The source of THz radiation based on dielectric waveguide excited by sequence of electron bunches

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1 Journal of Physics: Conference Series PAPER OPEN ACCESS The source of THz radiation based on dielectric waveguide excited by sequence of electron bunches To cite this article: A M Altark and A D Kanareykin 26 J. Phys.: Conf. Ser Related content - THz radiation under non-central propagation of electrons through periodical channel A A Ponoarenko, K V Lekotsev, A A Tishchenko et al. - Heterostructure terahertz devices Victor Ryzhii - Coherent control of the THz radiation in an inhoogenous plasa channel Xiao-Bo Zhang, Xin Qiao, Li-Hong Cheng et al. View the article online for updates and enhanceents. This content was downloaded fro IP address on 07/04/28 at 07:32

2 RREPS25 Journal of Physics: Conference Series 732 (26) 27 doi: / /732/1/27 The source of THz radiation based on dielectric waveguide excited by sequence of electron bunches A M Altark 1 and A D Kanareykin 1,2 1 St. Petersburg Electrotechnical University, St. Petersburg, 5 Prof. Popov St., , St. Petersburg, Russia 2 Euclid Techlabs LLC 400 Professional Dr. Ste. 220 Gaithersburg, MD 20879, USA E-ail: aaltark@ail.ru Abstract. We present a new ethod for excitation of THz Cherenkov radiation in a dielectric waveguide by relativistic electron bunches. A sequence of bunches generates onochroatic radiation. The frequency of radiation is defined by the distance between the bunches. The studies were carried by using the newly updated BBU-3000 code which perits taking into account a nuber of additional options: an external quadrupole focusing syste, group velocity of the wakefield, and the dielectric aterial loss factor. In this paper, we present our algorith for optiizing the nuber and sequential positions of bunches for generation of narrow band high power THz radiation. 1. Introduction Many applications benefit fro the iaging capabilities of high power THz radiation. Laser driven THz eitters, solid state oscillators (high frequency diodes), and QCL (quantu cascade lasers) are currently considered as viable THz sources [1,2]. At the sae tie, a high power narrow bandwidth THz source can also be ipleented using coherent Cherenkov radiation (CCR) generated by an electron bunch train passing through an all-etal or dielectric slow wave structure. In [3] observation of coherent Cerenkov radiation in the terahertz regie eitted by a relativistic electron pulse train passing through a dielectric lined cylindrical waveguide was presented. With the proposed technique, selective excitation of odes beyond the fundaental were deonstrated by the use of the appropriate bunch spacing [3]. Use of bunch train with adjustable spacing to selectively excite high order odes in a ultiode structure was deonstrated in [4]. Narrowband THz generation by a bunch train in a ultiode dielectric loaded wakefield structure was studied experientally. The structure paraeters (geoetry and perittivity) provided a wide spacing between the odes in the frequency doain and a nuber of different frequencies were individually excited by a tunable bunch train [4]. In this paper we consider THz radiation based on a dielectric lined waveguide (DLW). THz radiation is produced by sequence of relativistic electron bunches passing along the DLW, figure 1. This work was initiated by the experiental studies presented in [4]. The bunch train was produced following three ajor steps: energy odulation of long bunch by self-wakefield generation, energy odulation conversion into spatial odulation by using a chicane technique, and finally THz radiation by the electron bunch train passing through the DLW. This schee allows tuning the bunch separation in the train providing coherent THz generation and ode selectivity as well. The spectru of THz Content fro this work ay be used under the ters of the Creative Coons Attribution 3.0 licence. Any further distribution of this work ust aintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by Ltd 1

3 RREPS25 Journal of Physics: Conference Series 732 (26) 27 doi: / /732/1/27 radiation is defined by both the DLW paraeters and bunch separation of the train. In this paper, we present our results of nuerical odelling of the THz radiation generation by the train consisting of 100 µ long electron bunches taking in account the group velocity effects and loss factor at the propagation distance of the wave envelope in the DLW. We also considered the selectivity of the first three TM odes generated by the bunch train of up to 10 bunches. Special attention has been paid to the generation of the TM ode in the 0.8 THz frequency range. Monochroaticity and field aplitude of the first three THz odes have been studied as well. 2. Proble forulation We studied the wakefield created by Gaussian bunch train by using the Green function for DLW, figure 1. All bunches in train are ultrarelativistic so the phase velocities of each ode are close to speed of light Vz c, 1. If the radial offsets of the bunches are equal to zero, we can take into account only the TM-odes with a high value of the longitudinal coponent of electric field and no transverse deflecting force for relativistic electrons. The charge distribution in bunch train is described by the equation: l 2 N Q n p exp 2 2z n 1 2 z where Q-charge of single bunch, is the rs length of a single bunch, z position behind the first bunch, l n is the position of the n th bunch in the train. z V t is the axial z Figure 1. The bunch train passing along the axis of cylindrical dielectric waveguide. We take into account the attenuation coefficient and group velocity for each TM ode. The Green function is defined by equation: kz if L vg c 0 if 0 L1 vg c cos exp 1 GEz E, 0 where E, kz Vz,, vg -aplitude, wave nuber, attenuation coefficient and group velocity of TM 0 -ode, L - way passed by bunch train in waveguide, Vz depends on losses in waveguide according to: 1 1 1, diel wall, 2Q v Q Q Q w g w c. The attenuation coefficient diel wall where Q, Q quality factors caused by losses for TM 0 -ode in dielectric volue and etallic wall respectively. The group velocities significantly liit the length of wave pocket: 2

