The electron emission control method for a RF gun is considered. According to the main idea of the method, the additional resonance system is created in a cathode region where the RF field strength could be varied using the external pulse equipment. The additional resonance system is composed of a coaxial cavity coupled with a RF gun cylindrical cavity via an axial hole. Computed results of radiofrequency and electrodynamic performances of such a two-cavity system and results of the RF gun model pilot study are presented in. Results of particle dynamics simulation are described

Khodak, I.V.

Kushnir, V.A.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

METHOD OF ELECTRON EMISSION CONTROL IN RF GUNSI.V. Khodak, V.A. KushnirNational Science Center 'Kharkov Institute of Physics & Technology'1, Akademicheskaya St., NSC KIPT, 61108 Kharkov, Ukrainee-mail: kushnir@kipt.kharkov.uaThe electron emission control method for a RF gun is considered. According to the main idea of the method, the ad-ditional resonance system is created in a cathode region where the RF field strength could be varied using the exter-nal pulse equipment. The additional resonance system is composed of a coaxial cavity coupled with a RF gun cylin-drical cavity via an axial hole. Computed results of radiofrequency and electrodynamic performances of such a two-cavity system and results of the RF gun model pilot study are presented in. Results of particle dynamics simulation are described.PACS numbers: 29.25.Bx, 41.75.Lx1  INTRODUCTIONIt is known that radiofrequency electron sources (RF guns)  are  efficient  tools  to  obtain  bunched  electron beams of a high intensity and high brightness. The emis-sion current duration at the RF gun output is determined by the RF power pulse duration. The problem of emis-sion control is appeared in cases when a current pulse has to be differed considerably from the gun RF power pulse. This problem is solved, for instance, in laser driv-en RF guns [1]. The current pulse duration in these guns is in synchronism with the laser pulse duration and can be both of picosecond duration range [2] and nanosec-ond duration range [3]. There is also one more type of RF  guns  where  the  current  pulse  duration  is  in  a nanosecond range - RF gun with metal-dielectric cath-ode [4].At the same time, the problem of emission control is not solved for wide-used thermionic RF guns. The cath-ode is fixed directly in the RF field of a high strength in these guns. The current pulse duration at the output of such guns is actually in synchronism with the RF power pulse. Due to the transient that takes place, application of such a RF gun in a single-section electron linac with one RF source sometimes causes the degradation of out-put beam parameters.In the paper the method of emission control in RF guns and the way of its implementation are proposed. In the course of the problem realization, the on-axis elec-tric field was calculated numerically for the gun reso-nance model, gun operating modes were determined us-ing computer simulation. Frequency and time character-istics of the gun resonance model were researched ex-perimentally.2  PROBLEM CONCEPTBeam parameters at the RF gun output including the particle capture factor are determined by on-axis electric RF field  distribution [5].  The core  of  the method we have proposed consists in the fast (comparing with RF pulse duration) varying of electric field distribution in the  cathode  region where  the  additional  coaxial  reso-nance system is created for this purpose. Changing the fundamental  frequency  of  the  coaxial  cavity  with  the use  of  an  external  pulsed  equipment  one  can  change electric field distribution in the gun cavity on the whole and in particular, in the cathode region. Having tuned the  coaxial  cavity  on  the  fundamental  resonance  fre-quency of the cylindric cavity f0, we receive two oscilla-tion modes that can exist in the system. In case of fun-damental  studies  frequencies  of  cavities  are  differed considerably, the coaxial cavity presence has no influ-ence on the frequency and electric field distribution in the main cylindric cavity. Each of these three states is featured by their own electric field distribution.It is supposed to design the coaxial cavity tuning us-ing p-i-n diode based RF switchers that vary length of the coaxial cavity. The given concept was put in basis for the calculation, computer simulation and experimen-tal research of the gun resonance model.3  CALCULATION AND NUMERICAL SIM-ULATIONWe  investigated  the  S  band  two-cavity  resonance model (Fig. 1) where the coaxial cavity is directly cou-pled with a gun cylindric E010 cavity via a cathode-side axial hole and can be tuned on the resonance frequency of the cylindric cavity f0. The cylindric cavity length in the model Zcav was chosen closer to real RF guns. An axial conductor of the coaxial cavity can be used as a cathode placeholder.Fig. 1. Resonance model geometry.It is known [6] that for the fulfilling of the frequency equality, the coaxial cavity length  l should be equal to ≈ nλ0/4,  where n = 1,3,5,… .  