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Physics 862 Accelerator System Introduction to RF and Microwave

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Physics 862 Accelerator System Introduction to RF and Microwave Physics 862 Accelerator System Introduction to RF and Microwave Alireza Nassiri Adjunct Professor of ECE Outline • Three lectures: • Lecture 1 – today o Introduction o Overview of RF power generation • Lecture 2 – Wednesday, November 7 o Power transport – part one • Lecture 3 – Wednesday, November 7 o Power transport – part two o Introduction to low-level rf and controls • Homework A. Nassiri PHY 862 Accelerator Systems 2 Electromagnetic Spectrum • All the electromagnetic waves travel with the same velocity (i.e. 3 X 108 m/s) in the free space with different frequencies. The arrangement of electromagnetic radiations according to wavelength or the frequency is referred as electromagnetic spectrum. • As we know electromagnetic spectrum has no definite upper or lower limit and various regions of EM spectrum do not have sharply defined boundaries. • The electromagnetic spectrum types, their frequency, wavelength, source and applications have been outlined in the table below. As mentioned EM waves include electric wave, radio wave, microwave, infrared, visible light,ultra violet,X-rays,gamma rays and cosmic rays. A. Nassiri PHY 862 Accelerator Systems 3 Electromagnetic Spectrum EM Wave Type Frequency (Hz) Wavelength (m) Source Applications Electric wave 50 to 60 5 x 106 to 6 x 106 Weak radiation from AC circuit Lighting Radio wave 3 x 104 to 3 x 109 1 x 10-1 to 104 Oscillating circuits Radio communication, TV Microwave 3 x108 to 3 x 1011 1 x 10-3 to 1 Oscillating current in special vacuum tubes, Radar, TV, Satellite communication, remote sensing. Gunn, IMPATT, Tunnel diodes Infrared 1 x 1013 to 4 x 1014 7.5 x 10-7 to 3 x 10-5 Excitation of atoms and molecules Gives information on the structure of molecules and of external atomic electron shells, remote sensing. Visible light 4 x 1014 to 8 x 1014 3.75 x 10-7 to 7.5 x 10-7 Excitation of atoms and vacuum spark Gives information on the structure of molecules and of external atomic electron shells, remote sensing Ultra violet 8 x 1014 to 1 x 1016 3 x 10-8 to 3.75 x 10-7 Excitation of atoms and vacuum spark Gives information on the structure of molecules and of external atomic electron shells, remote sensing Bombardment of high atomic number X-rays therapy, industrial radiography, medical radiography, X-ray 1 x 1016 to 3 x 1019 1 x 10-10 to 3 x 10-8 target by electrons crystallography Gamma Ray 3 x 1019 to 5 x 1020 6 x 10-15 to 1 x 10-10 Emitted by radioactive substances Gives information about the structure of atomic nuclei Stars from Cosmic Ray > 1020 < 10-11 other galaxies A. Nassiri PHY 862 Accelerator Systems 4 Introduction • RF transmitter for accelerating cavities Cavity tuning loop Amplitude loop Phase loop RF Distribution Control System Four Components 1. RF Source 2. Power supply Pre- Power Master Oscillator LLRF 3. Transport (WG system) amp. Amplifier 4. LLRF • Power Supply • Modulator Termination Load RF source Accelerating Cavity What is the Overall Efficiency? Later in the lecture. A. Nassiri PHY 862 Accelerator Systems 5 Glossary of Terms as relates to RF • Frequency range o Klystrons are dominant and used above 300 MHz o Other devices such as IOT, Diacrode, Tetrode and SSA below 300 MHz • Peak power o Is related to energy gain in an accelerator as well as the overall length of a given accelerator system. High peak power typically results in arcing within the accelerating structures. • Average power o Is defined as the product of peak power and DF in pulsed systems o For CW systems, the output power is equal to the average power. Define the amount of heat produced by the system • Gain o Defines by rf drive. Klystrons, in general, have a high gain ~50 dB ( i.e. less drive power). IOTs are low gain devices- ~20 dB ( i.e., more drive power). • Phase Stability o Klystron is a voltage driven device and the rf phase is stable if the voltage is stable. A. Nassiri PHY 862 Accelerator Systems 6 Glossary of Terms as relates to RF • Decibel (dB) o 푑퐵푚=10 퐿표푔10 ("PmW") o 푑퐵=10 퐿표푔10 (푃1/푃2) o 푑퐵=20 퐿표푔10 (푉1/푉2) o 푑퐵푉=20 퐿표푔10 ("VVrms") o 푑퐵µ푉=20 퐿표푔10 (푉µ푉푟푚푠) o 푑퐵푐=10 퐿표푔10 (푃푐푎푟푟푖푒푟/푃푠푖푔푛푎푙) • dBm, W (푥푑퐵푚/10) 푥푑퐵푚 = 10 퐿표푔10 PmW 푃푚푊 = 10 0 dBm = 1 mW 30 dBm = 1 W 60 dBm = 1 kW 90 dBm = 1 MW A. Nassiri PHY 862 Accelerator Systems 7 Glossary of Terms as relates to RF (푥 /10) 푥푑퐵=10 퐿표푔10 (푃/푃푟푒푓) ↔ 푃∕푃푟푒푓=10 푑퐵 A. Nassiri PHY 862 Accelerator Systems 8 RF Power Sources • Two principal classes of microwave vacuum devices are in common use today: o Linear-beam tubes o Crossed-field tubes Linear Beam Devices Klystrons Hybrid O-type TWT Multi- cavity Two - cavity Twystron Helix Helix Ring-bar BWO TWT Reflex Laddertron Coupled cavity TWT A. Nassiri PHY 862 Accelerator Systems 9 Linear Beam Devices • In a linear-beam