Picosecond to Nanosecond Measurements at High Repetition Rate

Since FY2012, SSRL is now scheduling three to four three-day periods each year dedicated to running SPEAR3 in hybrid low-alpha operation.  In this mode the SPEAR3 ring has 1-4 camshaft pulses with very low current, and pulse duration of 5-20 picoseconds, for timing measurements.  The rest of the buckets are filled to provide 100-200 mA current for other users not involved in timing experiments.  The last low-alpha operation period in FY2013 is scheduled for June 14-16, and high repetition rate (HRR) measurements will be conducted at Beam Line 10-2 and Beam Line 13-3 during that period.

Admittedly this is a somewhat disruptive mode for the synchrotron to run in. Since that's the case, why would SSRL find it advantageous to spend valuable beam time in low-alpha operation?

Many phenomena that are the subjects of scientifically interesting and technologically important investigations today, from understanding the mechanisms of charge separation in photosynthetic systems to the interaction of the electronic, atomic and magnetic lattices in multiferroics, from demagnetization of magnetic memory bits to martensitic phase transition, take place in less than a nanosecond.  Response of a material to external stimulus (properties and behaviors) is intimately tied to the interaction between its electronic and atomic structure (or molecular arrangement).  Often a small distortion of one of the lattices (electronic, atomic or spin) profoundly influences the other, resulting in dramatic change in properties and the emergence of novel functionality. Larger distortions result in phase transitions and chemical changes. Understanding this synergy addresses many of DOE’s grand challenges: obtaining a deeper understanding of chemical catalysis, correlated electron systems and emergent behavior and the potential to tailor material properties by design. These phenomena have been studied extensively with optical pump – optical probe measurements for more than a decade.  But x-ray techniques are unsurpassed in probing the electronic and molecular structure of materials, and ever since the birth of x-ray free electron lasers (e.g., FLASH and LCLS) investigation of dynamical changes in materials at sub-nanosecond scales is growing quickly.

Very intense x-ray pulses at FELs often, however, significantly distort (and sometimes destroy) the samples.  They act as strong pumps as well as probes, making isolation of the response of the material to the original pumps difficult.  In order to mitigate this problem, pump-probe experiments at FELs tend to use very short pulses.  But the response of many interesting systems, including photosystem II and domain switching in ferroelectrics, is a hundred to several tens of thousand times slower than the length of an FEL pulse.  A weakly perturbing probe, with longer pulses but with much higher repetition frequency, complements pump-probe experiments at LCLS and other FELs very well.  Thus several synchrotrons are developing HRR pump-probe programs and beam lines, including the crab cavity program in the APS upgrade.

Low-Alpha Operation:  Electrons lose energy when they produce synchrotron radiation; this energy is replaced by radio frequency (RF) accelerating cavities.  In SPEAR3, the electrons lose about 1/3000 of their energy each turn (~1 MeV out of 3 GeV).  The individual electrons in a storage ring bunch oscillate in a longitudinal position around the center of the bunch and oscillate in energy about the nominal electron energy.  The energy and longitudinal oscillations are coupled through the slope of the RF accelerating voltage, VRF’=dVRF/dt, at the bunch position, and by the optics parameter, α, also known as the momentum compaction factor. 

This leads to two possibilities for decreasing the equilibrium bunch lengths – decrease α or increase VRFVRF can be increased by increasing the RF voltage, the RF frequency, or both.  The SPEAR3 accelerator group is looking into a hardware upgrade which would increase these parameters.  But in accelerator physics studies in 2006 it was discovered that existing magnets in the upgraded SPEAR3 ring can be tweaked to lower α, while keeping the higher components of α still small, without additional magnets or hardware.  (Low alpha program at ALS will require installation of additional magnets and hardware.)

As α is reduced and the bunch gets shorter, the peak current increases.  The high peak current leads the bunch to interact with its own synchrotron radiation, generating coherent synchrotron radiation (CSR) in a process similar to that in an FEL.  At first this interaction distorts the shape and lengthens the bunches, ultimately making them unstable.  The short bunches in low-alpha operation come at a cost of lower current and degraded horizontal emittance. 

By running SPEAR3 in low-alpha mode, we are able to explore the science with short pulses for a broad spectrum of applications at time structures that complement the LCLS.

Apurva Mehta and James Safranek

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