Abstract
In this paper, we explore a method to manipulate low energy electron bunches in a space charge dominated regime, and we use this method to design low energy linac bunch compressors to compress electron bunches in a space charge dominated regime. In the method, we use the space charge effects instead of avoiding them; i.e., we use the space charge forces to generate the required energy chirp instead of the ordinary method which uses the rf accelerating system to generate the chirp. We redefine the concepts of the dispersion function and beta functions in a space charge dominated regime to guide the optimization. Using this method, we study the low energy (5--22 MeV) linac bunch compressor design to produce short ($\ensuremath{\sim}150\text{ }\mathrm{fs}$) and small size ($\ensuremath{\sim}30\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$) bunches for the electron beam slicing project. The low energy linac bunch compressors work in a space charge dominated regime, and the bunches at the downstream of the gun have a negative energy chirp due to the space charge effects. To provide compression for the negative energy chirped bunch, we design a positive ${R}_{56}$ dispersive section using a four-dipole chicane with several quadrupole magnets. We have designed low energy linac bunch compressors with different photocathode rf guns. For example, one linac bunch compressor with the BNL photocathode electron rf gun has achieved a low energy bunch with the 166 fs rms bunch length, 28 and $31\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ rms beam size in the vertical and horizontal directions, respectively, at 5 MeV with 50 pC charge. Another example with LBNL's very-high frequency gun has achieved a low energy bunch with the 128 fs rms bunch length, 42 and $25\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ rms beam size in the vertical and horizontal directions, respectively, at 22 MeV with 200 pC charge.
Highlights
Very short bunches of high-brightness electron beams with very high peak currents are required in many systems, e.g., free-electron laser (FEL) systems or collider systems
The key ideas of this method to manipulate low energy electron bunches in the space charge dominated regime are as follows: We use the space charge effects instead of avoiding them to generate the energy chirp; we define a virtual reference particle whose six phase space coordinates are taken as the average values of all the bunch particles, and with this definition we redefine the concepts of the dispersion function and beta functions in the space charge dominated regime to guide the optimization; we split the linac compressor into two sections, the gun section and the compressor section, and calculate the simulation of these two sections separately
We find that the negative energy chirp of the bunch increases with the increase of the bunch charge due to the space charge forces increasing with the increase of bunch charge
Summary
Very short bunches of high-brightness electron beams with very high peak currents are required in many systems, e.g., free-electron laser (FEL) systems or collider systems. We design the unconventional low energy (5–22 MeV) linac bunch compressors which work in the space charge effect dominated regime to obtain the required short (∼150 fs), focused bunches (∼30 μm) with 50–200 pC charge. The key ideas of this method to manipulate low energy electron bunches in the space charge dominated regime are as follows: We use the space charge effects instead of avoiding them to generate the energy chirp; we define a virtual reference particle whose six phase space coordinates are taken as the average values of all the bunch particles, and with this definition we redefine the concepts of the dispersion function and beta functions in the space charge dominated regime to guide the optimization; we split the linac compressor into two sections, the gun section and the compressor section, and calculate the simulation of these two sections separately. V B we change the linac compressor’s rf gun from the BNL photocathode rf gun to LBNL’s very-high frequency (VHF) gun [13] and redesign the linac electron bunch compressor by applying the IMPACT-T with both space charge effects and CSR effects considered
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