Beam Extraction and Dump

For NLC, there is a chicane in extraction beam line to separate the electron beam and photons (synchrotron, beamstrahlung, bremmstrahlung radiations) with a common dump. As mentioned before, the dump is a huge neutron sources for severe background.

Recently (Nov.1999) K. Yokoya summarized requirements for a dump line as follows;

(1) FF optics and available space: Final focus optics is based on the JLC design study and the horizontal crossing angle is $\pm 4$m rad. A beam line for $\pm 10$m from IP is occupied by QC1 and QC2. So, any optical element for a dump line must be located at the upstream of 10m from IP.

(2) Beam parameters: Beam energy is assumed to be 500GeV for the most severe case. However, a design for 250GeV beam energy must be done separately because of its larger emmitance. For the beam parameters, JLC-C shall be chosen for the largest emmitance, which are listed in Table 2.

 

Beam energy 500GeV

$$500/14 \times 10^{-8}$$ norm.emit. x/y

$$\beta_x/\beta_y$$ 13mm / 0.2mm

$$\sigma_z \mu$$

After collisions, beam parameters were estimated by CAIN, and they are listed in Table 3.

 

$$1600/20 \times 10^{-8}$$ norm.emit. x/y

$$\beta_x/\beta_y$$ 4mm / 0.15mm

$$\alpha_x/\alpha_y$$ 1.5/0.0 ( nonzero because of beam-beam lens. )

(3) Beam diagnostic: At the dump line, there should be three measurements of (a) energy spectrum ( within a peak ), (b) polarization and (c) positrons in pair creation.

(4) Energy spectrum: For the measurement of beam energy, a bending magnet must be needed. The magnet should be vertical bend to prevent the positrons(c) from bending (horizontally) into the final focus line. NLC uses a horizontal bend because of its large crossing angle.

(5) Focussing optics: Extracted (disrupted) beams must be focused in order to be collided with lasers (polarized laser and laser wire etc.) for (a) and (b). The Rayleigh length(Z) of the laser is chosen to be equal to the bunch length of electron beam, $\sigma_z$=150$\mu$m. Since the minimal spot size of the $\lambda$=1$\mu$m laser is 3.5$\mu$m, the beam spot sizes, $\sigma_x, \sigma_y$, must be the same, 3.5$\mu$m. Taking accout of about twice emmitance at 250GeV beam, $\beta_x$ and $\beta_y$ should be 0.4m and 30m, respectively.

For the measurement of energy spectrum (a) with 0.1% resolution, a vertical dispersion must be reqiured to be $\eta_y$=3.5mm, where the corresponding spatial spread is 3.6$\mu$m ( = 0.1%$\times \eta_y$ ) as the same as the laser spot size. An orbit slope should be zero there for the polarization measurement,$\eta_y' =0$.

The above parameters must be optimized.