tube, as the name implies, the electron beam and the circuit elements with which it interacts are arranged linearly. • In such a device, a voltage applied to an anode accelerates electrons drawn from a cathode, creating a beam of kinetic energy. • Power supply potential energy is converted to kinetic energy in the electron beam as it travels to- ward the microwave circuit. • A portion of this kinetic energy is transferred to micro-wave energy as RF waves slow down the electrons. The remaining beam energy is either dissipated as heat or returned to the power supply at the collector. • Because electrons will repel one another, there usually is an applied magnetic focusing field to maintain the beam during the interaction process. A. Nassiri PHY 862 Accelerator Systems 10 Cross-Field Devices • The magnetron is the pioneering device of the family of crossed-field tubes. • Although the physical appearance differs from that of linear-beam tubes, which are usually circular in format, the major difference is in the interaction physics that requires a magnetic field at right angles to the applied electric field. • Whereas the linear-beam tube sometimes requires a magnetic field to maintain the beam, the crossed-field tube always requires a magnetic focusing field. Crossed-field devices Distributed emission Injected beam tube MBWO Magnetron Crossed-field amplifier Carcinotron Voltage tunable Crossed field amplifier magnetron A. Nassiri PHY 862 Accelerator Systems 11 Commercial RF Sources Tetrodes & Diacrodes available from industry 10000 peak < 1 1000 ms 100 Power kW per single tube Power 10 0 100 200 300 400 500 Frequency MHz A. Nassiri PHY 862 Accelerator Systems 12 Amplifier Class Class A Class B Class C Operative curve Operative curve Operative curve Output Signal Output Signal Less than 180⁰ Unsused Output Unsused area Signal area Input Input Input Signal Signal Signal C Efficiency Amplifier Class Description AB B A 100% Class-A Full cycle 360⁰ of conduction 75% 50% Class-AB More than 180⁰ of conduction 25% 0% Class-B Half cycle 180⁰ of conduction A AB B C Class-C Less than 180⁰ of conduction 360⁰ 270⁰ 180⁰ 90⁰ 0⁰ 2π 3π/4 π Conduction Angle 0 A. Nassiri PHY 862 Accelerator Systems 13 Grid Vacuum Tubes • The physical construction of a vacuum tube causes the output power and available gain to decrease with increasing frequency. The principal limitations faced by grid-based devices include the following: o Physical size. Ideally, the RF voltages between electrodes should be uniform, but this condition cannot be realized unless the major electrode dimensions are significantly less than 1/4 wavelength at the operating frequency. This restriction presents no problems at VHF, but as the operating frequency increases into the microwave range, severe restrictions are placed on the physical size of individual tube elements. o Electron transit time. Inter electrode spacing, principally between the grid and the cathode, must be scaled inversely with frequency to avoid problems associated with electron transit time. Possible adverse conditions include: 1) excessive loading of the drive source, 2) reduction in power gain, 3) back-heating of the cathode as a result of electron bombardment, and 4) reduced conversion efficiency. o Voltage standoff. High-power tubes operate at high voltages. This presents significant problems for microwave vacuum tubes. For example, at 1 GHz the grid-cathode spacing must not exceed a few mils. This places restrictions on the operating voltages that may be applied to the individual elements. o Circulating currents. Substantial RF currents may develop as a result of the inherent inter electrode capacitances and stray inductances/capacitances of the device. Significant heating of the grid, connecting leads, and vacuum seals may result. o Heat dissipation. Because the elements of a microwave grid tube must be kept small, power dissipation is limited. A. Nassiri PHY 862 Accelerator Systems 14 Tetrode • Vacuum tube based on intensity modulation of a electron beam • Typical parameters: • Frequency: accelerator applications up 300 to 400MHz • Finite electron drift time limits the achievable gain at higher frequencies • Limiter gain of ~15 dB mean that thigh power tetrode amplifiers need 2-3 stage of amplification, which drives up the cost and results in complicated amplifier systems. Grounded Grid Grounded Cathode A. Nassiri PHY 862 Accelerator Systems 15 Planar Triode • The envelope is made of ceramic, with metal members penetrating the ceramic to provide for connection points. The metal members are shaped either as disks or as disks with cylindrical projections. • The cathode is typically oxide-coated and indirectly heated. The key design objective for a cathode is high emission density and long tube life. Low-temperature emitters are preferred because high cathode temperatures typically result in more evaporation and shorter life.
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