitted by a very hot cathode (T ≤ 1500 ◦ C), following individual helical paths and arranged in a hollow electron beam (beam
Fig. 2. Resonant cavity.
current IB ≤ 10 A) by means of suitable static magnetic fields in the gun and cavity regions (Table 2), are accelerated at relativistic velocity through an extremely high cathode-to-collector voltage (VK = 500–700 kV). The electron beam releases part of its kinetic energy to a self excited TEmn electric field in a resonant cavity (Fig. 2) delimited by two Bragg reflectors [4] able to assure a cavity high Q-factor (Q ∼ = 4000). The operational CARM frequency ω, higher than the cut-off frequency of its cavity for the TEmn mode, is obtained by a Doppler up-shift of the relativistic electron cyclotron frequency ˝C in the cavity region by a factor s ≤ 2 , being the relativistic factor, which value is strictly dependent on the cathode voltage VK . For the best performances of the CARM, the ratio between axial and longitudinal electron velocities at the cavity input is set to: ˛=
v⊥ 1 = v||
(1)
The electron beam quality, measured by the electrons longitudinal velocity spread v /v , is crucial for the optimum performances of the CARM. While transferring energy to the TEmn field the electrons lose kinetic energy, hence their relativistic factor decreases and so does their longitudinal velocity v . For limited variation, according to the dispersion relation − k v = ˝C , the consequent relativistic cyclotron frequency increase is compensated by the v decrease, so that the CARM output frequency does not significantly changes (auto-resonance effect). At the cavity output the residual electron beam energy is uniformly distributed on the CARM collector. In the second step of the ENEA CARM development, a depressed collector could be added to the pre-prototype in order to increase its efficiency. 4. The gun region The CARM gun region includes the cathode and the accelerating anode. A diode-type gun has been chosen because it assures a lower velocity spread of the generated electron beam The minimum distance of 110 mm in vacuum between these two geometrically complementary electrodes and their shapes have been accurately optimised [6] by numerical simulation with the commercial code CST in order to limit the resulting electric field, which calculated contour plot is shown in Fig. 3, to a safe maximum value of 8 kV/mm, a value detected around the emitting ring. Simulation work is still in progress to further reduce this value. The start of the electrons gyromotion with cyclotron angular frequency ˝G =
Fig. 1. CARM simplified schematic.
e · BG m
(2)
where BG ≤ 0.05 T is the static axial magnetic field in the gun region and m = m0 the electrons relativistic mass, is determined by an electric field kick close to the input of the drift tube. Along the drift tube, connecting the anode to the resonant cavity, the axial magnetic field is smoothly increased (Fig. 4) up to the value
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Fig. 6. Electron beam shape.
Table 1 Electron beam main parameters. Fig. 3. Electric field in the gun region.
Beam radius (in the cavity) Beam energy Beam thickness Beam current Velocity ratio Longitudinal velocity spread
4.5–5.5 mm 500–700 keV 100–200 m 5–10 A 0.45–0.55 <0.5%
6. The electron beam
Fig. 4. Longitudinal magnetic field qualitative profile.
BC ≤ 7 T it has in the cavity region, thus determining the increase of the transverse energy of the electrons in an adiabatic context. The increasing magnetic field produces also a reduction of the beam diameter. An accurate choice of the frequency Doppler shift factor s ≤ 2 allows a reasonable value of the magnetic field in the cavity region limited to less than 7 T, then available by relatively cheap magnetic coils, provided that the electron beam quality meets the theoretical v /v ≤ 0.5% optimised value. 5. The emitting ring The emitting ring is an integrating part of the cathode (Fig. 5) and lays in the upper flat region of this electrode, so that all the emitted electrons are subject to the same accelerating electric field value and follow paths with similar lengths. Consequently the electrons longitudinal velocity spread is limited, as confirmed by the simulation results [7], to less than 0.5%, then assuring a high efficiency energy transfer to the electromagnetic field in the resonant cavity. Rare earth hexaborides, with an average work function of 2.4 eV in the 1000–1500 ◦ C temperature range, can be efficiently used for the realisation of the circular, 50 mm diameter, 1 mm wide emitting ring. The expected emitter surface roughness (≤1 m) does not influence the velocity spread. The operational temperature of the emitter will be limited to a maximum of 1300 ◦ C in order to extend its lifetime to a minimum of 10,000 h and to reduce its evaporation rate.
