Non-π mode pre-bunching system IPNS, KEK: H. Miyatake, M. Okada, K. Niki, Y. Hirayama, N. Imai, H. Ishiyama, S.C. Jeong, I. Katayama, M. Oyaizu, Y.X. Watanabe Nishina Center, RIKEN: S. Arai ASRC, JAEA: H. Makii, Y. Wakabayashi M. Okada et al., PRST,15(2012) : 2nd RISP W.S. H. Miyatake, IPNS, KEK

Compact pre-buncher coupled with a beam chopper at the injection beam line of the accelerators Intense pulsed-alpha beams (> pμA) sharp time-structure (~20 ns in FWTM) of the pulsed beam enough beam suppression (less than ) during ~200 ns just before the beam-on period Experimental requirements at the TRIAC facility in C(α,γ) 16 O reaction cross section measurement in the low energy region with astrophysical interest

Experimental Setup TRIAC : Tokai Radioactive Ion Accelerator Complex (Now DIAC: Daejon Ion Accelerator Complex) Charger Breeder (ECR Ion source) : provide high intense α beam E α = 0.14 – 1.4 MeV / u → E cm = 0.4 – 4.2 MeV ( 12 C +  ) γ-ray detector Charge Breeder (18GHz ECR Ion Source) Chopper Pre-buncher 26MHz-SCRFQ Bunch interval 38.5 ns 2-4 MHz-beam Bunch interval ns

Single-gap buncher merit –compact design –arbitrary potential waves –unrestricted bunching frequency demerit –high bunching potential –beam distortion due to the leakage electric field Beam gnd electrodepotential electrode (V/cm) (cm) attenuation length ~10 cm < 30 cm (βλ of 2MHz, 2keV/u beam)

Two-gap buncher π-mode operation: tube length=βλ/2 Merit –lower bunching potential –no-leakage field Demerit –fixed bunching frequency –limited wave form of the bunching potential gnd electrodes Beam potential electrode (drift tube) Ｌ (=βλ/2) Saw-tooth wave bunching with 1st, 2nd, and 3rd harmonic frequencies is not possible by single two-gap buncher. Three two-gap bunchers are needed.

Bunching potential to the particles Assuming the effective voltages to the 1st and 2nd gap, V 1st and V 2nd, are expressed by the 3rd order harmonics π-mode operation →θ=π (fix !!)

Effective synthesized acceleration voltage, V total : If we adjust the effective potential V n and the phase φ n, Pseudo saw-tooth potential is realized in the single two-gap buncher with non-π drift length Precise digital wave technique is required: Arbitrary Function Generator Module

Optimization of the drift length in terms of the power dissipation P min = 140 deg. cf. P min = two-gap bunchers

βλ non-π mode two-gap pre-buncher particles in these region should be chopped.

Entrance Exit Longitudinal and Transverse Emittance of Simulated Beam at Entrance and Exit of SCRFQ Particle motion in the prebuncher : TRACEP (S. Yamada, 1990) Particle motion in the SCRFQ: PARMTEQ (K. R. Crandall et al., 1988) acceptance of SCRFQ (291π mmmr) Bunching gain =5.4 Back ground beam: ~20%

Beam Chopper Electric deflector with short rise/fall time constant around 40 ns →At the exit of SCRFQ linac, a fast chopper would be applicable: two-mode Tandem chopper (ISAC) with high voltage (~40 kV). →impossible for the installation → At the entrance of the SCRFQ, a single gap deflector would be applicable: chopping voltage (~2 kV) → suffer from a long tail (~10 cm) of the effective field. Chopper of multi-gap (layer) electrodes perpendicular to the beam direction Chopper-On Merit Gap space becomes short. → Required chopping voltage becomes low. Voltage is applied alternately on the layer electrode → Effective field length becomes short Demerit Some fraction of beam particles would be lost by the layer electrodes

variable drift tube (7 to 15 cm) gap (10 mm) inner diameter (40 mm)total length (267 mm)

rise/fall time constant: 50 ns chopping time: 160 ns for 2MHz bunching Beam suppression ~ Multi-layer beam chopper beam 2 keV/u width of each layer: 10 mm thickness of each layer: 0.1 mm gap between two layers: 1.9 mm applied chopping voltage: 160 V estimated tail of the field: ~1.5 mm transparency: 95% 40 mm grounded layers potential applied layers