The  value  l = 3λ0/4 was chosen taking into account real conditions of the experi-   ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №3.Серия: Ядерно-физические исследования (38), с. 68-70.68ment.  Besides,  fundamental  resonance  frequencies  for higher  n values  will  be  closer  to  each  other  that  can cause  their  undesirable  intersection  under  real  design conditions.Calculation of on-axis electric field distribution held using a SUPERFISH code [7] has shown that the reso-nance  model  frequency  is  lower  of  f0 ≅ 2.8 GHz  for l < 3λ0/4 and  electric  field  distribution corresponds  to 'zero' oscillation mode (Fig. 2, curve '0'). The resonance model frequency is higher of f0 for l > 3λ0/4 and electric field distribution corresponds to the 'pi' oscillation mode (Fig. 2, curve 'pi'). The increasing of the field absolute value in the cathode plane (position Z=0 in Fig. 2) fea-tures both oscillation modes. According to the calcula-tion, the RF power in both cases is dissipated consider-ably on the inner and outer cylindric walls of the coaxial cavity that decreases the quality factor of the whole sys-tem. The quality factor value is decreased in 4 times in both cases.00.10.20.30.40.50.60.70.80.910 5 10 15 20 25 30 35 40 45Z , m mE z/Ezmax'off-tune ''0 ''pi 'Fig. 2. Calculated electric field distributions.Values of l that differ considerably from n3λ0/4 cor-respond to the total coaxial cavity detuning relatively to f0.  Let  us call  this as 'off-tune'  state.  In this case,  the electric field distribution in the system corresponds to the case of electric field distribution in a simple single-cavity RF gun without any additional  cavities (Fig. 2, ''off-tune' curve).It should be noted that the frequency difference be-tween  '0' and 'pi' oscillation modes is small (∼ 9 MHz) that corresponds to the coaxial cavity length variation in few tenth of millimeter. This fact should be taken into account when designing the coaxial cavity tuning sys-tem.To define possible RF gun operating modes particle beam dynamics in the model was simulated for each of obtained  electric  field  distributions  using  PARMELA code [8]. It was supposed that particles are emitted dur-ing the whole half-period of an accelerating field. The model geometry was changed a little to determine cor-rect simulation results. The output hole of 25 mm in di-ameter, which does not modify the electric field distri-bution in the cathode region, was added in the cylindric cavity on the wall that is opposite from the coaxial cavi-ty. Simulation results are summarized in Table 1.Simulation shows the absence of accelerating parti-cles at the exit of such a resonance system when per-forming the 'pi' oscillation mode. Particle beam parame-ters in 'off-tune' mode are incident to a single-cavity RF gun. Comparing results obtained for 'zero' and 'off-tune' states one can show that the particle beam in '0' oscilla-tion mode has a more narrow energy spread and emit-tance value of one order less then in the 'off-tune' state.Table 1'Off-tune' '0' − modeWmax, MeV 0.64 0.88εn, mm⋅mrad 3.8 0.17∆W/W, % (70% of particles) 51 32∆ϕ, °26 38N/Ne 0.7 1This is agreed with the electron dynamic calculation [5] for various axial electric field amplitudes and will per-mit also to obtain electron beams of higher brightness.Thus,  the  summary  that  can  be  made on  basis  of simulation results is as follows. The 'pi' oscillation mode switches the RF gun in the 'closed' state when a current is absent at the RF gun output. The preferred mode is the switching between '0' and 'pi' oscillation modes that corresponds to 'open' and 'closed' RF gun states respec-tively. Such operating mode can be recommended for wide-used thermionic RF guns.4  RF MEASUREMENSTo define the possibility of switching between oscil-lation  modes  is  feasible,  there  were  measured  ampli-tude-frequency and pulse performances of the resonance model.  All  measurements  were  held  according  to  the block diagram shown in Fig. 3.1 2 5 64783Fig. 3. Measuring layout (1-frequency meter, 2-RF generator, 3-pulser, 4- RF switcher, 5-automatic  system of resonance frequency measurement, 6-os-cilloscope, 7-cavity, 8-detector).The automatic system of resonator frequency mea-surement [9] was used in the measuring setup (Fig. 3, pos. 5). On-axis electric field distributions in cylindric cavity were measured for each of the three above- de-fined  cases  and  amplitude-frequency  response  of  the system. The electric field was measured using the reso-nant perturbation technique. Measured dependences are ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №3.