Test of two Nb superstructure prototypes

An alternative layout of the TESLA linear collider (Technical Design Report, DESY Report No. 2001-011, 2001), based on weakly coupled multicell superconducting structures (superstructures), significantly reduces the investment cost due to a simplification in the rf system of the main accelerator. In January 1999, preparation of the beam test of the superstructure began in order to prove the feasibility of this layout. Progress in the preparation was reported frequently in Proceedings of TESLA Collaboration Meetings. Last year, two superstructures were installed in the Tesla Test Facility linac at DESY to experimentally verify methods to balance the accelerating gradient in a weakly coupled system, the stability of the energy gain for the entire train of bunches in macropulses, and the damping of higher order modes. We present results of the first cold and beam test of these two Nb prototypes.

Figure 1

SST-II: the left subunit is assembled in a He vessel with cold tuner. The right one is shown with the single FPC needed to transfer rf power to 18 cells.

Figure 2

Computed energy deviation for the TESLA beam: 2820 bunches, $3.2\mathrm{nC}/\mathrm{bunch}$, spacing 337 ns.

Figure 3

Peak-to-peak field flatness. (a) P1, 92% and (b) P2, 94%.

Figure 4

Relative deviations (zigzag lines) from ideal linear spectrum shifts (straight lines) as measured for the eight highest modes of both prototypes.

Figure 5

Prototype of $2\times 7$ cells. Field probes: A1–A4. HOM couplers: HOM1–HOM3.

Figure 6

Signals from field probes (P1 upper diagram, P2 lower diagram) measured during the acceleration of 530 bunches, $q=4\mathrm{nC}$, $t_b=1\mu \mathrm{s}$ at $15\mathrm{MV}/\mathrm{m}$.

Figure 7

Prototype P2. Oscillations of signals (normalized) during the acceleration.

Figure 8

Spectrum of the energy modulation as measured at the end of the linac.

Figure 9

BPM signals without the excitation of the deflecting dipole mode.

Figure 10

BPM signals with the excitation of the deflecting dipole mode. Note different scales for both signals in the $x$ plane.

Figure 11

Deflection measured for the different optics setting.

Figure 12

Normalized position of the deflected beam in the $x\mathrm{\text{−}}y$ plane.

Figure 13

Damping of dipoles with $(R/Q)\ge 1\Omega /{\mathrm{cm}}^2$.

Figure 14

Damping of modes with $(R/Q)<1\Omega /{\mathrm{cm}}^{2n}$.