Fig. 5. Cathode and emitting ring.
The electron beam shape (Fig. 6) has been evaluated and optimised by simulation [7] with the CST Particle Studio (Stationary Particle Tracking Solver) code. The resulting main parameters of the optimised beam are given in Table 1. This set of parameters will be used to simulate the beam–wave interaction through both commercially available and homedeveloped particle-in-cell (PIC) codes. In parallel a purely analytical optimisation work is presently in progress [8]. 7. The high Q resonant cavity The resonant cavity (Fig. 2) is obtained by delimiting a straight section of a smooth circular waveguide, which cut-off frequency is significantly lower than the CARM operational frequency, by two high efficiency Bragg reflectors [5]. The waveguide diameter and length are set by an acceptably safe power losses density limit PD ≤ 2 kW/cm2 on the cavity walls at maximum performances, while the operating mode is determined by the cavity magnetic field, the Doppler shift factor and by the beam radius, energy and quality. The main geometric parameters of a TE92 resonant cavity have been preliminarily optimised [9] according to the coupled mode theory formulation given in Ref. [10]. In Fig. 7 the computed reflectivity for five modes of the up-stream reflector, with a basic waveguide diameter D = 15 mm, sinusoidal corrugation amplitude
Fig. 7. Reflectivity of the up stream Bragg reflector.
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4 Table 2 Magnetic coils essential parameters.
Inner radius (mm) Length (mm) Magnetic field (T)
10. Conclusions Gun coil
Cavity coil
450 150 ≤0.05
60 500 ≤7
A = 25 m, period T = 651 m and length L = 450 mm is shown. With these geometrical parameters an optimum reflectivity of 96% and a 3 dB bandwidth of about 0.8 GHz for the TE92 mode have been evaluated. The nearest modes in competition, TM91 and TM92 , are safely distanced of about 2 GHz from the central TE92 mode. In any case an accurate dimensioning of the resonant cavity and a pretty good quality of the electron beam should not allow the generation of these spurious modes. Optimisation work is still in progress mainly to reduce the overall length of the corrugated section of the two Bragg reflectors by investigating different corrugation profiles and parameters. A cold test of the real cavity is also foreseen in order to verify its oscillation frequency and mode. Adjustments will be made in case of discrepancy from the expected ones. 8. The magnetic coils The two main CARM magnetic coils: the gun and the cavity coils, have been very preliminarily dimensioned according to the foreseen CARM performances and dimensions. Their resulting main parameters are summarised in Table 2. A more accurate analysis is presently in progress for a detailed definition of all their electromagnetic and mechanical parameters. 9. The high voltage modulator unit For the first step of the CARM development schedule, this unit will have an output voltage in the range 500–700 kV with a pulse length up to 100 s, rise time R ≤ 1 s, voltage overshoot less than 2%, voltage drop, ripple and stability less than 0.1% during the pulse flat top. The maximum energy delivered in case of a CARM inner electric breakdown is limited to 10 J. The analysed unit [11] is made by an AC/DC, voltage regulated, voltage converter with an input voltage of 20 kV ac, 50 Hz, three phases. A two secondary windings, star and delta connected, step down transformer (20 kV/2 × 1650 V AC) feeds two, series connected, thyristor bridges delivering a maximum output voltage of 4.3 kV DC. This converter feeds the DC/DC pulse modulator made by eight IGBT modules connected to the eight primary windings of a pulse transformers (transformer ratio 4/87.5) which secondary windings are series connected for generating a maximum overall output voltage of 700 kV. The required accuracy of the pulsed output voltage is obtained by a PID digital controller based on the pulse width modulation (PWM) technique. The configuration, layout and parameters of this unit have been preliminarily optimised by a MATLAB/Simulink model. The chosen configuration allows high flexibility of pulse amplitude and length.