Серия: Ядерно-физические исследования (34), с. 69-70.69shown in Fig. 4 and Fig. 5.The amplitude-frequency response was measured in the state when the coaxial cavity length was equal to 3λ/4 within accuracy of ±0.05 mm.Comparing measured electric field distributions with calculated ones one could note that these experimental curves have the same pattern as for calculated ones. A small spread in the field amplitude values in the cathode plane (position Z=0 in Fig. 5) explains the coaxial cavi-ty  length  increasing  up  to  9λ0/4 caused  by  the  RF switcher.00.10.20.30.40.50.60.70.80.912791 2796 2801 2806 2811 2816 2821f, M H zU/UmaxFig. 4. Amplitude-frequency response.0 .70 .750 .80 .850 .90 .9510 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5Z , mmE z/Ezmax'off-tune''0 ''pi 'Fig. 5. Measured electric field distributions.There was used RF switcher based on КА520 p-i-n diodes. Initially the system was tuned on 'pi' oscillation mode that corresponds to the 'closed' state of a gun. Po-sition of the p-i-n diode switcher was choosing so that state of the system was changed to 'off-tune' state after applying bias voltage. Then diode was fed by pulse volt-age. The obtained pulse from detector (Fig. 6) permitted to confirm experimentally the fact of oscillation mode control in the two-cavity resonance system.Fig. 6. Rise time of oscillations (t = 1µs/div).The rise time of oscillations was defined for such a two-cavity model. According to the pulse curve, the rise time is ∼ 1 µs that is confirmed by estimations made us-ing the equivalent circuit method. This points to the os-cillation rise time in the system are determined mainly by the high-quality factor value of the cylindric cavity and actually does not depend on the low-quality factor value of the coaxial cavity.It should be noted that the resonance system being in '0' or 'pi' oscillation mode has a low-quality factor value. This  fact  has  no  special  sense  for  the  'pi'  oscillation mode because the gun is in the 'closed' state. However, it will require that the gun should be fed by additional RF power in '0'  oscillation mode in order to obtain a qualitative electron beam at the gun output. Besides, the low-quality factor value causes the pass-band broaden-ing of the resonance system. This should be taken into account when defining the range of adjustment of p-i-n diode switchers.5  CONCLUSIONInvestigations  have  shown  that  switching  between electric field distributions in the RF gun can be made using  an  additional  electrically  tuned  coaxial  cavity. This  permits  to  obtain  few  RF-gun  operating  modes, one of which corresponds to its 'closed' state. Hence, the beam at RF gun output can be pulse modulated that cor-responds to diode- to- triode RF gun conversion.It is supposed to carry out the full-scale experiment with an electron beam using one of existing in our insti-tute RF guns.REFERENCES1. R.L.Sheffield. Photocathode RF Guns  // AIP Con-ference  Proceeding  184,  New  York,  1989,  v. 2, p. 1501-1531.2. C.Travier  //  Nuclear Instruments  and Methods in  Physics Research. 1994, v. A340, p. 26-39.3. N.I.Ayzatsky,  A.N.Dovbnya,  V.A.Kushnir  et.  al. Laser driven radiofrequency electron guns //  Proc.  of  XIII  Workshop on  Charged  Particle  Accelera-tors, Dubna, 1993, v. II, p. 111-115. (in Russian).4. N.I.Ayzatsky, E.Z.Biller, I.V.Khodak et. al. Metal-dielectric Cathode in Radiofrequency Electron Gun //  Technical Physics Letters.  1998, v. 24, No. 10, p. 762.5. V.A.Kushnir,  V.V.Mitrochenko.  Electron  Dynam-ics Investigation in RF Guns // Problems of Atomic  Science  and  Technology,  Issue:  Nuclear  Physics Research (29,30). 1997, № 2, 3. p. 96-98. (in Rus-sian).6. Jerome L. Altman.  Microwave  Circuits.  Moscow: «Mir», 1968, p. 298-303. (in Russian).7. J.H.Billen  and  L.M.Young.  POISSON  /SUPER-FISH on PC compatibles // Proc. 1993 Particle Ac-celerator  Conff.,  Washington  (USA), 1993, p. 790-792.8. L.M.Young.  PARMELA //  Los  Alamos  National  Laboratory, LA-UR-96-1835 (preprint), Los Alam-70os, 1996, 93 p.9. V.A.Kushnir,  V.V.Mitrochenko.  Method of  Auto-matic Locking a Resonant Frequency of Cavities //  Pribory  i  technika  experimenta. 1998,  №  2, p. 67-70. (in Russian).ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №3.Серия: Ядерно-физические исследования (34), с. 71-70.7172

Method of electron emission control in RF guns

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Method of electron emission control in RF guns

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Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)