Control circuit of the pre-bunching system Arbitrary function generator (AFG) is a key device –Bunching wave form is described with 0.5 ns step. –Numerical input table for the AFG is adjusted by monitoring the rf output. –Ideal wave form can be synthesized by taking account the transit time factor of the actual bunching electrode Triggers for the pre-buncher and the chopper are synchronized to the master generator for the accelerator complex.

BUNCHER OFF and CHOPPER OFF 13 micro-bunches in one bunching cycle (2 MHz) for the 26 MHz SCRFQ BUNCHER ON and CHOPPER OFF bunched into the central micro-bunch bunching gain: 5.46 width of the central bunch: 12 ns in FWTM background bunches: 20 % to total BUNCHER ON and CHOPPER ON beam suppression factor during 250 ns before the central bunch: 5 x bunching gain: 4.85 * beam loss (14%) due to the beam chopper time width of the central bunch: 13 ns in FWTM Beam test (1.1 MeV/u)

Summary Single two-gap pre-buncher of the non-π mode operation becomes available for the pseudo saw-tooth beam bunching. –It is realized by the precise wave digitized technique, AFG module. The multi-layer beam chopper for the low-energy beam is suitable for the sharp beam-chopping with relatively low voltage (160V) and short effective field length (1.5 mm). Combining the two-gap pre-buncher and the multi-layer beam chopper, beam suppression of with bunching gain of ~5 are achieved. This performance has been satisfied with the experimental conditions MeV de-excited γ-rays from the α-capture state in 16 O, by bombarding 6 pμA, 701 keV/u bunched α beams Induced γ-rays by neutrons from (α, n) reactions of 13 C, contaminant of the 12 C-target.

Thank you

exp. 12 C(α,γ) 16 O reaction cross section measurement in the low energy region with astrophysical interest –Background neutrons from 13 C(α,n) 16 O smear out the prompt γ-rays in the DC-beam experiment –Pulsed intense alpha beams (~pμA) are essential Enough beam suppression (less than ) during ~200 ns beam off period Sharp structure (~20ns in FWTM) of the pulsed beam induced γ- rays by neutrons from 13 C(α,n) 16 O

Saw-tooth wave bunching with two-gap buncher Pseudo saw-tooth synthesized from 1st, 2nd, and 3rd harmonic waves ? 3 two-gap bunchers in π-mode are ideal, but expensive and need space. Possibility of a single two-gap buncher in non π-mode operation

BUNCHER ON bunched into the central micro-bunch bunching gain = 5.46 time width of the central bunch: 12 ns in FWTM background bunches: 20 % to total BUNCHER ON and CHOPPER ON beam suppression factor during 250 ns before the central bunch: 5 x bunching gain = 4.85 * beam loss due to the beam chopper time width of the central bunch: 13 ns in FWTM background bunches: 20 % to total

Installation Multi-layer beam chopper variable frequency saw-tooth wave two-gap pre-buncher

Chopping the background bunches 12 micro-bunches should be chopped clearly in every 13 micro-bunches Electric deflector with short rise/fall time (~38 ns) At the exit of the SCRFQ (178 keV/u) ? → two-mode tandem chopper at ISAC * need much space for the TRIAC At the entrance of the SCRFQ (2 keV/u) ? →single gap deflector suffers from the long tail of the effective field Chopper of multi-gap (layer) electrodes perpendicular to the beam direction gap space becomes short →required deflection voltage becomes low effective field length becomes short, if the voltage is applied alternately on the layer electrode some fraction of beam particles would be lost by the layer electrodes

Transit Time Factor