Recent Accelerator Activities

• Short Bunch
• Single Bunch
• Coupling Control
• 1.5 GeV Operation
• Top-Up

Short Bunch – this mode is available by request for special shifts

- coherent synchrotron radiation first observed January 23, 2007

Electrons traveling in a curved trajectory radiate photons over a broad spectrum, the total power radiated is proportional to the number of electrons. If the bunch length of the stored electrons is on the same order as the radiation, coherent synchrotron radiation (CSR) is created with a large increase in the power emitted – proportional to the square of the number of electrons. Because CSR occurs primarily at very long wavelengths, where the frequency is around 1 THz, the infrared beamlines stand to benefit from this mode of operation.

CSR can be created during two different modes. The first mode is a multi-bucket, very short bunch mode where the bunch length is on the order of a few ps. The second mode takes advantage of an instability to cause multi-bunching within a single bunch. This mode produces shotgun bursts of CSR and it the mode used at the CLS.

Bunch lengths are measured using the streak camera on the XSR facility diagnostic beamline. The bunch length in the CLS for stored beam during normal operations is approximately 38 ps. This is ~ 1 cm.

To decrease the bunch length, the three families of quadrupoles are changed to decrease the dispersion from 0.15 m to less than -0.5 m in the straights. Tables of the quad values are determined from the DIMAD model. As the dispersion decreases, the momentum compaction decreases from 0.0038 to about -0.00050, crossing through 0.0 at dispersions of approx. -0.525. The horizontal and vertical tunes are held constant.

Short bunch experiments were initially performed during June and July of 2006 with 20 mA current spread over 210 buckets. Though no CSR was observed, the bunch length was reduced to 6 ps.

Streak camera image of 8-ps bunches (1.5% coupling)

The single-bunch mode was tested in January 2007. With a stored current of 12 mA, bunch length of about 20 ps (bunch lengthening due to potential wall distortion) the Far IR beamline saw 4-orders of magnitude power increase!

Note βy/βx ≈ 35 at the XSR cf. ≈ 0.3 in the ID straights. Since beam dimensions are proportional to β½ the vertical size is hugely amplified at the XSR where the beam images are produced. The opposite effect happens horizontally with the 1- σ beamsize in the straights close to 0.5 mm.

The vertical emittance is further reduced by increasing the vertical tune by a small amount (4.28 -> 4.32). The effective coupling with the larger tune decreased (June 2007) from the best-corrected value of 0.1% to 0.065%.  

Local Coupling: On March 5th 2007, a setup was achieved where the beamsize was held constant at the XSR but reduced around straights 7, 8, 9 and 10. The decrease in beamsize was inferred from a decrease in the stored beam lifetime.

DIMAD simulations indicate that is should be possible to increase the beamsize in some ID straights while decreasing it in others by varying the coupling and vertical dispersion around the ring. This arrangement could provide the maximum brightness to a few beamlines while keeping the overall lifetime large. A local coupling correction scheme was first tried on Jan 24, 2006  by holding the amount of coupling constant at the XSR and increasing it around the ring. At 175 mA the lifetime was increased from 9.9 hrs to 10.5 hrs while the coupling at the XSR was held at 0.31%. Measurements of the vertical dispersion clearly show the reduced vertical shifts near the XSR.

Vertical dispersion plots for no correction, a global correction to 0.3% and a local correction.  Note the scale between no correction and the global correction.

This is a promising early result, and more elaborate schemes will be tried in upcoming machine studies. Local corrections should be easier when all 36 SQs are active sometime in the future. Decreased (increased) beamsize can be confirmed by the SM beamline.

 

1.5 GeV Operation - Achieved November 7, 2005

Operating the storage ring at electron energies at or below 1.9 GeV had been requested by the PGM beamline group for beamline commissioning. Since an early design goal of the CLS was to run the storage ring at energies as low as 1.5 GeV it was decided to setup the machine for 1.5 GeV operations. At this operating energy optical photons (so-called green-beam) can be produced by the PGM undulator. This was useful for alignment and commissioning.

Green-beam at the PGM

There are pros and cons to low (electron) energy operation. Advantages include lower emittance (  ~ 5 nm-rad) and larger possible beam currents, both leading to increased brightness. However, in this mode the maximum x-ray energy is lower, so it is most beneficial to soft and medium x-ray beamlines. Lower electron energy and higher current densities will result in shorter beam lifetimes. As well, beam instabilities are more destructive at lower operating energies.

