Quadrupole Magnets

The quadrupoles provide some of the focusing properties of the spectrometer and to a large extent its acceptance. Operating limits imposed on the quads are as follows: 1850A for Q2 and Q3 and 3250A for Q1.

All three quadrupoles for the HRS spectrometer are warm iron superconducting magnets. The soft iron around the superconducting coil enhances the field at the coil center and reduces stray fields. The basic parameters for the first quadrupole, Q1, are an effective length of 0.9 $m$, useful aperture of 0.3 $m$ and a field gradient of 9.5 T/m. To achieve the lowest possible angle setting of the HRS spectrometer (with respect to the beam line) the incident electron beam passes through a notch in the outer yoke of Q1 when the spectrometer is at its smallest angle of 12.5$^\circ$ . The other two quadrupoles Q2 and Q3, are essentially identical with an effective (magnetic) length of about 1.8 meter, a useful aperture of 0.6 $m$ and a field gradient of 3.5 T/m.

The maximum operating currents (assuming a 4 GeV/c momentum particle) for the quadrupoles are about 3000 A, 1700 A, and 1600 A, for Q1, Q2, and Q3, respectively. This will render pole field values of 1.2, 1.0, and 1.0 T, respectively. The energy stored in the quadrupole fields is sufficient to cause an unrecoverable quench if all the energy stored is dumped into the magnets. Therefore a quench protection circuit is incorporated. However, a quench can only happen if the cryomagnets have a helium level below the coil 60% during operation.

The operating current to the Q1 quadrupole coils is provided by Danfysik System 8000 power supplies, which can operate up to 3500 A current and 5 V. The power supplies will be cooled with a combined maximum water flow of 45 liters per minute.

In addition to the main quadrupole windings, all quadrupoles have multipole windings. To further optimize focusing properties of the HRS magnet system, it was intended to operate including some of these multipole trim coils in order to reduce higher order aberrations. The operating current for these multipole corrections is small, only (the multipole corrections are typically less than 2% of the main quadrupole field), of order 50 A, and will be provided by thirty two Lakeshore power supplies. These power supplies can operate up to 100 A current and 30 V voltage. Since the sextupoles were inadvertently installed rotated 90 $^\circ$ from their correct orientation, these trim coils are now considered useless and there are at present no plans to use them.