Linac

The linear accelerator (injector) for SOLARIS has the six S-band warm linear travelling wave structures (Linac sections) supplied by three RF power units (RFU). The first RF Unit (RFU 1) feeds the two linac sections, Linac 1a and Linac 1b. The second and third RF units, RFU 2 and RFU 3, feeds Linac 2a, Linac 2b and Linac 3a, Linac 3b (see Figure 1). The RF thermionic gun with a BaO cathode was chosen as an electron source. The SOLARIS injector operates at the electron beam energy of 550 MeV with option for a full energy (1.5 GeV) upgrade with supplementary RF stations and linac sections. The linear injector operates in ultra-high vacuum environment and initially with RF repetition rate of a few Hertz. The beam is transported to the storage ring through a transfer line. The total length of the SOLARIS injector is about 40 meters.

Figure 1: Schematic layout of Solaris injector

 

Table 1: The beam parameters at the Injector exit
Energy Gain (max.) [MeV]    550    
Charge of one electron bunch [nC] 0.2
Beam Current [mA] 600
Emittance (norm, rms) horizontal/vertical mm mRad 3.111/2.175
Energy Spread [keV] 400
Bunch length [ps] 14


ELECTRON GUN SYSTEM
 

The gun system for SOLARIS is depicted in Figure 2. The beam is transported from the gun through a chopper section to fit a time structure of 100 MHz buckets in the storage ring. The RF gun with the thermionic cathode suffers from having a long low-energy tail in the electron bunch and an energy filter (EF) was therefore designed. The beam is bent by two 60° bending magnets to achieve a point with dispersion where a slit can filter out the low-energy electrons. To adjust the dispersion a focusing quadrupole magnet is placed in the middle. The focusing is controlled by four additional quadrupole magnets. In total the energy filter is one DBA-cell. To focus the beam a solenoid (Sol) is placed straight after the gun and another one before EF. Diagnostic components (CT, YAG, FC) are inserted to have a control on beam parameters.


Figure 2: Gun System assembly

 

LINAC SECTIONS

 

The travelling wave accelerating structures work in S-band frequency range (2998.5 MHz). 2π/3 mode provides a good efficiency.  The single resonators (‘cups') are sized individually so that the structure has a constant field gradient. Each structure gives an energy gain of 100 MeV.

Figure 3: Linac sections for Solaris
 

Table 2: General specification of linac section
Operating Frequency [MHz] 2998.5   
Max. Input Power (peak) [MW] 100
Max. Input Pulse Duration [µs] 0.75
Max. Repetition Rate [Hz] 100
Max. Input VSWR 1.10
Q Factor 12500
Shunt Impedance (U2/P) [MΩ] 250
Accelerating Gradient [MV/m] 20
Energy Gain [MeV] 100
Operating Temperature [°C] 40 (±0.1)
Dimensions (length x width x height) [mm] 5276 x 188 x 389

 

RF POWER UNITS
 

RF power units for Solaris injector includes S-band 35 MW klystrons fed by high pulsed power solid state modulators based on the solid state switches operating in parallel. The klystron amplifiers provide 4.5µs RF output pulses which travel through the waveguide system into the linac sections. Over-coupled cavities (SLED – SLac Energy Doubler) are used for enhancing RF peak power at the expense of RF pulse width. The pulse width is compressed (4,5µs -> 0,75µs ) and output RF peak power is increased (35MW -> 200MW) without increasing the average input power consumption. Power divider (PD) (3dB hybrid coupler) divides the power from the SLED equally between the two linac sections. One RF power unit supplying the two linac sections forms, together with SLED cavities and waveguide system, the one accelerating unit of SOLARIS injector (Fig. 5).


Figure 4. RF power units for Solaris injector
 

Figure. 5: The Solaris injector includes three accelerating units with the same configuration
 

Table 3: The main operational parameters for the RF units

 
RF Frequency [MHz] 2998.5   
RF Peak Power from Klystron [MW] 35
RF Average Power from Klystron [kW] 16
Klystron Pulse Voltage (max.) [kV] 282
Klystron Pulse Current (max.) [A] 315
Pulse length (top) [µs] 4.5
PRF range [Hz] 0-100

 

Table 4. General Specification of SLED unit
Operating Frequency [MHz] 2998.5
Max. Input Power (peak) [MW] 35
Max. Input Pulse Duration [µs] 4.5
Max. Repetiton Rate[Hz] 100
Max. Output Power (peak)[Mw] 200
Output Pulse Duration [µs] 0.75
Max. Input VSWR 1.10
Q Factor 98000
Coupling Value b 6


FOCUSING, DIAGNOSTIC AND CORRECTOR SYSTEM
 

The intersections  between the linac sections are designed for quadrupole magnets and diagnostics. To obtain sufficient focusing the doublets of quadrupole magnets will be used in the first and the second intersections and after the last linac section. The diagnostic and corrector systems were designed. For beam position monitors stripline BPMs were chosen which are inserted in the intersections between Linac 1b and 2a, Linac 2a and Linac 2b and two BPMs between Linac 3b and the transfer line. The YAG screens as well as current transformers are used, too. There are also corrector magnets along the injector (see Figure 1).
 

TRANSFER LINE
 

The transfer line consists of two dipole magnets which bends the beam in vertical plane and six quadrupole magnets focusing the beam vertically and horizontally. The TL is ended by septum magnet which connects TL with the storage ring (see Figure 6).

 

Figure 6. Transfer Line assembly
 

More about linac in article Injector System for the Polish Synchrotron Radiation Facility SOLARIS.