Early attempts to ramp down (manually) the beam from 2.9 GeV to lower energies resulted in a total loss of beam current around 2.5 GeV. This ramp down technique should work under computer control but will not be attempted in the near future. Instead, 1.5 GeV settings for the booster, BTS transfer line and storage ring are saved in a machine setup file for easy re-installation.

1.5 GeV setup in about 1.5 hours: 1.5 GeV operation of the booster (demonstrated several years ago) can be achieved within a few minutes. Setting up the BTS line required about 15 minutes to establish the beam at the injection point of the storage ring. This was accomplished by scaling the excitation currents of all the magnets (dipoles, quadrupoles and steering magnets) to the new energy. Some readjustment of the dipole magnets was required to properly align the beam in the horizontal direction. A small adjustment to a single vertical steering magnet was required in the vertical direction. The storage ring magnets and injection kickers were also scaled to the new beam energy. Through "tweaking" the injection septum, the storage ring dipole magnets and the quadrupole families, beam was injected and stored in less than one hour. After beam was stored, orbit correction was applied and the appropriate betatron tunes established. Beam size, as observed in the OSR, was significantly reduced as expected for the small beam emittance. Stored current up to 500 mA is achievable.

1.5 GeV operations is now available for future applications as required by the User community.

 

Top-Up Mode

Initial investigations into a “top-up” mode of operation were carried out during the evening of November 28/05. In this mode the stored beam current is maintained at a near constant level using frequent injection. Ultimately, photon shutters are left open, ID gaps are left closed (as they are) and photon beams are continuously delivered to Users.

Preliminary investigations will address safety issues concerning injection with the shutters open. One concern is the possibility that injected electrons will somehow be transported down a photon beamline. This could only occur if an upstream dipole magnet was shorted out. To avoid this most improbable condition top-up injection will only occur with beam stored in the ring. Stored beam will indicate that all dipoles are working correctly. More probable causes for concern are from radiation shower produced by injected electrons hitting the aperture of the small gap vacuum chambers (i.e., straight 11) and from stray injected electrons producing bremsstrahlung that can be transported into any of the front end enclosures of the experimental floor. Detailed radiation measurements will be taken under a variety of injection conditions to evaluate the radiation levels in the FOEs and on the experimental floor. This will include measurements with the ID gaps open and closed and possible collimating of the injected beam using the electron beam scrapers in the injection straight.

Another concern is possible damage to the permanent magnets in the undulators due to radiation created by the injected beam. During normal injection the undulator gaps are open and radiation damage is minimized. With the ID gaps closed the scrapers can be used to limit the electron beam loses to the injection area. Eventually collimators may be installed in the injection line to reduce the emittance of the injected beam, increase the injection efficiency and reduce radiation from stray electrons.

The advantages of top-up injection include improvement to the X-ray beam stability through a constant heat load on the X-ray optics and the reduction of current dependent systematics of the storage ring beam diagnostics (resulting in improved beam quality). Details of the injection frequency and top-up injection software will be worked out in future runs. The injection frequency will depend on the beam lifetime and the desired variation in stored beam currents (e.g. 0.1%, 1%, etc.). During injection and until the beam is sufficiently damped the stored beam will undergo an effective emittance blowup and orbit oscillations. Timing signals will be made available to Users in order to gate out data acquisition during the 10s of ms when the beam quality is unsuitable. Gating data acquisition may not be suitable for some Users and the use of top-up will have to be determined by the User community.

June 25/07 Update. Work on top-up mode continues: The delay was due to difficulty with the injection system. For injection four kicker magnets and two septum magnets (on one trigger) need to be fired. During a normal injection, these magnets can take up to a minute to warm-up. However, the kickers cannot be left on continuously as they perturb the stored beam. It turns out that the kickers do not need a warm-up time, so the solution is to separate the triggers to the kickers and septum. This way, the septum can be left on continuously.
*** Update: the triggers have been separated and are working as expected. The septa are left on continuously, the kickers are turned on for injection.

Sept 14/07 Update. It was shown early on that leaving the septum magnets on was disturbing the stored beam. The injection triggers were recombined and it was instead found that a small adjustment to the septum magnet power supply would enable beam to be injected with the cold magnets. We have shown that the beam can be held at 100 +/-0.5 mA with single injections approximately every 30 seconds. These tests were done with the undulator gaps wide open. The next step is to resume radiation measurements while injecting with the gaps closed.

Updated March 31, 2009

Last modified: 2009-04-01 12:04:45