Beam position

Page maintainer: Sergey

This page is considered done. It been reviewed by Alexander. There may be missing elements, but they are all flagged and the text has no errors.

Intro

The HERA electron (positron) beam goes through the gaseous HERMES target. Beam line is assumed to be straight inside the target cell, except for the polarized 2002-2005 running, when a strong transverse target holding field was present. Typical variation of beam XY-offsets for a given year of data taking is a (very) few hundred microns, depending on the orbit tuning. It is continuously measured by two sets of Beam Position Monitors (BPMs) installed around the beam pipe upstream and downstream of HERMES.

Essentials

Beam position for the "transverse" target data (2002-2005)

In the presence of a strong transverse target holding field beam is deflected by ~1.3mrad and displaced by ~150 $\mu$ m in the horizontal plane along it's way through the ~40cm long target cell, so the beam line looks like an arc with an apex around +11cm in the HERMES coordinate system. There is also a ~1.6mm difference in beam X-offset between target magnet ON and OFF runs. In the offline data productions the (average) beam offset is taken into account by the TMC code (see here for the shifts observed in earlier productions). The (varying) slope can also be simulated in the Monte-Carlo.

Beam position for the whole HERA II Run (2002-2007)

All modern HRC productions of the HERA II Run are reprocessed by HTC code, which gives a more generic approach to the beam line offset accounting problem. This approach is suitable for any target field configuration ever used at HERMES. Besides, all the modern productions use identical HERMES coordinate system for this whole period. This coordinate system is coupled to the HERMES dipole spectrometer magnet yoke. Year-average beam offsets in this coordinate system are presented in the following table, they are assumed to be accurate to ~100 $\mu$ m:

HRC/HTC	$\mu$ DST	Target Magnet	X-offset	Y-offset
02d.0/1		ON	-100 $\mu$ m	+300 $\mu$ m
-"-		OFF	+1500 $\mu$ m	-"-
03d.0/1		ON	0 $\mu$ m	+1700 $\mu$ m
-"-		OFF	+1600 $\mu$ m	-"-
04d.0/1	04d1	ON	-400 $\mu$ m	+1500 $\mu$ m
-"-	-"-	OFF	+1200 $\mu$ m	-"-
05d.0/1	05d1	ON	+2800 $\mu$ m	-"-
-"-	-"-	OFF	+1200 $\mu$ m	-"-
06e.0/1 (e-)		ON	+350 $\mu$ m	+300 $\mu$ m
-"-		OFF	-"-	-"-
06e.0/1 (e+)		ON	+750 $\mu$ m	+500 $\mu$ m
-"-		OFF	-"-	-"-
07c.3/6	07c2	ON	-"-	+600 $\mu$ m
-"-	-"-	OFF	-"-	-"-

Since the same coordinate system is used in all cases, relative beam shifts between different years are meaningful and reflect small displacements of the HERMES platform after moving in and out of service position, as well as a different beam orbit tuning.

It should also be noted that anticipated relative shift of top vs bottom HERMES detector halves in this coordinate system does not exceed ~200 $\mu$ m in both X- and Y-directions.

Both X- and Y-slopes of the beam line are for now assumed to be 0 (for the transverse running beam slopes at the apex are set to 0). This must be a good enough approximation (anticipated gaussian error ~200 $\mu$ rad in both X- and Y-directions). Survey measurements conducted in 2004-2007 confirm that HERMES spectrometer magnet was originally very well aligned with respect to the HERA machine plane (to better than 100 $\mu$ rad) and the quadrupoles in the beam line. On the other hand presently used HERMES coordinate system based on these measurements is aligned with the spectrometer magnet yoke with accuracy ~50 $\mu$ rad.

HTC calculates track-to-beam vertex for every track, taking into account year-average beam line offsets as well as beam curvature in case of transverse running. Beam line in the vertex finding procedure is (conservatively) smeared by 200 $\mu$ m in both X- and Y-directions in order to take into account typical orbit variation during a given year of data taking, as well as a finite instant beam profile size. Vertex position as well as track parameters and also beam offsets and slopes at the expected vertex are present in the output of all modern $\mu$ DST productions.

Use of BPM measurements in the Recoil tracking

Beam position is used in the Recoil detector track fitting. The beam position calculated using dedicated Recoil beam finder in the Recoil coordinate system is compared with the beam position determined with the use of BPMs. From this comparison connection between the Recoil coordinate system and the BPM coordinate system is determined and information from BPMs which is available for each run is used in data productions.

More info

Elke's mail on calculation of beam position from BPM measurement 2007/12/18

Paper: Design of a Local IP Orbit Feedback at HERA-E

Other beam related pages

Code Repository