Gyrotron Development Review: Operational Principle and Interaction Structure

Transcription

1 MIT International Journal of Eletronis and Communiation Engineering, Vol. 4, No. 1, January 014, pp ISSN () MIT Publiations Gyrotron Development Review: Operational Priniple and Interation Struture Akhilesh Shukla Department of E&C Engg. Moradabad Institute of Tehnology Manish Saxena Assoiate Professor, ASH Moradabad Institute of Tehnology Nitin Kumar CEERI Pilani, Rajasthan, INDIA Kshitij Shinghal Department of E&C Engg. Moradabad Institute of Tehnology V. K. Pandey NIET Greater Noida, U.P., INDIA ABSTRACT A gyro-monotron or simply gyrotron is a powerful soure of eletromagneti waves of very short wavelength (in the order of a few millimeters). The gyrotron shows several unique advantages as a high power soure ompare to the other millimeter/ sub millimeter wave soures either semiondutor based devies or vauum based tube devies. A suffiiently long pulse width, high effiieny and high output power are the key requirements for the development of fusion gyrotrons. In the present artile, the more emphasis has been given on general nature of developing good gyrotrons. Keywords: Vauum tubes, nulear fusion, Cylotron Resonane Heating, phase bunhing, plasma heating. I. INTRODUCTION The mirowave vauum tube is apable to generate very high eletromagneti power at a very high frequeny range. In ase of suh type of devie, due to the high mobility of eletrons, size is not a restrition at a high frequeny and thus vauum devie an operate up to very high frequenies with very good power handling apability [1, ]. In all kinds of the mirowave vauum tubes, the radiations are generated by the highly aelerated eletrons. To generate the radiation, the eletrons are gathered in the miro-bunhes and this phenomenon is alled bunhing ourring in all kinds of the mirowave tubes in various forms. The researh on the gyrotron mirowave tube was initiated by the demand of high power, high frequeny eletromagneti wave soure in the magnetially onfined plasma fusion appliation. Sine the initial phase of gyrotron development, new thrust areas have been explored by the several researh groups. At present, there is a strong interest in developing powerful soures of the terahertz radiation for numerous sientifi and tehnologial appliations as well as spetrosopy, plasma diagnostis, ommuniation, mediine et. The gyrotron development is presently going mainly in the development of quasi-optial megawatt gyrotron in the frequeny range of GHz for the plasma fusion researh, espeially for the eletron ylotron heating and urrent drive in the tokamak systems [3]. The seond diretion is the development of medium power gyrotrons in the frequeny range of 4-84 GHz for industrial and heating appliations [4]. The third diretion is the development of sub-terahertz and terahertz gyrotrons for the spetrosopi and medial appliations (f e 0.3 THz) [5]. One more diretion is important to be mentioned as the appliations of gyrotrons in the seurity and atmospheri siene. The researh in the field of gyrotron development was initiated by the plasma fusion ommunity due to the requirement of high power RF soures for the Eletron Cylotron Resonane Heating (ECRH). Now the gyrotron is a signature devie in the plasma fusion due to its remarkable apability of generating MW s of output power in the millimeter wave band. Due to the global energy risis and environmental degradation, lean, effiient and sustainable soures of energy are highly required.