4 RREPS25 Journal of Physics: Conference Series 732 (26) 27 doi: / /732/1/27 v g d dk z where P z U - power flow through cross section, energy stored per length of waveguide for TM 0 - ode. The wakefield (resulted THz radiation) is calculated by a convolution of the Green function with the charge density of the bunch train: z P U z, Ez. E G s p s ds 3. Calculation of THz radiation We perfored a calculation for a DLW with paraeters fro table 1 in the updated BBU-3000 code [5,6] with a ultibunch odule. BBU-3000 is used for calculation of wakefield and bea dynaics for different geoetries of DLW based on the analytic for of the Green function, figure 2. Figure 2. Capabilities of BBU-3000 code. Table 1. Paraeters of waveguide and bunch Paraeter Value Inner dielectric radius (u) 600 Outer dielectric radius (u) 850 Length (c) 4 Epsilon 3.8 Loss tangent of dielectric 0.0 Conductivity of wall (S/) Charge (nc) 1 σ z (u) 100 For the THz band we chose a dielectric waveguide with a sall aperture and thin dielectric layer (250 u), because the frequency intervals in the discrete spectru of Green functions grow as the thickness of the dielectric layer is reduced, so that it is possible to adjust frequencies over a large band. The paraeters of the TM odes are presented in table 2. The ain purpose of using a bunch train is the selection of only one ode with the desired frequency without exciting any of the other odes. The ost interesting case is the possibility to generate radiation consisting only of the TM ode. 3

5 RREPS25 Journal of Physics: Conference Series 732 (26) 27 doi: / /732/1/27 Table 2. Properties of TM odes Mode frequency, GHz Vg/c Attenuation coefficient (1/c) TM TM TM Bunch train spectru Green function spectru (a) TM - ode TM - ode TM - ode TM - ode (b) Bunch train (c) result radiation radiation is not fored Figure 3. THz radiation based on TM ode: (a) Aplitude Spectru distribution of Green-function and bunch train (a) and for result radiation (b), Wakefield created by bunch train (c), distances between bunches are equal c. The radiation is characterized by four ain paraeters: frequency, aplitude, onochroaticity and nuber of periods. Monochroaticity is defined as the ratio of the aplitude of the selected ode to the su of the aplitudes of all radiated odes. In figure 3 the calculated results are shown for radiation with the frequency of TM ode. Fro figure 3 (a) we can see that aplitude spectru distributions of the bunch train and the Green function are covered in frequency by this ode. Aplitude spectru distribution of result radiation contain principallytm ode, figure 3 (b). It is possible only for a unique distance between bunches: c. As we can see fro figure 3 (c) the nuber of wavelengths of result radiation is liited by influence of the group velocities. The high 4

6 RREPS25 Journal of Physics: Conference Series 732 (26) 27 doi: / /732/1/27 value of group velocity eans to the decrease of wave pocket for each ode. Therefore, the length of the result radiation is defined by the axial value of the group velocity (0.616c for TM ode). The figure 4 show dependence of radiation paraeters of TM odes as a function of nuber of bunches in train. It is shown that there is possibility to generate onochroatic radiations based on TM and TM odes by adjusting distances between the bunches. The aplitude of wakefield depend on nuber of bunches only for TM ode, figure 4 (a). Monochroaticity, in particular, does not depend on bunch nuber in train for TM and TM odes, figure 4 (b). It is possible to excite TM odes with different distances between the bunches, but the excitation of TM ode is possible only with train consisting 5, 7 and 8 bunches with single value of distance between bunches c. Aplitude, MV/ TM, f =149 GHz TM, f = 439 GHz TM, f =768 GHz (a) Nuber of bunches Figure 4. Calculations results for TM odes Nuber of bunches 4. Conclusion In conclusion, we have nuerically odeled narrowband THz generation by a bunch train in a ultiode dielectric loaded structure. We showed that for a bunch train for 8 bunches passing through the selected dielectric lined waveguide the length of the wave envelope at 768 GHz does not exceed 1 c for the bunch separation of 765 µ. This liitation is defined by the group velocity value for that particular ode. It was also deonstrated that the THz radiation onochroaticity and field aplitude are reduced for higher frequencies. The radiation onochroaticity is in the range of 0.55 caused by the weak lower TM and TM ode generation, figure 4 (b). Monochroaticity (b) TM, f =149 GHz TM, f = 439 GHz TM, f =768 GHz Acknowledgents. for Basic Research. This work was supported by grant fro the Russian Foundation References [1] Gallerano G P, Biedron S 2004 Proc. of FEL 216 [2] Lang M, Deninge A 22 Photonik Int 36 [3] Andonian G, Willias O, Wei X et al 21 Applied Physics Letters [4] Antipov S, Jing C, Baryshev S et al 25 Proc. of IPAC 1929 [5] Schoessow P, Jing C, Kanareykin et al 21 Proc. of PAC WEP111 [6] Kanareykin A, Altark A 24 Journal of Physics Conference Series