Emittance diagnostics will involve two diagnostic beamlines to image the electron beam with infrared (IR) and ultraviolet (UV) synchrotron radiation (SR) from bending dipoles [1]. Placed in locations of different optical functions, the beamlines will provide experimental access to both horizontal and vertical emittance and to beam energy spread.

Bunch lengthening with harmonic cavities is essential for machine performance and time resolved measurements with synchrotron radiation for individual longitudinal bunch distributions present another capability of the diagnostic beamlines.

Diagnostic Beamline Layout

Layout diagnostic BL R3

The diagnostic beamline images SR from a bending magnet onto a CCD camera. See figure for a schematic beamline layout. The bending magnet is the first matching cell dipole in achromat 20 of the MAX IV 3 GeV storage ring. The electron beam path is indicated in red, the SR path is shown in blue. The distance from the center of the bending magnet to the planar SiC mirror is 1.85 m.

Vertical Beam Size

Visible and near-UV SR can be used to resolve vertical beam sizes at the few µm scale. This has been shown in a collaboration with the Swiss Light sources by imaging with pi-polarized SR [2] and with the obstacle diffractometer method [3], see figure below for a schematic.

Schematic diagnostic BLsImages of the electron beam with SR in the IR-vis-UV range are highly dominated by effects inherent to SR emission and diffraction. These effects are theoretically predictable and have been modelled in the Synchrotron Radiation Workshop (SRW) [4,5]. In this way both the horizontal and vertical electron beam sizes are derived from imaged SR.

The two figures below show examples of vertical intensity profiles of imaged SR, measured and calculated, for a vertical beam size of 11.5 µm. In the right figure a diffraction obstacle causes a well-predictable diffraction pattern.

Pi polarizationSigma polarization

Horizontal Beam Size

Horizontal beam sizeDue to the low horizontal emittance of 328 pm rad the horizontal beam size is less than 30 µm at the beamline locations in the MAX IV 3 GeV ring. Such low beam sizes can be resolved by imaging SR with a wide horizontal opening angle and at wavelength in the near IR. The figure shows measured and calculated horizontal SR intensity profiles for a horizontal beam size of 24.5 µm. Measurements have indicated a slight beamline misalignment that is currently limiting horizontal beam size resolution.

Longitudinal

Longitudinal bunch profileMeasurements on the longitudinal bunch profile are important to verify bunch lengthening by harmonic cavities and to diagnose beam instabilities. Currently an optical sampling oscilloscope is used for multi-turn single-bunch diagnostics. The figure shows a measured longitudinal bunch profile with 55 ps rms bunch length taken at low electron beam current (< 1mA).

Ongoing activities

At present, one diagnostic beamline (in achromat 20) is under commissioning and minor mechanical alignment remains to achieve the targeted horizontal beam size resolution. Furthermore, refinements of the theoretical model will be developed to account for varying electron beam optic functions through the bending magnet.

The other diagnostic beamline in the 3 GeV storage ring (in achromat 2) is delayed due to a mechanical problem with one vacuum chamber. A longer shutdown periode is required to continue the installation.

Two diagnostic beamlines in the MAX IV 1.5 GeV storage ring are currently being prepared for installation and will go into commissioning along with the storage ring after the 2016 summer shutdown.

 

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Last updated: September 22, 2016

 

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