Detailed Design Report

Transcription

1 Detailed Design Report Chapter 4 MAX IV Injector 4.6. Acceleration MAX IV Facility

2 CHAPTER 4.6. ACCELERATION 1(10) 4.6. Acceleration 4.6. Acceleration RF Units Accelerator Units RF Distribution and Synchronisation Waveguide System Temperature Stabilisation... 9

3 CHAPTER 4.6. ACCELERATION 2(10) 4.6. Acceleration The MAX IV linear accelerator will be constructed in a very modular way. 17 stations are foreseen, each with a high power RF unit and a number of linear accelerator structures. In addition, an 18th high power RF unit will provide RF power to the electron sources and the first Linac section. All 18 RF units are to be constructed in an identical way. Figure 4.6.1: Block diagram showing the major components of the MAX IV linac RF system RF Units The MAX IV linear accelerator will be divided into 18 RF stations. Each station consists of an S-band klystron with modulator and electronics for control and interlock. The klystron generates pulsed RF power from high voltage power delivered by the modulator. The modulator converts AC line power into high voltage pulse power. Its main components are a high voltage power supply, a high voltage pulse unit and a pulse transformer. An interlock system protects the RF unit and the linac in case of failure. To achieve the MAX IV energy stability the RF accelerating fields must be stable. Pulse to pulse random variations can not be corrected by feedback and therefore gives the upper limits on the phase and amplitude noise levels of individual components such as klystrons and modulators. The jitter tolerance specifies the pulse-to-pulse variations that are acceptable in the linac phase and amplitude parameters. For the Thales klystron TH2100, as an example, the phase jitter is 4 for 1 % variation of klystron voltage (280 kv).

4 CHAPTER 4.6. ACCELERATION 3(10) Figur.4.6.2: Schematic layout of RF Unit. RF unit Symbol ΔI/I 0 < % ΔE/E 0 < 0.1% Unit mean rf phase rf unit 0 ϕ 0 S-band deg mean rf phase rf unit1 ϕ 1 S-band deg mean rf phase rf unit 2 ϕ x S-band deg mean rf phase rf unit 3-9 ϕ 2 S-band deg mean rf phase rf unit ϕ 3 S-band deg mean rf voltage rf unit 0 V0/V0 % mean rf voltage rf unit 1 V1/V1 % mean rf voltage rf unit 3 Vx/Vx % mean rf voltage rf unit 3-9 V2/V2 % mean rf voltage rf unit V3/V3 % Tabell 4.6.1: The voltage and phase tolerances per rf unit for unit 3-9 and are 8, larger assuming random errors. All tolerances are rms values.

5 CHAPTER 4.6. ACCELERATION 4(10) Solid state switching technology will be used for the klystron high voltage pulse unit to achieve the desired demands on stability and low operational cost. The pulse shape must be as rectangular as possible. The raise and fall time should be as short as possible to maximize the total efficiency. The pulse to pulse stability must be better than ±0.01 %. An output RF power of 35 to 37 MW with a pulse length of 4,5 μs and a repetition rate of 100 Hz is needed. All klystrons can be operated in saturation, with no low level RF amplitude control, and having global phase control. Total klystron power margin is 1-2 RF units to allow for klystron failure and maintenance. A comprehensive interlock system is required for the reliable and safe operation of the RF system. Internal protection interlocks must be provided for each RF unit that fully ensures safe operation of the equipment. External interlock connection(s) as part of the protection chain of the RF unit must be provided for use in the MAX-lab installation. External interlock(s) will operate at the level of switching off completely the pulsed high voltage from the RF unit, and could be used for example, from a remote emergency stop button or from a klystron waveguide arc detector. The control system for the RF unit will include an API for the complete control and status including interlock signals of the equipment and a TANGO ( device server/servers for GNU/Linux or Windows XP. The source code for the TANGO device serves will be included and will have all necessary commands and attributes for the complete remote control of the RF unit. The control interface and protection of each RF unit will be made using the Ethernet protocol having minimum 10ms response time. There will be a local control and display panel showing operational data and the interlock status of a RF unit under all operating conditions. The RF unit shall be designed for compact size, but still keeping a reasonable level of serviceability. One side of the unit will be placed close to a tunnel wall, so all parts needing service must be accessible from the other three sides. Compact RF units with easy access of replaceable units are preferred. An oil tray must be included that fits below the high voltage pulse transformer tank and has a minimum volume 105 % of the tank oil volume so that all oil will be collected in case of a leak Accelerator Units The MAX IV linear accelerator will be constructed in a modular way. The 18 RF stations are foreseen to feed 35 linear structures and two RF guns. See the fig.xx., each consisting of a high power RF unit followed by a SLED unit, Power divider and two Linac sections. The 35 Linac sections will serve as a full energy injector with electron beam energy in excess of 3 GeV. 17 of all 18 Accelerating units are to be constructed in an identical way. The first one will have some differences.

