Higher order mode damper for low energy RHIC electron cooler superconducting radio frequency booster cavity

Abstract

To improve RHIC luminosity for heavy ion beam energies below 10 GeV/nucleon, the Low Energy RHIC electron Cooler (LEReC) is currently under commissioning at BNL. The Linac of LEReC is designed to deliver a 1.6 MeV to 2.6 MeV electron beam, with rms dp/p less than 5e-4. A 704 MHz superconducting radio frequency (SRF) booster cavity in this Linac provides up to 2.2 MeV accelerating voltage. With such a low energy and very demanding energy spread requirement, control of Higher Order Modes (HOMs) in the cavities becomes critical and needs to be carefully evaluated to ensure minimum impact on the beam. In this paper, we report the multiphysics design of the HOM damper for this cavity to meet the energy spread requirement, as well as experimental results of the cavity with and without the HOM damper.DOI:

Design and test of 704 MHz and 2.1 GHz normal conducting cavities for low energy RHIC electron cooler

“…If we shift the 3.2808 GHz mode 0.5 MHz away from the harmonic of 9 MHz, it gives voltage fluctuation of 0.57 kV, corresponding to ±2.9⋅10 -4 dp/p. Similar to [2], this 0.5 MHz difference can be reached by carefully choosing the electron beam kinetic energy, shown in Figure 5 (top plot). Any kinetic energy, for which the plot is above the dotted line, can guarantee that TM 011 2π/3 mode is 0.5 MHz away from the harmonic of 9 MHz.…”

“…As described in [2], 32 gaussian laser pulses with 0.6 mm rms length are stacked together with a 0.75 mm interval to form a 24 mm "flat-top" pulse. It is used to drive a DC photoemission gun [11] with a multi-alkali (CsK 2 Sb or NaK 2 Sb) cathode.…”

“…The electron linac of LEReC consists of a DC photoemission gun, one 704 MHz SRF booster cavity [2], and three normal conducting cavities: a 2.1 GHz third harmonic cavity, a 704 MHz cavity [3], and a 9 MHz cavity. The 704 MHz SRF booster cavity accelerates 400 keV bunches from the DC gun near the top of the RF wave, with an accelerating voltage up to 2.2 MV.…”

Stable operation of a high-voltage high-current dc photoemission gun for the bunched beam electron cooler in RHIC

“…The required LEReC beam parameters for cooling at the lowest BES energy are summarized in Table I. A photoemission electron gun injector, followed by a superconducting rf cavity for acceleration [23][24][25][26][27][28], was adapted as an electron source for LEReC. This choice took advantage of the high-voltage electron gun experience and the excellent beam quality achieved at Cornell University [29][30][31][32][33][34][35][36][37], in Japan [38][39][40][41][42][43][44] and at Jefferson National Lab [45][46][47][48].…”

High-brightness electron beams for linac-based bunched beam electron cooling

“…Design and commissioning of the rf cavities are described in Refs. [15][16][17][18][19][20]. The optics of the entire transport line has been designed and optimized to deliver electron bunches of different energies with an electron beam quality suitable for cooling [21].…”