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Towards GV/m single-cycle THz pulses from undulators Vitaliy Goryashko 2016 Outline Accelerator physics in Uppsala and FREIA Laboratory Work on a linac-based THz source Concept of generation of single-cycle pulses with undulators

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Towards GV/m single-cycle THz pulses from undulators

Vitaliy Goryashko

2016

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Vitaliy Goryashko Single-cycle THz pulses

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Outline

  • Accelerator physics in Uppsala and FREIA Laboratory
  • Work on a linac-based THz source
  • Concept of generation of single-cycle pulses with

undulators

  • Optimal tapering
  • Proposal for a THz Light at Uppsala
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Vitaliy Goryashko Single-cycle THz pulses

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Background: accelerator physics in Uppsala

1940's: The(odore) Svedberg proposes to build a cyclotron

  • Gustaf Werner synchro-cyclotron (1947 – present)

– nuclear physics & cancer treatment

  • CELSIUS ring (1984 – 2005)

– nuclear physics

  • External

– CTF3/CLIC at CERN (since 2005) – FLASH/XFEL at DESY (since 2008) – ESS (since 2009)

  • FREIA laboratory (since 2011)
  • Skandion clinic (2015)

– cancer treatment

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European Spallation Source (ESS), Sweden

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FREIA

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FREIA: Facility for Research Instrumentation & Accelerator Development

cryogenics

  • liquid helium
  • liquid nitrogen

control room

  • equipment controls
  • data acquisition

radio-frequency (RF) power sources 3 bunkers with test stands horizontal cryostat vertical cryostat Competent and motivated staff

collaboration of physics (IFA) and engineering (Teknikum).

Funded by KAWS, Government, Uppsala Univ.

State-of-the-art Equipment

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Low-energy excitations: D. N. Basov et al., Rev. of Mod. Phys. 2011

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Combined THz/X-ray source: old design

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How to generate GV/m single- cycle pulses with MeV bunches?

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Single-cycle synchrotron radiation

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Single-cycle radiation from a segmented undulator

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Single-cycle radiation from a segmented undulator: cont’d

Magnetic field of segments

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Single-cycle radiation from a segmented undulator If instead of increasing the distance between the segments I will decrease it, I will recover Takashi’s tapered undulator.

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What is the optimum taper profile? Can one generate just one cycle?

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Simple reasoning from Takashi

total field is a convolution of the current density and the field of a single electron Suppose that the time profile of the current density is the same as the single-electron field profile The convolution boils down to the inverse Fourier transform of the spectral power.

Let us optimize ()

via undulator tapering

in order to get as few cycles as possible.

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0, = −

  • 2
  • 2
  • (′) sin #$

1 + ' − ′ #

  • ( )*+ ,- .$

/0/ /0/

On-axis field in a tapered undulator

  • E. Saldin et al. "A simple method for the determination of the structure of ultrashort

relativistic electron bunches." NIMA 539.3 (2005): 499-526.

2 = 3 34 , 34 = 3 2

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3 = 3 2 1 + 2

  • (4) ,

Stationary phase method

stationary point

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I equate the spectral power of radiation from a tapered undulator 5 to that of the quasi-half-cycle pulse

It turns out that the minimum number of cycles is 1.5. Optimal taper profile

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Radiation from the optimally-tapered undulator

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Proposal for a THz Light Source in Uppsala

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Wish list for intense THz radiation.

Parameter Quasi-half-cycle pulses for time- resolved experiments Narrowband pulses for frequency-resolved experiments Spectral range (THz) 1.5-15 1.5-15 Pulse duration (ps) 0.1-1 1-10 Pulse energy (mJ) 1000 100 Peak electric field (GV/m) 1 0.1 Relative bandwidth FWHM 100% 10% Repetition rate (kHz) 1-100 1-100

+ Polarization control, pump-probe configuration

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The source

  • it covers the spectral range from 5 to 15 THz, exceeding that
  • f laser-based sources;
  • polarization variable from linear to circular or elliptical;
  • tunability of the central frequency and bandwidth;
  • mutli-kilohertz repetition rate;
  • light carrying orbital angular momentum.
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Source 1: quasi-half-cycle pulses

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Source 2: multi-cycle pump and single-cycle probe Source 2a Source 2b