6 CHAPTER 4.6. ACCELERATION 5(10) Figure 4.6.3: Schematic layout of one accelerator unit. The linac sections are 5.2 m long travelling wave S-band structures, operating at GHz. They are constant gradient structures with an on-axis load. Therefore the last six of their 154 cells are coated with an absorbing material and add little to the total accelerating voltage. The fill-time of the structures is 740 ns. The reason to choose 5.3 m long accelerating sections as compared to the well know 3 m structure, is to reduced number of couplers, input windows or valves and a simpler power distribution system. Basic parameters Length 5.2 m Shunt Impedance 51.5 M/m Attenuation db Frequency /-.03 GHz Mode 2 / 3 Typ constant gradient

7 CHAPTER 4.6. ACCELERATION 6(10) SLED system will be used to provide the necessary accelerating gradient. SLED has the advantage that it will not increase the average power consumed by the accelerator. A relatively long output power pulse of 4,5 µs from each klystron is feeding a SLED system that deliver a short high power RF pulse to two linac structures. Each SLED system consists of two cylindrical cavities in the high Q TE 015 mode. The theoretical value of Q 0 for copper cavities is , and values around have been achieved in practice. Basic parameters Operating Frequency ± 0.5 MHz Cavity Resonant Mode TE 015 Measured Unloaded Q ± 5000 Coupling Value (β) 5 Input power (peak) Input power (average) Cooling 40 MW max 36 kw (PRF 200 Hz, 4.5 µs) Water circuit Operating Temperature 40 ± 2 C Input VSWR 1.10:1 with matched load on output port The RF power delivered from the SLED cavities to the linac structures is seen in Fig The RF power is normalised to the klystron power. 8 6 Et (, 0. ) t Figure 4.6.4: RF output power delivered to the linac structures normalised to the klystron power for the parameters above.

