P. R. Sarma

Design of improved magnetic channel for superconducting cyclotrons by the random search method

Magnetic channels are used in cyclotrons and accelerators for extraction of ion beams. Passive magnetic channels use iron bars to lower the magnetic field locally and set up a field gradient. In order that the beam quality does not deteriorate, this field gradient should be highly uniform. We have used the random search method for improving the field quality of a magnetic channel by modifying the geometric parameters of a conventional channel.

A new method of finding the pole profile in quadrupole magnets for obtaining high field quality

The field quality in quadrupole magnets depends mainly on the pole profile. One often uses a circular pole of optimized radius to reduce the field errors. Hyperbolic profiles which are modified at the pole edges are also used. In the design of these magnets one first chooses a profile and then calculates the error harmonics. Here we have described a new method in which we choose the error harmonics first and then find the profile thus ensuring that the field quality is good. The method can also be applied to other multipole magnets.

Modification of cosθ coil shape in superconducting dipole magnets for reducing the coil size

In coil-dominated superconducting dipole magnets the coil size and hence the cost of the magnet can be reduced by optimizing the shape of the coil. We have used the variational calculus and a random search technique to show that the coil shape markedly deviates from the conventional cosθ design when one reduces the coil size while holding the field and field quality to specified values. A block-coil dipole giving a field of 7.0T has been designed on the basis of the optimization. With iron yoke this can give a field of 8.4T.

A coil shape in superconducting quadrupole magnets for obtaining the perfect field

A cos(2θ) current distribution over a cylindrical surface gives an ideal quadrupolar field in so-called cos(2θ) quadrupoles. A constant current in a coil configuration generated by two perpendicular intersecting ellipses also produces the perfect quadrupole field. We have found that an ideal quadrupole field can be obtained with a coil shape, different from the intersecting ellipses. Block-coil quadrupoles have been designed based on the coil shape as well as the intersecting ellipse shape.

New method of designing pole profile in combined function magnets of high field quality

Combined function magnets are often used in synchrotrons and beam lines. The field quality of such magnets depends mainly on the pole profile. A new method is described for determining the finite pole shapes of such magnets for obtaining field qualities of 1×10−4 or better. Profiles of the typical quadrupole-sextupole and dipole-quadrupole magnets are presented.

A simple design of sextupole magnets with optimized circular pole faces

A circular pole face for a sextupole magnet makes the mechanical design simple to fabricate. We have optimized the radius of the circular face to minimize the field error. We have obtained a thumb rule that the optimized radius for a sextupole should be 0.56 times the half-aperture of the magnet.

Triple drift ion beam bunching systems for enhanced bunching efficiency

To obtain high efficiency in bunching ions one generally uses a multiharmonic buncher or a double drift buncher. Here we have explored the possiblity of using a triple drift bunching system to obtain still higher bunching efficiency. It consists of three bunchers separated in space. The first buncher is excited with the fundamental rf wave whereas the other two bunchers are driven by the second harmonic. We have optimized the parameters of such a system and have shown that by using only two rf frequencies one can obtain bunching efficiency higher than what can be obtained with a multiharmonic buncher consisting of nine harmonics.

Abrupt variation in ion current with biased disk voltage in the electron cyclotron resonance ion source

The performance of the biased disk in the 6.4GHz electron cyclotron resonance ion source at VECC, Kolkata was studied at a pressure of ∼1×10−7Torr. We observed an abrupt variation of beam current with bias voltage. For low negative bias voltages (from 0 to −5V) the beam current gradually decreases and then shows a sudden change in magnitude at a voltage of about −6V. The higher the charge state, the higher is the magnitude of this change. For H+ also the current changes abruptly, but the jump is from a higher to a lower current. This indicates a sudden change in the charge-state distribution of the beam, likely corresponding to a suddenly improved confinement.

A method of designing magnetic channels of improved field quality for superconducting cyclotrons

Magnetic channels are used in the extraction system of superconducting cyclotrons for focusing and extracting the high energy beam. In order to preserve the beam quality, the field gradient produced by the channels should be constant. We have evolved a procedure based on the linear programming technique for optimizing the geometry of magnetic channels. With this, we have reduced the deviation in the field gradient not only in the median plane but through the beam aperture.