2 MIT International Journal of Eletronis and Communiation Engineering, Vol. 4, No. 1, January 014, pp ISSN () MIT Publiations The storage of the petrohemials will be depleted in 50 to 80 years and the use of oal is very problemati for the environment. The nulear fission reators are dangerous due to the nulear wastage and radioative leakage. The other non-onventional energy generation methods like solar ells or wind mills are not still in the ondition to fulfill the global energy requirement ompletely in near future due to the ost and prodution rate. The nulear fusion is the main soure of energy in the universe (like in Sun). Researh on the ontrolled nulear fusion is going on sine several deades and at present the plasma physis ommunity is in the ondition to build a pratial nulear fusion reator. To start the nulear fusion, two things that is heating of fusion fuel upto the temperature of billions of degree entigrade and onfinement of generated plasma, are must. Tokamak is a mahine to onfine the hot plasma magnetially [6] and gyrotron is used as a high power RF soure in the heating (ECRH) of plasma upto the temperature required for nulear fusion reation level. Figure 1 shows the gyrotron assembly, transmission line, launher, and tokamak system for ITER. The gyrotron frequeny is deided on the basis of ECRH frequeny ourring in the eletrons of the plasma in the tokamak hamber due to the onfinement magneti field. Other types of RF soures like klystrons, gridded tubes, et., are used in the Lower Hybrid Heating (LHH) and Ion Cylotron Resonane Heating (ICRH). High effiieny, high output power and long pulse width are the key requirements for the development of fusion gyrotrons. Considerable efforts are being made to improve the effiieny of gyrotron greater than 50% by using the single stage or multistage depressed olletor [7, 8]. Table 1 presents some gyrotrons developed speially for the industrial appliations. The major areas of appliations of THz gyrotrons are ESR and solid state NMR spetrosopy. People are also working on other potential areas of appliations of gyrotron as a THz soure like radioative material detetion, seurity, et. The bulky and heavy part of THz gyrotron is the superonduting magnet system whih also inludes ryostat. To redue the required magneti field, mostly the THz gyrotrons are operated at higher harmoni (mostly at seond harmoni). II. OPERATIONAL PRINCIPLE The gyrotron is the soure of millimeter as well as THz radiation. The devie fills the gap between lasers and onventional mirowave tubes as a radiation soure (Figure ). This devie ontains the advantages of both lasers and onventional tubes. The energy transfer mehanism is similar to the onventional tubes (multi-photon emission) and the size of interation struture does not depend on the radiation wavelength like lasers (fastwave operation) [10]. The large size of the interation struture makes the gyrotron more advantageous ompared to the onventional slow-wave mirowave tubes in ase of power handling apability at high frequenies. Gyrotron is a fast-wave devie (that is v ph>, where v ph is the RF phase veloity and is the veloity of light in free spae) and in priniple the fast-wave an propagate in free spae, while in ase of onventional tubes, a slow-wave struture is required to slow down the RF (also alled slow-wave devies, v ph < ). Aording to the saling law, the transverse dimension of the interation struture, required to slow down the RF, depends inversely on the radiation frequeny (f), whih makes the interation struture impratial at high frequenies due to very, very small size. In ase of gyrotron, the slow-wave struture is not required and a simple ylindrial waveguide type of interation struture an beused for the RF propagation. The radius of the ylindrial avity (R ) ' is given by R χ λ π, where m, n 0 χ ' is the n th root of derivative of m th order, first kind Bessel funtion. The proper operating mode seletion enhanes the value of (the value of inreases for high order modes), whih again inreases the interation avity radius.figure 3 shows a omparison between the different interation strutures at 30 GHz frequeny. Fig. 1. Gyrotron assembly with transmission lines in ITER Tokamak system [9]. Table 1. Gyrotrons developed speifially for industrial appliations Manufaturer IAP Varian (CPI) Mitsubishi Frequeny (GHz) Power (kw) Effiieny (%) Voltage (kv) Beam urrent (A) Cavity mode TE 0 TE 0 TE 0 Magnet power (kw) Fig.. Comparison of average power among onventional tubes, lasers and gyrotrons

