Electron bunch diagnostic development

Particle accelerators are a vital tool in the investigation of the structure of matter. However, those utilising conventional radio-frequency (RF) technology are large (m-km) and expensive due to the limitation of the magnitude of the electric field (<100MV/m), which drives the acceleration. Plasma-based accelerators offer the possibility of achieving accelerating field strengths one thousand times larger. Consequently the scale of the facilities reduce to such an extent that they might be supported within an individual university or hospital.

Plasma-based particle acceleration utilising a laser to drive the accelerating structure, the wakefield, has developed extensively since its proposal in 1979[1], including the introduction of a capillary-discharge guiding structure[2], and has recorded electron bunches with percent level energy spread and GeV energies[3]. These have high current (tens of pC within a few tens of femtoseconds) also making them desirable for use in compact FELs [4]. Additionally the transverse motion of the accelerating electrons can generate a high-brightness, coherent X-ray source [5].

One of the remaining challenges is the stability and reproducibility of the acceleration and therefore of the final bunch properties. Precise control of the injection of electrons into the accelerating wakefield is necessary if this is to be improved. It has been proposed that his can be achieved by tailoring the gas target[6][7].

The FLA/PWA group aims to build on the idea of the capillary-discharge target by developing targets with integrated gas jets. In addition modification of the capillary shaping will be used to extend the interaction length and control the transverse motion of the electrons to tune the properties of the resulting betatron oscillation. With these targets we will investigate the effect of varying parameters on bunch qualities and will determine the required condition for optimised energy gain, compression and emittance.

In order to achieve this we require diagnostics capable of measuring the femtosecond-duration laser-plasma accelerated electron bunches in a single shot. We propose to utilise similar approaches to those employed in radio-frequency (RF) accelerator diagnostics, modifying them to provide high temporal resolution and independence from multiple shot measurement.

Transition radiation (TR) is broadband radiation that is generated whenever a charge crosses the interface between two materials with different dielectric constant[8]. This radiation has been used within the RF accelerator community predominantly as a method of measuring the longitudinally-integrated transverse beam profile and the total charge of the bunch. For long electron bunch durations TR is incoherent at visible wavelengths and the intensity of the radiation scales linearly with the charge of the bunch. For shorter bunch durations, the coherent region of the spectrum extends down to shorter wavelengths. The intensity of the coherent radiation is quadratically related to the charge in the bunch, but is also dependent upon the bunch form factor, the Fourier transform of the temporal profile of the bunch.

In FLA/PWA we are developing transition radiation diagnostics towards several goals:
a. Determination of the bunch form factor and temporal profile using a broadband spectrometer for the measurement of coherent light and the transition between coherent and incoherent spectral[9].
b. Beam transverse profile and charge measurements using incoherent light at optical wavelengths through spatial filtering.
c. Determination of the bunch duration using spikes within the incoherent spectrum[10].

Preliminary tests have been performed during experiments conducted at the ASTRA Gemini laser facility of the STFC Rutherford Appleton Laboratory in 2012 and 2014. Here spectral and profile measurements were made covering the wavelength ranges 400-1000 nm and 5-20 microns. In addition an integrating current transformer was used to measure the peak charge of the bunch. Below are some preliminary data from the most recent experiment:

Modulated spectra of transition radiation generated by a laser-plasma accelerated electron bunch propagating through an aluminium foil.

Near-field of the transition radiation for the same electron bunch.

[1] T. Tajima and J. Dawson, PRL (1979) 4, 267-270
[2] D. J. Spence and S. M. Hooker, PRE (2000) 63, 015401
[3] W. P. Leemans et al., Nat. Phys. (2006) 2, 696-699
[4] F. Gruener et al., Appl. Phys. B (2007) 86, 431-435
[5] S. Kneip et al., Nat. Phys. (2010) 6, 980-983
[6] S. Bulanov et al., PRE (1998) 58, R5257-R5260
[7] A. Gonsalves et al., Nat. Phys. (2011) 7, 862-866
[8] V. L. Ginzburg and I. M. Frank, Zh. Eksp. Teor. Fiz (1946) 16, 15; P. Goldsmith and J. V. Jelley, Philosophical Magazine (1959) 4,836
[9] S. Wesch et al., Nucl. Instr. Meth. Phys. Res. Sec. A (2011), 665, 40-47
[10] P. Catravas et al., PRL (1999) 82, 5261-5264