ARIES IFE Final Focusing Magnets L. Bromberg Magnet Technology group at MIT MIT Plasma Science and Fusion Center ARIES Meeting UCSD April 23, 2002

Talk Novel superconducting materials Magnesium diboride End effects Design Window

Magnesium Diboride Maybe If field > 7-8 T Avoid Nb 3 Sn

Quadrupole arrays Large amounts of work to optimize single quadrupoles We have developed optimal designs for quadrupole arrays Minimization of field errors Simpler construction (planar) Applicable away from magnet ends What about the ends? How close can you put these magnets together? sin (2  )cos (2  )

IRE design of array

End effects We have developed requirements for matching currents to multipole required For quadrupoles, two solutions with sin (uniform current density) or cosine (linear variation in current density) However, most of the field errors are introduced at the ends Because of symmetry, only 2*n harmonics, n > 1 (n = 1 is quadrupole) Scalar potential of the magnetic field outside the quadrupoles decays as  sin (k x x) sin (k y y) exp (- (k x +k y ) z) Ansatz: Have the current density vary as j (z ) ~ exp(- (k x +k y ) z)

Treatment of the magnets ends Using j (z) ~ exp (- (k x + k y ) z ), together with. j = 0 For the sin(2  ) case: j z = j 0 c j x = j 0 x (k x + k y ) exp (- (k x + k y ) z ) For the cos (2  ) case: j z = j 0 x/a exp (- (k x + k y ) z ) j x = j 0 (x 2 - a 2 )/2a (k x + k y ) exp (- (k x + k y ) z )

End effects… work in progress We have preliminary evaluated the octopole component of a 3-D system using such magnet end current density Problems with normalization How to measure, where? integral along the path  ~  a n r n ( n = 2 is quadrupole)  n ~ a n dl Fastest e-folding of field with quadrupole array ~ a/ 

Final focused plasma Multiple beam paths… Final focus magnets Well protected, high field, compact magnets Good vacuum (10 -6 Torr) Clean metal surfaces (no insulating surfaces) Plasma source for neutralization Differential pumping Shielding Debris removal Compact axially and radially

Final optics Toughest magnet: 8 T 0.18 m beam tube radius This is a LARGE MAGNET Through out NbTi. HTS OK Maybe Nb 3 Sn

HTS based Tough magnet

LTS based Tough Magnet

Conclusions Design window Good Bad We want to be in the blue region