A CARM device, a promising high power source in the ECRH frequency range required for DEMO, is in an advanced phase of study in ENEA. The attained simulation results show the concrete feasibility of this source. Preliminary informal contacts with worldwide leader factories are in progress to evidence any possible technological difficulties and to search for possible solutions. Acknowledgments This work has been performed in the frame of the Enabling Research of the EFDA Work Programme 2014 for the implementation of the Fusion Roadmap. The authors would like to acknowledge the ENEA CARM task force: L. Amicucci, M. Aquilini, A. Bruschi (IFP, CNR, Milano), P. Buratti, E. Campana, M. Carpanese, F. Causa, S. Ceccuzzi, F. Ciocci, G. Dattoli, D. De Meis, A. De Stefano, S. Di Giovenale, L. Di Pace, E. Di Palma, D. Di Girolamo (CREATE, Napoli), A. Doria, A. Fastelli, G.P. Gallerano, S. Garavaglia (IFP, CNR, Milano), E. Giovenale, A. Lampasi, G. Maffia, L. Mezi, F. Mirizzi (CREATE, Napoli), A. Petralia, P. Petrolini, B. Raspante, G.L. Ravera, G. Rocchi, E. Sabia, G. Schettini (Università Roma Tre, Roma), A. Simonetta (IFP, CNR, Milan), I. Spassovsky, A.A. Tuccillo, P. Zito for its effective contribution to the preliminary analysis of the proposed CARM device. References [1] E. Poli, G. Tardini, H. Zohm, E. Fable, D. Farina, L. Figini, et al., ECRH efficiency in DEMO plasmas, Nucl. Fusion 53 (2013) 013011. [2] V.E. Zapevalov, M.A. Moiseyev, N.A. Zavolsky, Numerical simulation of processes at the cavity of high power 300 GHz gyrotrons, in: Proc. of the IRMMW-THz Conference, Mainz, Germany, 2013. [3] K. Sakamoto, Y. Oda, T. Kariya, R. Minami, R. Ikeda, K. Kajiwara, et al., Preliminary results of a 300 GHz, short pulse high order mode gyrotron, in: Proc. of the IRMMW-THz Conference, Tucson, AZ, USA, 2014. [4] V.L. Bratman, N.S. Ginzburg, G.S. Nusinovich, M.L. Petelin, P.S. Strelkov, Relativistic gyrotrons and cyclotron autoresonance masers, Int. J. Electron. 51 (1981) 541–567. [5] C.K. Chong, D.B. McDermott, M.M. Razeghi, N.C. Luhmann Jr., J. Pretterebner, D. Wagner, et al., Bragg reflectors, IEEE Trans. Plasma Sci. 20 (June (3)) (1992). [6] G. Dattoli, L. Giannessi, J.V. Rau, A. Petralia, M. Artioli, M. Quattromini, et al., High voltage electron gun for high average power FEL, in: Proc. of the 19th Int. Conference on High Power Particle Beams, Karlsruhe, Germany, September 30–October 4, 2012. [7] I. Spassovsky, E. Di Palma, V. Surrenti, G. Dattoli, A. Doria, G.P. Gallerano, et al., Preliminary design of a thermionic gun to generate a relativistic beam for driving a high power millimeter wave oscillator, J. Vac. Sci. Technol. B (2015) (in press). [8] G. Dattoli, E. Di Palma, A. Doria, E. Sabia, I. Spassovsky, S. Ceccuzzi, et al., The Gyrotron, CARM and FEL devices: an analytical unified formulation for the small signal analysis, in: Proc. of the IVECS Conference, Saint-Petersburg, Russia, June 30–July 04, 2014, 2015 (in press). [9] S. Ceccuzzi, P. Buratti, G. Dattoli, E. Di Palma, A. Doria, G.P. Gallerano, et al., Initial steps towards a Bragg reflector for a 250 GHz CARM experiment, in: Proc. of the 9th Int. Workshop on Strong Microwaves and Terahertz Waves, Nizhn Novgorod, Russia, 24–30 July, 2014. [10] V.L. Bratman, G. Denisov, N.S. Ginzburg, M.L. Petelin, FEL’s with Bragg reflection resonators. Cyclotron Autoresonance Masers versus Ubitrons, IEEE J. Quantum Electron. 19 (3) (1983). [11] P. Zito, G. Maffia, A. Lampasi, Conceptual design of a pulsed high voltage, high precision power supply for a Cyclotron Auto-Resonance Maser (CARM) for plasma heating. Proc. of the 28th SOFT, Sept. 29th – Oct 3rd 2014, S. Sebastian, Spain (in press).
Please cite this article in press as: F. Mirizzi, et al., A high frequency, high power CARM proposal for the DEMO ECRH system, Fusion Eng. Des. (2015), http://dx.doi.org/10.1016/j.fusengdes.2015.02.026