3 MIT International Journal of Eletronis and Communiation Engineering, Vol. 4, No. 1, January 014, pp ISSN () MIT Publiations beam-wave interation, a simple ylindrial waveguide type of resonator struture is used (due to the radiation emission mehanism), whih is designed to support a partiular TE m,n,q mode, where q is the axial index of TE mode. The designed ylindrial waveguide type of resonator struture is tapered on both sides, down tapered at input side and up tapered at output side (Figure 4b). The designed interation resonator struture (simply alled interation struture or avity) operates (or resonates) very near to the ut-off to support the refletions at input taper side. The resonant frequeny for the interation struture is given as [10]: Fig. 3. Comparison of interation struture sizes for helix TWT, oupled avity TWT and gyrotron. The power transfer mehanism takes plae in the gyrotron interation avity by the azimuthal relativisti phase bunhing. An eletron beam onfined by the strong external stati magneti field travels in ylotron motion through a ylindrial waveguide. The ylindrial waveguide is overed by the input taper (dereasing radius) on the first end and the output taper (inreasing radius) on the seond end. This kind of struture is alled open ylindrial resonator avity. The input taper behaves like a ut-off region while the output taper behaves like a semi ut-off region. The ylindrial waveguide (also alled middle setion) is designed to support a partiular mode and some noise signals (also alled natural frequenies) sustaining inside the ylindrial waveguide in the form of a partiular mode. This partiular mode makes a standing wave, like a Gaussian profile, in the middle setion due to the omplete refletion at the input taper end and partial refletion at the output taper end. The eletrons emitted from the irular one type of dispenser athode under the temperature limited region, aquire the gyrating motion due to the ross eletri and magneti fields. This whole arrangement is alled Magnetron Injetion Gun (MIG) and the nomenlature omes from the type of eletron motion on the surfae of athode like the eletron motion in a magnetron osillator [10]. The emitted eletrons are ompressed under the influene of Gaussian type of stati magneti field and form a hollowannular eletron beam. The gyration type of motion in the eletrons is formed due to the ross eletri and magneti fields, while the hollow beam formation takes plae due to the ring type of athode. The hollow eletron beam is launhed at a partiular maximum of the TE mode in the interation avity for maximum interation effiieny. III. INTERACTION MECHANISM The RF radiation generated in the gyrotron by the interation between the gyrating eletron beam and the osillating RF eletri field during the resonane between the gyration motion (ylotron motion) of eletrons and the RF osillations is generally, alled beam-wave interation. Figs. 4 & 5 explain the bunhing proess and the energy transfer mehanism. For the f r π χ R + qπ L wherel is the length of interation avity middle setion. For the beam-wave interation at interation struture, a resonane ondition between the gyrating motion of eletrons (a kind of osillation) and RF osillation must be fulfilled. If Ù is the angular frequeny of eletrons gyrating motion (ylotron motion), s is the harmoni and ù is the angular frequeny of RF osillation, then the resonane ondition is given as: (1) ω sω. () In ase of gyrotron the eletron beam moves under the influene of very high potential and thus the relativisti effet plays a ruial role in the beam-wave interation mehanism. Due to the very high energy of eletron beams (and thus very high veloity) a kind of frequeny shift ours in the RF due to the relativisti Doppler effet. In ase of gyrotron the Doppler frequeny shift or simply Doppler shift is given as [11, 1]: ω kv sω (3) z z wherek z is the axial wave number of RF osillation and v z is the axial veloity of gyrating eletrons. In ase of gyrotron, the standing wave is formed at the middle setion oftheinteration struture as disussed earlier and thus k z «k, where k is the transverse wave number. Similarly, in ase of gyrating eletrons v z is always kept below the v (transverse veloity of eletrons) and thus the Doppler shift term (k z v z ) beomes very small and eq n (3) an be simplified as eq n (). From this disussion it is lear that the angular frequeny of the eletrons gyration motion always is smaller than the RF angular frequeny and this effet is very important in the effetive beam-wave interation. The resonane phenomena between the RF and gyrating eletron beam an be understood in better way by the dispersion diagram. The dispersion relation for a ylindrial waveguide is given as: where k χ R ω k k and z + (4) k z π L. By using eqsn (3) and (4),

4 MIT International Journal of Eletronis and Communiation Engineering, Vol. 4, No. 1, January 014, pp ISSN () MIT Publiations the dispersion diagram for the gyrotron interation struture an be represented as shown in Figure 6. The straight lines in the Figure 6 show the eletron ylotron harmoni modes (for s 1,, 3 and 4) and the paraboli urve indiates the waveguide mode. The mathing point of the eletron ylotron harmoni mode and waveguide mode is the resonane point between the eletron beam and the RF. It an be seen from Figure 6 that the resonane point is very lose to the ut-off of the ylindrial interation struture. To generate the ylotron motion in the eletrons, a very high order magneti field is required. By using eq n (), the relation for the required magneti field at avity enter (B 0 ) is obtained as follows. The ylotron motion of an eletron in the presene of magneti field B o is given as: γ Ω 1 v 1 Be 0 m γ (5) 0 V 1+ ( b kv ) 511 (6) wherem 0 is the rest