8 CHAPTER 4.6. ACCELERATION 7(10) RF Distribution and Synchronisation The beam energy spread is related to RF phase and amplitude variations. Cooling temperature, air condition and modulator high voltage jitter changes the beam energy. The long term beam energy drift and phase drift will be reduced by using accurate cooling and air conditioning control system. Short term jitter will be reduced by putting high demands on the modulators. The injector RF distributions system should be designed for maximal phase stability relative to the bunches in the linac. The average position of the beam near crest of the accelerating voltage must be kept stable to a fraction of an S-band wavelength or about 1 (rms). This is achieved by couple out RF power from the first klystron before the SLED cavities which is distributed along an nitrogen-filled rigid coaxial transmission line mechanically attached to the linac supports at each coupling points. About 300W RF power are coupled out at the positions of each klystron for driving the klystrons. A remote controlled phase shifter is needed at each klystron to adjust the RF phase. When the linac tunnel changes length due to temperature, both the linac and the transmission line changes the same length. At each coupling point there is an expansion joint that absorbs the length changes. This design is based on the fact that the RF wave propagates with almost the same speed (0.25 % difference) in an nitrogen-filled coaxial transmission line as the electron bunches in the linac. When the linac tunnel changes length the RF phase will follow the bunches. Changes in the coaxial line s dielectric constant are the primary cause of phase length variation. Using an approximate equation from SLAC for their 3 km long linac scaled to a 300 m long line the change in phase length relative to the beam is given by: d T θ ( θ ) = 2.5dT ( C) dp ( Torr) ε ε where θt is the total equivalent phase length at S-band in degrees of electrical phase, ε is the relative dielectric constant (teflon supports plus nitrogen gas) T ε is the temperature of the dielectric and p ε is the N 2 gas pressure inside the coaxial line. To achieve the demanded phase stability a temperature stability of <±0,13 C and pressure stability of >±0,5 Torr are needed. In order to minimize losses due to skin depth we want to use a coaxial line with a diameter as large as possible without supporting the TE 11 mode (a higher-order mode will increase loss and VSWR and has a different propagation velocity than the TEM mode). Only the a + b TEM wave can propagate when the wavelengths is longer than λ C = π ( ) where a is 2 the inner and b the outer radius of the line. If we use the EIA 1 5/8 (a=16,87 and b=38,79 mm) standard coaxial rigid line, the cut off wavelength is 0,087 m (3,43 GHz) which is lower than 0,01m (3 GHz), only the TEM mode will propagate. The maximum power that could be distributed along the line is 2,35 MW. With 20 linac units of 12,6 meter length and 545 W coupled out from the line at each klystron position a

9 CHAPTER 4.6. ACCELERATION 8(10) power of 20 kw is needed from the first klystron. Assuming a 10 m long low loss coaxial cables (Aircom plus ) with an attenuation of 2,6 db there will be 300 W available at each klystron. At each klystron input there are remote controlled variable phase shifters and attenuators. 3 GHz VCO oscillator phase locked to laser High power phase shifter Main Drive Line High power phase shifter Akroma 9 sections 9 sections Figure 4: Blockdiagram of the linac RF system showing the main drive line that supplies the klystrons with drive power.

10 CHAPTER 4.6. ACCELERATION 9(10) RF unit Coupling factor [db] RF unit Coupling factor [db] Table 4.1: The coupling factors for the couplers placed along the main drive line is shown in this table. The master oscillator is the heart of the linac which determines its performance. It should be a RF source with long term frequency stability and low phase noise. Femtosecond lasers push the limits of the phase noise spectrum Waveguide System Because of the very high microwave power used in the linac feeding system the waveguides must be placed under vacuum (<10-8 bar). The high power waveguide network must be adjusted to be equal in phase length, or to differ by only integral numbers of wavelengths Temperature Stabilisation As a general design goal energy stability should be achieved by putting high demands on temperature stability on critical RF components and using klystron modulators with high pulse to pulse voltage stability. To meet the different demands on temperature stability the water temperature control in the linac tunnels will consist of two regulation systems, a

11 CHAPTER 4.6. ACCELERATION 10(10) klystron gallery secondary system and a linac station fine cooling system. The RF system including accelerating structures and SLED cavities require a stability of ±0.05 C at a water temperature of C. This stability is achieved using a closed-loop water system at each station that provides constant water temperature to the SLED, accelerating structures, waveguides and loads. The klystron gallery secondary system supplies 25 C temperature regulated water with a stability of ±0.5 C to components in the klystron gallery such as klystron collector, - body, and focusing coils and other RF devices that needs cooling. Also cooling water for the magnet power supplies and beam focusing magnets are supplied from this system. Reservoir Reservoir Reservoir Reservoir Linac RF Linac RF Linac RF Linac RF Pump Pump Pump Pump Heater Heater Heater Heater Temp. Temp. Temp. Temp. Klystron Components, magnets and magnet power supplies in four Temp. Reservoir Max IV linac primary cooling system Figure 1: Block diagram showing one water cooling block consisting of four linac stations. Each acceleration station, consisting of SLED, accelerating structures, waveguides and loads, has its own fine cooling system and the other devices have a common temperature regulation system. To avoid that fast temperature changes on the primary water supply is affecting the fine cooling temperature stability reservoir tanks have to be added in the cooling system, see figure 1.