LU13525/Wootton Observation of Picometer Vertical Emittance with a Vertical Undulator K.P. Wootton, 1, * M.J. Boland, 1, 2 R. Dowd, 2 Y.-R.E. Tan, 2 B.C.C. Cowie, 2 Y. Papaphilippou, 3 G.N. Taylor, 1 and R.P. Rassool 1 1 School of Physics, The University of Melbourne, Melbourne, VIC, 3010, Australia. 2 Australian Synchrotron, 800 Blackburn Rd, Clayton, VIC, 3168, Australia. 3 European Organization for Nuclear Research (CERN), BE Department, 1211 Geneva 23, Switzerland. (Dated: October 10, 2012) Using a vertical undulator, picometer vertical electron beam emittances have been observed at the Australian Synchrotron storage ring. An APPLE-II type undulator was phased to produce a horizontal magnetic field, which creates a synchrotron radiation field that is very sensitive to the vertical electron beam emittance. The measured ratios of undulator spectral peak heights are evaluated by fitting to simulations of the apparatus. With this apparatus immediately available at most existing electron and positron storage rings, we find this to be an appropriate and novel vertical emittance diagnostic. PACS numbers: 29.27.-a, 41.75.Ht, 41.60.-m, 41.85.Lc In recent years, storage ring light sources and damping rings have produced electron and positron beams of diminishing vertical size [1– 3]. Beams of unprecedented small size are de- manded particularly for electron-positron col- liders at both the energy [4] and intensity fron- tiers [5]. With vertical dimensions of several µm, direct measurement of beam size is ap- proaching diffraction limits of visible light and hard x-ray diagnostics [6]. We report on the de- velopment of a new vertical electron beam size measurement technique which utilizes a verti- cal undulator. Vertical undulators are rare [7] – typically beams are deflected in the horizon- tal plane. In this work, an elliptically polar- ized undulator was phased as a vertical undu- lator. We present direct observation of µm ver- tical beam sizes corresponding to picometer ra- dian (pm rad) vertical beam emittances, and re- mark that this technique is immediately achiev- able using existing photon beamlines of electron storage ring light sources. The spectral and angular profile of undula- tor radiation is especially sensitive to the trans- verse emittance [8], defined as an envelope in position-angle phase space of the electron beam ensemble. Projections of the angular profile of undulator harmonics can be used to character- ize the beam emittance. This has been demon- strated using a soft x-ray undulator producing a vertical field [9], referred to as a horizontal undulator because the beam is deflected in that transverse direction. Previous analytical descriptions and model- ing of undulator brilliance focused on determi- nation of the absolute photon beam brilliance [8], and identified the brilliance of even harmon- ics as especially sensitive to the transverse emit- tance [10], using a horizontal undulator. The absolute photon beam brilliance is a difficult quantity to measure. A novel technique has been previously proposed, measuring the ratio of intensities of the first and second undulator harmonics and comparing with simulations of photon brilliance [11, 12]. We present measure- ments of the stored electron beam size by ex- panding this idea and taking the ratios of many adjacent pairs of odd and even harmonics. This technique shares similarities with an- other projection measurement of vertical emit- tance [1, 13] but differs by passing, as opposed to masking, the on-axis null radiation field. Il- lustrated in Fig. 1 are simulated transverse pro- files of the intensity of undulator radiation. In-