mass of eletron, γ is the relativisti fator, e is the harge on eletron, v is the veloity of eletrons and V b is the eletron beam voltage. Using eqs n () and (5), one an write: () Fig. 4. (a) Eletrons in random phase in helially moving (b) Beam interation struture, () eletrons in bunhed phase in helially moving beam. Fig. 5. CRM instability (Phase Bunhing) ω s Be 0 0 m γ (7) B 0 m0 f π γ (8) se where ω πf. Simplifying eq n (7) by putting the values of m 0 and e, one an write: f( GHz) γ B0 ( T) 8s The eletron beam enters into the interation avity with ylotron motion at a partiular radial position, whih is deided by the peak of eletri field intensity of a partiular mode (also alled operating mode) in radial diretion. The eletrons are distributed randomly on their larmor paths as shown in Figs. 4 & 5 before entering into the avity. Here, the interation mehanism an be disussed under two onditions in the following paragraphs. (a) Side view (b) (9) Fig. 6. Dispersion diagram for gyrotron interation for different harmonis. Case 1ω Ω : Consider a beamlet from the gyrating eletron beam (e.g., beamlet 3 as shown in Fig. 1.9a). The beamlet enters into the interation avity and half of eletrons (red olor in Fig. 5) aquire aelerations and the rest half (green olor) feel the retardations in the presene of eletri field. The relativisti fator γ inreases for red eletrons and dereases for green eletrons and thus by eq n (5), the eletron ylotron frequeny dereases for eah red eletron and inreases for eah green eletron. Thus after several rotations the eletrons get bunhed together (Figs. 4 and 5). In this mehanism half of the eletrons give the energy to RF in the form of radiation, while half gain the energy from RF and thus the net energy transfer to the RF is zero. Case ω>ω : If the eletron ylotron frequeny is slightly less than the RF frequeny then the bunhed eletrons always move in retarding phase on their larmor paths. The bunhed eletrons in retarding phases ontinuously emit the oherent radiations whih amplifies the RF signal in the interation avity. The eletron ylotron frequeny diretly depends on the stati magneti field at the avity enter (B 0 ) and thus the applied magneti field is always kept below the alulated value from

5 MIT International Journal of Eletronis and Communiation Engineering, Vol. 4, No. 1, January 014, pp ISSN () MIT Publiations eq n (9). This phenomenon is alled detuning in the gyrotron ommunity and essential for the eletron bunhing in retarding phase. The relativisti fator plays a major role in the bunhing and the energy transfer mehanism and thus this bunhing proess is also alled relativisti phase bunhing. IV. CONCLUSION At present gyrotron is a mature devie as a radiation soure in a very wide frequeny range and theory, design and development of the devie has been established up to a level. But still suffiient researh is remaining in the improvement of the existing gyrotrons and also in the new kind of gyrotrons like multi-beam gyrotron, frequeny tunable gyrotron, oaxial gyrotron, large orbit gyrotron et. Pulse width enhanement, improvement in depressed olletor, performane of the quasi optial mode launher,stability in the output power and frequeny, high harmoni operation with high effiieny et. are the key issues for the improvement of gyrotron performane. REFERENCES [1] M. Thumm, Progress in gyrotron development. Fusion Eng.Design 66 68, (003). [] Mikhail Yu. Glyavin et.al.,thz, Gyrotrons, Status and Possible Optimizations, Terahertz Siene and Tehnology, Vol. 5, No., June 01. [3] S. Mitsudo, H. Hoshizuki, T. Idehara, T. Saito, Development ofmaterial proessing system by using a 300 GHz CW gyrotron. IOP. J. Phys. Confer. Ser. 51, (006). [4] Y. Makino, T. Ueno, T. Matsumoto, S. Miyake, Fabriation of bulk eramis by high power millimeter wave radiation. J. Appl. Phys. 40, (001). [5] T. Kikunaga, H. Asano, Y. Yasojima, F. Sato, T. Tsukamoto, A 8 GHz gyrotron with permanent magnet system. Int. J. Elet.79, (1995). [6] H. Asano, T. Kikunaga, K. Hemmi, F. Sato, T. Tsukamoto, A8 GHz gyrotron with a permanent magnet system for industry appliations. Proeedings of the 1st International Conferene Infrared and Millimeter Waves, Paper AM5, July [7] V.S. Bajaj, et.al., 50 GHz CW gyrotron osillator for dynaminulear polarization in biologial solid state NMR. J. Magn. Resonan. 189, (007). [8] T. Tatsukawa, T. Maeda, H. Sasai, T. Idehara, M. Mekata, T. Saito, T. Kanemaki, ESR spetrometer with a wide frequenyrange using a gyrotron as a radiation power soure. Int. J. Infrared Millimeter Waves 16, (1995). [9] N. Kumar, M.K. Alaria, U. Singh, A. Bera, T.P. Singh A.K.Sinha. Design of beam tunnel for 4 GHz, 00 kw gyrotron, J. Infrared Millimeter Terahertz Wave 31, (010). [10] V.L. Granatstein, B. Levush, B.G. Danly, R.K. Parker, A quarter entury of gyrotron researh and development, IEEE Tr. Plasma Si. 5, (1997). [11] M. Thumm, State of the Art of High Power Gyro-Devies and Free Eletron Masers Update 004 (FZK, KIT, Germany, 004). [1] U. Singh, A. Bera, R.R. Rao, A.K. Sinha, Synthesized parametersof MIG for 00 kw, 4 GHz gyrotron, Int. J. Inf. Millimeter Terahertz Wave 31, (010).