- Demonstrate SASE gain to saturation at 600-800 nm
- Operate with ATF linac beam (72-85 MeV)
- Fit in available space in ATF -- 4 m long
- Must achieve gain length of <0.4 m
- Must control e-beam walk-off to < 50 mm
- Need strong focusing
- Need ultra-precise alignment techniques

**VISA Magnetic Structure**

Halbach-type Pure-Permanent Magnet Undulator with Distributed Strong Focusing

| |

Undulator magnet length [m] | 4 |

Undulator type | Planar permanent magnet |

Magnet material | NdFeB |

40 mm x 10 mm x 4.45 mm | |

Thickness of magnet block (z) [mm] | 4.45 ± 0.05 |

Length of magnet block (x) [mm] | 30 ± 0.1 |

Height of magnet block (y) [mm] | 10 ± 0.1 |

Separation between magnet blocks [mm] | 0.05 ± 0.025 |

Total Number of magnet blocks | 1760 |

Undulator period l_{w}[mm] |
18 |

Full gap g [mm] | 6 |

Undulator field B_{max} [T] |
0.75 |

Undulator parameter K | 1.26 |

a_{w} |
0.89 |

Number of periods per segment | 55 |

Number of segments | 4 |

segment magnet length [m] | 0.99 |

Number of periods N_{w} |
220 |

Wiggle plane | horizontal |