Magnetron: Magnetron 1 Cross Sectional Sketch of Coaxial Cavity Cylindrical Magnetron: Cross Sectional Sketch of Coaxial Cavity Cylindrical Magnetron 2 PowerPoint Presentation: 3 Cavity Magnetron Cavity Magnetrons: Cavity Magnetrons PH0101 Unit 2 Lecture 5 4 Figure 2 Major elements in the Magnetron oscillator PowerPoint Presentation: PH0101 Unit 2 Lecture 5 5 Cavity Magnetrons PowerPoint Presentation: PH0101 Unit 2 Lecture 5 6 Anode Assembly Construction: Construction Each cavity in the anode acts as an inductor and the interaction space acts as a capacitor. These two form a parallel resonant circuit and its resonant frequency depends on the value of L of the cavity and the C of the slot. The frequency of the microwaves generated by the magnetron oscillator depends on the frequency of the RF oscillations existing in the resonant cavities. PH0101 Unit 2 Lecture 5 7 PowerPoint Presentation: PH0101 Unit 2 Lecture 5 8 Forms of Cavity: 1.slot- type 2. vane- type 3. rising sun- type 4. hole-and-slot- type Magnetron Cavity Resonator: Magnetron Cavity Resonator 9 PowerPoint Presentation: PH0101 Unit 2 Lecture 5 10 Tuning A Magnetron: Tuning A Magnetron PH0101 Unit 2 Lecture 5 11 Basic Magnetron Operation : Basic Magnetron Operation Operation of Magnetron can be subdivided into four phases 1phase : Production and acceleration of an electron beam in a dc field 2phase : Velocity-modulation of the electron beam 3phase : Formation of electron bunches by velocity modulation (here in form of a “Space-Charge Wheel”) 4phase : Dispense energy to the ac field 12 PHASE 1: PHASE 1 Magnetron is a cross field device as the electric field between the anode and the cathode is radial whereas the magnetic field produced by a permanent magnet is axial. A high DC potential can be applied between the cathode and anode which produces the radial electric field. Depending on the relative strengths of the electric and magnetic fields, the electrons emitted from the cathode and moving towards the anode will traverse through the interaction space In the absence of magnetic field ( B = 0), the electron travel straight from the cathode to the anode due to the radial electric field force acting on it 13 Phase 2: Phase 2 If the magnetic field strength is increased slightly, due to lateral force the electron path bends. If the strength of the magnetic field is made sufficiently high then the electrons can be prevented from reaching the anode. The magnetic field required to return electrons back to the cathode just grazing the surface of the anode is called the critical magnetic field ( B c ) or the cut off magnetic field. If the magnetic field is larger than the critical field ( B > B c ), the electron experiences a greater rotational force and may return back to the cathode quite faster. 14 PowerPoint Presentation: 15 PowerPoint Presentation: 16 The electron path under the influence of different strength of the magnetic field: The electron path under the influence of different strength of the magnetic field 17 Figure : Electron trajectories in the presence of crossed electric and magnetic fields (a) no magnetic field (b) small magnetic field (c) Magnetic field = Bc (d) Excessive magnetic field: Figure : Electron trajectories in the presence of crossed electric and magnetic fields (a) no magnetic field (b) small magnetic field (c) Magnetic field = Bc (d) Excessive magnetic field 18 Phase 3 and 4: Phase 3 and 4 At any particular instant, one set of alternate poles goes positive and the remaining set of alternate poles goes negative due to the RF oscillations in the cavities. As the electron approaches the anode, one set of alternate poles accelerates the electrons and turns back the electrons quickly to the cathode and the other set alternate poles retard the electrons, thereby transferring the energy from electrons to the RF signal. This process results in the bunching of electrons, the mechanism by which electron bunches are formed and by which electrons are kept in synchronism with the RF field is called phase focussing effect . PH0101 Unit 2 Lecture 5 19 PowerPoint Presentation: 20 Fig (v) Bunching of electrons in multicavity magnetron: Fig (v) Bunching of electrons in multicavity magnetron 21 Cross Sectional Sketch of Coaxial Cavity Cylindrical Magnetron: Cross Sectional Sketch of Coaxial Cavity Cylindrical Magnetron 22 Performance Characteristics: Performance Characteristics Power output : In excess of 250 kW ( Pulsed Mode), 10 mW (UHF band), 2 mW (X band), 8 kW (at 95 GHz) Frequency : 500 MHz – 12 GHz Duty cycle : 0.1 % Efficienc y: 40 % - 70 % PH0101 Unit 2 Lecture 5 23 Applications of Magnetron: Applications of Magnetron Pulsed radar is the single most important application with large pulse powers. Voltage tunable magnetrons are used in sweep oscillators in telemetry and in missile applications. Fixed frequency, CW magnetrons are used for industrial heating and microwave ovens. PH0101 Unit 2 Lecture 5 24 PowerPoint Presentation: PH0101 Unit 2 Lecture 5 25 Have a nice day.